Very Low Noise Preregulator for Benchtop Power Supply

[Kalvin]

Hi,

I am in the process of collecting ideas for a very low noise benchtop power supply to be used for developing precision analog circuits. It will provide three isolated channels: 2 * 0V .. 24V / 1A for the analog circuits and 1 * 0V .. 5V / 2A for digital circuits. The target for maximum noise voltage is less than 100uV. No high frequency switching noise is allowed.

The straight forward design would be to use rechargeable batteries and linear regulators. Batteries would not generate any ripple or hum, so essentially the power supply should be quite noiseless and with careful linear regulator selection and design the 100uV noise level should be realistic.

The other option is to use traditional design with transformer(s), rectifiers, capacitors and linear regulators. Selecting transformers with tapped output would reduce any heat problems associated to linear regulators. This design should be pretty noiseless as well, but requires some relay or switch logic for selecting proper transformer taps.

There are few designs floating around which utilize triac-controlled preregulator feeding the linear regulator. These design doesn't require tapped transformers and they should have a good efficiency and generate very little heat. However, I am not sure how noiseless these triac-controlled preregulators are.

Both the batteries and transformers are quite bulky, and I was thinking is there any clever way to reduce sized and weight and keep the noise level down.

Then I came up with a (quite complex) idea which might work: Let's take one power supply of 30V / 3A and three adjustable buck-regulators, one for each output channel. The buck-regulators will provide adjustable power supply to three power op amps driven by good quality 30kHz sine wave. Each power op amp will drive a 1:1 transformer driving a rectifier, filtering capacitors and the linear post-regulator. There will be required some opto-isolated feedback from the output back to the buck-regulators. Using the 30kHz sine wave will keep filtering capacitor size small and less bulky. This design is a mix of switching preregulator and linear regulator, and the 30kHz transformers are used for isolation and noise filtering.

How does this sound to you - other than this is a bit complex solution for a quite simple problem. However, this solution is quite flexible and scalable, and it will keep the size and generated heat down . Do you think that this solution would be able to provide low noise output, or do you think that buck-regulators might still emit too much switching noise to the secondary side of the 30kHz transformer so that the 100uV design target is difficult to achieve?

[mzzj]

Sounds complicated and you still have relatively poor efficiency.  Linear amplifier driving  ainewave is something like 60% max effiency plus all the other losses.

 I would go for brute force, no fancy tapped Transformers or anything. Just linear regulator with 100w cooling capacity. Maybe a capacitor multiplier as a preregulator..

[dannyf]

Then I came up with a (quite complex) idea which might work

Seems to be quite complicated a solution for a multi-faceted issue.

If you are concerned about ripple out of a smps, you can probably design a linear regulator with good PSRR at high frequencies. But in general, high speed high power feedback loops driving a dynamic load are hell.

[Liv]

Most low-noise will preregulator running on mains frequency. Implemented on the MOSFET, not on the triac. Since the switching frequency is low, you can specifically reduce the slew rate, and it will not affect much on efficiency. This is done, for example, a Agilent U8002A.

In my PSU I use multi-level output stage. This is similar to switching windings, but has excellent dynamics.

[Marco]

I prefer to go with 3 fixed independent flyback or forward converters, each followed by a buck converter.

If it's a simple flyback module why bother with the buck converter? The optocoupler feedback is trivial to adapt for variable output voltage.

[Marco]

A bang bang DCM fly back is always in the same bad shape ... but ripple at those frequencies is easy to clear up.

Why care about the efficiency of the flyback going down at lower voltages? The losses in Watt are still going to go down, so it won't become a problem for the supply ... and Kalvin clearly doesn't care that much about overall efficiency.

[blackdog]

Hi, 

Lets do it a different way...

In an old magazine i found this schematic...


I changed it a little and made a test circuit with a LT1083.
Q3 measures the diverence between the input and the output of a regulator, here a LT1083.



When the difference is large enough, then Q3 will give a pulse to the gate of the thyristor.
Now T1 stops the charging of the buffer capacitor C1.

It differs from a "normal" thyristor circuit is, that the charging is stopped, when sufficient energie is present in C1.
Charging will start again at each zero crossing and is switched off again when sufficient energie is in C1.
A typical thyristor circuit turns "random" on, this would create interference pulses, there are large coils necessary to suppress current spikes.

R4 and C3 controles the switching off, adjusti these values for minimum noise from the transformer.
If you make C3 to large, the noise( al little, much better than a normal Thyristor controler) is gone, but the dissipation in the IRF5210 wil be Higher.

Dutch site were i explane how its works, use google translate   
http://www.circuitsonline.net/forum/view/110029/11

And then this, do NOT use a normal bridge circuit if you want a high efficiency powersupply, use a LT4320 electronic Bridge controler.
http://www.bramcam.nl/NA/NA-01-PSU/LT4320

On the same website/topic you can see some testing i have done with a LT4320, its a beautiful IC!

It is very difficult to build a low-noise power supply with a switching pre-regulator (30Hkz - 2Mhz)
The switcher is like a lighthouse in a darkroom 

Just 2 cent's

Kind regarts,
Blackdog

[Kalvin]

Thanks guys for you insight and suggestions.

@mzzj: Yes, like you said the op amp generating a sine wave has quite low efficiency 60%, and the overall efficiency might be close to the original brute force approach.

Let's say the output voltage is set to 12V @ 1A. The dissipation in the brute force, direct battery-operated linear regulator is 12W, giving efficiency of 50%. With the lower output voltages the efficiency will drop even more. For example with the 5V output voltage the power loss in the linear regulator is 19W.

With a tapped transformer the voltage drop across the linear regulator would be typically 2-4 volts depending of the tapping available, giving typical dissipation of 2W - 4W. Using some sort of triac- or MOSFET-driven pre-regulator, one could reach similar dissipation of 2W - 4W.

@blueskull: I would like to avoid any SMPS if possible. Although my idea is to use a buck converter as an adjustable pre-regulator feeding the op amps seems counter-intuitive  in this regard, I guess that the op amp and the 1:1 transformer should filter out the switching noise quite well. Of course, I can be totally wrong here and the switching noise from the buck converter gets transferred though the op amp, transformer, rectifier, filtering and the linear post-regulator to the output destroying my limit for the 100uV output noise, as I don't have tested this yet. This is something I don't have experience and I am just guessing here.

@dannyf: My intention is to keep the feedback from the input of the linear post-regulator to the pre-regulating buck-converter as lazy as possible, and use the linear post-regulator as the main controlling device. In this way I should be able to avoid the hellish control loop. The linear post-regulator would be fed by a beefy capacitor providing enough energy during load transients and while the buck regulator is trying to catch up and keep the capacitor filled.

@Liv: Thank you for your suggestion. I need to take a look at  Agilent U8002A.

I tried to estimate the power loss of the op amp driven by a sine wave at 12V and 1A RMS. In this case the power supply voltage needs to be 16V as the power op amps require quite a good headroom. The typically efficiency for driving sine wave is said to be 60%. The direct approximation for the losses is 16V * 1A * 0.4 = 6.5W. This is not terribly bad unless my estimation is totally wrong. For 5V the losses in the op amp are 9V * 1A * 0.4 = 3.6W. At 24V the losses increase to 28V * 1A * 0.4 = 11W.

@blackdog: Nice design. I need to take some time studying it. I have been thinking something like http://cds.linear.com/docs/en/application-note/an32f.pdf figures 5 and 8. Those designs seem quite simple and encouraging.

[tszaboo]

Then I came up with a (quite complex) idea which might work: Let's take one power supply of 30V / 3A and three adjustable buck-regulators, one for each output channel. The buck-regulators will provide adjustable power supply to three power op amps driven by good quality 30kHz sine wave. Each power op amp will drive a 1:1 transformer driving a rectifier, filtering capacitors and the linear post-regulator. There will be required some opto-isolated feedback from the output back to the buck-regulators. Using the 30kHz sine wave will keep filtering capacitor size small and less bulky. This design is a mix of switching preregulator and linear regulator, and the 30kHz transformers are used for isolation and noise filtering.

How does this sound to you - other than this is a bit complex solution for a quite simple problem. However, this solution is quite flexible and scalable, and it will keep the size and generated heat down . Do you think that this solution would be able to provide low noise output, or do you think that buck-regulators might still emit too much switching noise to the secondary side of the 30kHz transformer so that the 100uV design target is difficult to achieve?

Linear already did very similar things some 27 years ago
http://cds.linear.com/docs/en/application-note/an29f.pdf
Funny how they have an answer to almost any analog problems ever existed. I see 30uV noise figures on page 5. I'm sure somehow you can adopt design to work in the 21 century.

[Kalvin]

Then I came up with a (quite complex) idea which might work: Let's take one power supply of 30V / 3A and three adjustable buck-regulators, one for each output channel. The buck-regulators will provide adjustable power supply to three power op amps driven by good quality 30kHz sine wave. Each power op amp will drive a 1:1 transformer driving a rectifier, filtering capacitors and the linear post-regulator. There will be required some opto-isolated feedback from the output back to the buck-regulators. Using the 30kHz sine wave will keep filtering capacitor size small and less bulky. This design is a mix of switching preregulator and linear regulator, and the 30kHz transformers are used for isolation and noise filtering.

How does this sound to you - other than this is a bit complex solution for a quite simple problem. However, this solution is quite flexible and scalable, and it will keep the size and generated heat down . Do you think that this solution would be able to provide low noise output, or do you think that buck-regulators might still emit too much switching noise to the secondary side of the 30kHz transformer so that the 100uV design target is difficult to achieve?

Linear already did very similar things some 27 years ago
http://cds.linear.com/docs/en/application-note/an29f.pdf
Funny how they have an answer to almost any analog problems ever existed. I see 30uV noise figures on page 5. I'm sure somehow you can adopt design to work in the 21 century.

Indeed. I have read this great application note and the original idea of using the sine wave came from the design "Ultralow Noise 5V to ±15V Converter" - the 30uV output noise is something to go for. However, I wanted to be able the adjust the secondary voltage in order to reduce the heat in the linear post regulator, thus the idea of using power op amps and variable supply voltage for the op amps = tracking pre-regulator. However, the application note states that the efficiency is only 30%.

[tszaboo]

Then I came up with a (quite complex) idea which might work: Let's take one power supply of 30V / 3A and three adjustable buck-regulators, one for each output channel. The buck-regulators will provide adjustable power supply to three power op amps driven by good quality 30kHz sine wave. Each power op amp will drive a 1:1 transformer driving a rectifier, filtering capacitors and the linear post-regulator. There will be required some opto-isolated feedback from the output back to the buck-regulators. Using the 30kHz sine wave will keep filtering capacitor size small and less bulky. This design is a mix of switching preregulator and linear regulator, and the 30kHz transformers are used for isolation and noise filtering.

How does this sound to you - other than this is a bit complex solution for a quite simple problem. However, this solution is quite flexible and scalable, and it will keep the size and generated heat down . Do you think that this solution would be able to provide low noise output, or do you think that buck-regulators might still emit too much switching noise to the secondary side of the 30kHz transformer so that the 100uV design target is difficult to achieve?

Linear already did very similar things some 27 years ago
http://cds.linear.com/docs/en/application-note/an29f.pdf
Funny how they have an answer to almost any analog problems ever existed. I see 30uV noise figures on page 5. I'm sure somehow you can adopt design to work in the 21 century.

Indeed. I have read this great application note and the original idea of using the sine wave came from the design "Ultralow Noise 5V to ±15V Converter" - the 30uV output noise is something to go for. However, I wanted to be able the adjust the secondary voltage in order to reduce the heat in the linear post regulator, thus the idea of using power op amps and variable supply voltage for the op amps = tracking pre-regulator. However, the application note states that the efficiency is only 30%.

Where exactly do you need this high current and low noise? I'm asking, because when I need a low noise power supply, the only way to do it is to have it onboard with the precision circuits. I mean it is kinda pointless to have a low noise circuit, and then connect it to it's PSU with 1 meter cable and create a bad PSR(R) with the cable. Even it is 4 wire PSU, you still have loop compensation and other problems. An onboard good old LM317 or a 78xx creates a power supply which is usually much better than any remote PSU.

[Kalvin]

Where exactly do you need this high current and low noise? I'm asking, because when I need a low noise power supply, the only way to do it is to have it onboard with the precision circuits. I mean it is kinda pointless to have a low noise circuit, and then connect it to it's PSU with 1 meter cable and create a bad PSR(R) with the cable. Even it is 4 wire PSU, you still have loop compensation and other problems. An onboard good old LM317 or a 78xx creates a power supply which is usually much better than any remote PSU.

The initial need is to have an adjustable, low noise benchtop power supply for precision analog systems prototyping and development. Like I stated in my first posting, using the rechargeable batteries are good starting point and I have used them with great success: No switching noise what so ever. In reality, actual power requirements are quite modest as a typical measurement circuit will consume only tens of milliamperes to 100mA and typical voltage range is +/- 5V ... +/- 15V. The dynamic range in the measurement systems is 80 dB or more, so the power supply must be noiseless. I was hoping that I could stretch my real requirements a bit and create a low noise power supply with a bit higher maximum current range up to 1A and voltage range up to 24V. Maybe this is too tall order anyway, and I will do something more conservative and split the design for two different power supplies altogether.

As you stated correctly, the onboard LM317 or any other similar regulator is hard to beat, and I am not really trying to do that - Typically these measurement devices will be portable, handheld devices, and they will be powered from rechargeable batteries. During the development and prototyping I would like to substitute the real power supply with something more flexible which would allow me to see how the system performs under different power conditions etc. etc.

My intention here is to collect information from the knowledgeable people on this forum, and trying to figure out whether my ideas are realizable. Maybe someone else will also learn from this discussion, as I will.

[blackdog]

Hi Kalvin, 

If you want a "Low Noise" Bench Supply, stay away from the schematic's like the LT designs.
Everything with "hard switching" even with a inductor in serie, will generate a lot of EM, been there tried that...

The design from Mark-Stuart (not mine) is one of the view you can use for low noise prereg and high efficiency.
The other one is a transformer with 3 a 4 taps...

And, if you want low noise, watch out how you layout the wiring, almost everyone ignores it...
Take some time and read my notes on the link on about my powersupply design.

http://www.circuitsonline.net/forum/view/110029/1/voeding


If you want "Low Noise", this design give you low noise, about 5uV @ 22Khz bandwith.
Ri for DC and good wiring far less than 0.0001 Ohm, AC 100Khz, less then 0.02 Ohm
No Power on / Power off transients, fast acting current control etc.
Forget at the moment, the FET at the output (Q7 IRFP2907)


Just another 2 Cents..

Kind regarts,
Blackdog

[dannyf]

How about just amplifying a voltage reference? Is it going to be noisier than all the complicated contraption?

[Kalvin]

Thank you for your valuable input. Especially i would like to thank blackdog for spending some time on this issue.

I will go for the design based on tapped transformer as it will be a simple, straightforward solution for the low noise power supply with easy thermal management.

In one of my earlier posts I was referring to LT1970A "500mA Power Op Amp with Adjustable Precision Current Limit". I will prototype the parts for this design.

[albert22]

Interesting design. But.
Please correct the link.
May be is: http://www.circuitsonline.net/
Is there an English version or should we use google translator ?
Thanks

[blackdog]

Hi Albert22,

Sorry, bad link 

This is the good one, sorry no English version... Google is your Friend 

http://www.circuitsonline.net/forum/view/110029/11/voeding

Kind regarts,
Blackdog

[timofonic]

Hi Albert22,

Sorry, bad link 

This is the good one, sorry no English version... Google is your Friend 

http://www.circuitsonline.net/forum/view/110029/11/voeding

Kind regarts,
Blackdog

Hello.

I need a real 30V PSU and low noise could help me to do certain practices that usually I need to do in our crowded class.

How did you draw that very pretty schematic? I'm forced to use Eagle at my vocational training school, it's ugly as hell.

Is that LT1021 expensive to get? What advantage has over discrete component or other part?

How much it it would cost the BOM?

Is that a relay in the second transformer?

Do you use two transformers?

Is a toroid transformer some improvement in the device?

Would you provide the schematic in some interoperable form? I'm still very slow drawing schematics

Sorry for too many questions. Thanks a lot for your project!

Best regards.

[blackdog]

Hi Circuiteromalaguito,

Tomorrow i wil awnser your questions, now iam tired :-)

Kind regarts,

Blackdog

PS
Schematic editor: http://www.abacom-online.de/uk/html/splan.html

[timofonic]

Hi Circuiteromalaguito,

Tomorrow i wil awnser your questions, now iam tired :-)

Kind regarts,

Blackdog

PS
Schematic editor: http://www.abacom-online.de/uk/html/splan.html

Hi Blackdog.

No problem! Thanks for your reply!

Nice schematic editor and CHEAP! Where's the trick compared to stuff like Eagle, DipTrace and Altium?

Best regards.

[timofonic]

Hi Circuiteromalaguito,

Tomorrow i wil awnser your questions, now iam tired :-)

Kind regarts,

Blackdog

PS
Schematic editor: http://www.abacom-online.de/uk/html/splan.html

 Hijo

Careros you there? Just a reminder, I don't wanna annoy you. Take your time!

Thanks!

[blackdog]

Hi,

Sorry for the delay...

1e
LT1021, you can use different 5V references, i do not know were you live, here in the netherlands i can buy what i want from many suppliers.
It is also posible to use a different reference like the LT1027-5V, LT1236-5V, TI REF5050 or an LTC6655-5V.
I have a lot of LT1021 on stock fore a project, but the LT1027 its even better in some specs.
A LT1021 here in the Netherlands cost about 8-Euro, a LT1027 about 12-Euro.

2e
BOM Costs...
I do not know, i dit not make a calculation...

3e
Relais
Yes its more than one relais, but i wil not use a relay's, it wil use a Powerfet preregulator like the schematic on de Circuitsonline website.

4e
I use two transformers or maybe one, it is bettet to use a toroid and lay some extra turns arround the toroid so u don't need T3.
T3 can be a verry smal transformer, about 2 Watt's, maybe it's easter just tho buy the transformer than put extra turns on the big one.

5e
Would you provide the schematic in some interoperable form? I'm still very slow drawing schematics
I do not understand this question, sorry...


Kind regarts,
Blackdog

[DanielS]

How about just amplifying a voltage reference? Is it going to be noisier than all the complicated contraption?

The reason why bench-top power supplies are "complicated contraptions" is because they have to protect loads from power-on/off glitches, be designed so their most likely failure modes are safe to avoid damaging the load if possible, be able to survive all manners of load faults, have clean responses to almost any sort of load transient, etc.

There is more going on inside a bench-top supply than merely scaling a reference.

[blackdog]

Hi DanielS,

"There is more going on inside a bench-top supply than merely scaling a reference"

I can not say it any better than you 

Kind regarts,
Blackdog

[timofonic]

Hi,

Sorry for the delay...

1e
LT1021, you can use different 5V references, i do not know were you live, here in the netherlands i can buy what i want from many suppliers.
It is also posible to use a different reference like the LT1027-5V, LT1236-5V, TI REF5050 or an LTC6655-5V.
I have a lot of LT1021 on stock fore a project, but the LT1027 its even better in some specs.
A LT1021 here in the Netherlands cost about 8-Euro, a LT1027 about 12-Euro.

2e
BOM Costs...
I do not know, i dit not make a calculation...

3e
Relais
Yes its more than one relais, but i wil not use a relay's, it wil use a Powerfet preregulator like the schematic on de Circuitsonline website.

4e
I use two transformers or maybe one, it is bettet to use a toroid and lay some extra turns arround the toroid so u don't need T3.
T3 can be a verry smal transformer, about 2 Watt's, maybe it's easter just tho buy the transformer than put extra turns on the big one.

5e
Would you provide the schematic in some interoperable form? I'm still very slow drawing schematics
I do not understand this question, sorry...


Kind regarts,
Blackdog

Cabmn you export the netlist in some way it can imported in Eagle and KiCad? What file formats can esplanade export to? Cadence ASCII? Protel? SPICE?

I think Spain has fewer supplies. There are llocal ones in my city, but they are laughably expensive.

[mrgregs]

Hi,

Sorry for the delay...

1e
LT1021, you can use different 5V references, i do not know were you live, here in the netherlands i can buy what i want from many suppliers.
It is also posible to use a different reference like the LT1027-5V, LT1236-5V, TI REF5050 or an LTC6655-5V.
I have a lot of LT1021 on stock fore a project, but the LT1027 its even better in some specs.
A LT1021 here in the Netherlands cost about 8-Euro, a LT1027 about 12-Euro.

2e
BOM Costs...
I do not know, i dit not make a calculation...

3e
Relais
Yes its more than one relais, but i wil not use a relay's, it wil use a Powerfet preregulator like the schematic on de Circuitsonline website.

4e
I use two transformers or maybe one, it is bettet to use a toroid and lay some extra turns arround the toroid so u don't need T3.
T3 can be a verry smal transformer, about 2 Watt's, maybe it's easter just tho buy the transformer than put extra turns on the big one.

5e
Would you provide the schematic in some interoperable form? I'm still very slow drawing schematics
I do not understand this question, sorry...


Kind regarts,
Blackdog

Cabmn you export the netlist in some way it can imported in Eagle and KiCad? What file formats can esplanade export to? Cadence ASCII? Protel? SPICE?

I think Spain has fewer supplies. There are llocal ones in my city, but they are laughably expensive.

Farnell has a Spanish storefront (although it's still UK/Liege stock), and have the LT1027 for 8.29 euros. I don't know what the P&P is, or if it's like in the UK where it's free shipping over £20, as I don't speak Spanish, sorry:( (I just replaced the "uk" at the beginning of the URL with "es")

[Electroplated]

Hi Kalvin,

My intention here is to collect information from the knowledgeable people on this forum, and trying to figure out whether my ideas are realizable. Maybe someone else will also learn from this discussion, as I will.
[/quote]

My two bobs worth regarding switchers are pre regs, I have a psu that uses a LM2576 as the pre reg, it tracks the linear output so it sits 5 v above the output. For a few weeks I fought with the noise spikes on the output, even though I had carefully designed the pcb there was a good 50mV noise. What I did was change the position of the catch diode so it is cathode end on through board, fitted a small ferrite bead to the longer anode.

I then fitted two 470n ceramic smd caps as close as possible to the pcb side of the switchers input and ground then added a common mode choke and some caps, I ended up with a rather low noise output, on full load it produces around 1mV p-p noise, thats at 25 volts @ 3 A, most of that is low level ripple.

So with careful planning of the pcb around the switcher and some filtering noise can be reduced, however, using leads from the psu to say a plug board for your project would require a small cap across the boards input to reduce any further noise picked up in the leads.

I also note from my painful experiments that if I connect the ground output of the psu to earth then noise levels rise, mostly common mode noise.

On another prototype pcb I have the switcher separate from the rest of the circuit, it feeds the linear part via two jumper cables that pass a toroid ring with two turns, then into a series of ceramic caps, early tests do show a reduction in switching noise but I need to fully evaluate the idea further.

Regarding the thyristor pre reg, its an old design that I first saw in the same mag way back in 94, I had to build it, it worked, I think I still have my modified version laying in the loft. ( last part added due to the image bringing back memories ).

EP

[IanJ]

Just for reference.........My own home made dual channel linear bench psu with switcher pre-regulator (LM2576).

My rigol scope in dual channel mode (an attempt at a differential probe setup), math function A-B, 50ohm in-line terminators fitted.

Ch.1 of the Psu set to 5vdc out and with a 1amp load.
Ch.1 & Ch.2 of the scope set to AC, 2mV / div, 20Mhz bandwidth limit on.
The math output scale set to 2mV / div.

The p-p noise is measured at approx. 4mV. Those repeating peaks are from the on-board switch-mode tracking pre-regulator.

PS. If I ever design and build another it won't be using a switcher, but I had tried a mosfet pre-reg at prototyping stage but had issues coming off heavy loads and the pre-reg not recovering properly.

Ian.

[JohnnyBerg]

PS. If I ever design and build another it won't be using a switcher, but I had tried a mosfet pre-reg at prototyping stage but had issues coming off heavy loads and the pre-reg not recovering properly.

Welcome to the club !

[T3sl4co1l]

Well...I've got a radio project with a DC-DC converter on the front end... just sayin'... 

[JohnnyBerg]

Yeah .. with relative steady loads, and fixed voltages it is doable. However, I always see switcher noise at the output

With quick transient loads, and a wide range of output voltage it becomes "a whole different cake", as we Dutch say 

[Electroplated]

Well...I've got a radio project with a DC-DC converter on the front end... just sayin'... 

And I have a few failed pro-type bench supplies that have an rf transmitter .....

Way back in 96 I came up with a rather simple pwm bench supply, it used a 555 in pwm mode and a mosfet, it was actually added to the magazine project shown scanned in this thread, minus the thyristor prereg, from memory I had it give 25, 50, 75 % duty cycle then the mosfet went full on, worked ok once I ironed out a few problems, sadly I loaned it out and never got it back, I did search through a box of old diagrams for the design not so long ago but gave up.

That's another odd task the venerable 555 could do :-)

Just for interest on efficiency, I have a dual linear supply rated at 0-30V @3A per channel, on the rear of the case it states its wattage at 250W, just sitting there powered on it draws 45W but one channel under full load shows it drawing almost 220W, mind you the toroid transformer in it is as big as a dinner plate, however my home brew psu under full load draws just on 110W, 6W idle.

Though the design works well due to the filtering I still think a multi tapped transformer is the only way to achieve a true low noise psu but having the transformer made to spec isn't cheap.
--

EP

[blackdog]

Hi Group,

I have done extensive testing with the preregulator on this picture, no switching noise, no dynamic problems...
The LT1083 is just a test regulator  and this wil be your regelator schematic, Q3 and component's wil be on the input and output of your design.


Dynamic loadregulation, its easy 
The load is changing between 0,5 and 10-Ampere, No Problemo!
Toptrace is buffer capacitor, bottom trace is the output of the regulator (LT1083), my own design has much less ripple, but i was not testing ripple here.


Choose the Fets for the current and the voltage you are using, and choose the capacitor C3 so for minimum noise from the transformer and the minimum losses in the Fets.
R13 is set to the minimum voltage in which the voltage regulator is working properly at the maximum current, this is usually between 3 and 5V.

Kind regarts,
Blackdog

[Electroplated]

Hi Group,

I have done extensive testing with the preregulator on this picture, no switching noise, no dynamic problems...
The LT1083 is just a test regulator  and this wil be your regelator schematic, Q3 and component's wil be on the input and output of your design.


Dynamic loadregulation, its easy 
The load is changing between 0,5 and 10-Ampere, No Problemo!
Toptrace is buffer capacitor, bottom trace is the output of the regulator (LT1083), my own design has much less ripple, but i was not testing ripple here.


Choose the Fets for the current and the voltage you are using, and choose the capacitor C3 so for minimum noise from the transformer and the minimum losses in the Fets.
R13 is set to the minimum voltage in which the voltage regulator is working properly at the maximum current, this is usually between 3 and 5V.

Kind regarts,
Blackdog

This is an interesting solution, I'm going to try this design myself after checking my parts draws, I have most of the parts at hand, thank you for the work you put in with this Blackdog.

--
EP

[JohnnyBerg]

@blackdog: doesn't this design heat up the transformer to extremes?
You draw the max current from the transformer at a point where the transformer cannot deliver that, due to the phase shift?
Or are you over dimensioning the transformer? 

[blackdog]

Hi JohnnyBerg, :-)


Why, al the "normal" preregulators whit a Fet or Thyristor do that!!!
This preregulator switches OFF!!! NO extra current, No extra Load, that's the beauty of this controller...

It is strange that every time I show this schematic, i always get the same comments, and every time I explain that this pre-reg is different than a thyristor or a switching FET.

When a new half-sine start, then it is normal, the FET into conduction, and the entire circuit reacts as if it is not present.
If the voltage across the regulator is sufficiently high, the FET is turned off and the transformer will see no more load.
There are no peak inrush currents with these pre-reg and wiht every half sinus it starts again loading the capacitor until it is enough.
Only shutdown phenomena, which you can set with the RC time of the proposed capacitor

Sorry for the bad english an i hope you guys understand it, bear with me, I'm a dyslexic monkey. 

Kind regarts,
Blackdog

[Electroplated]

Ok, I have built and tested Blackdogs circuit, so far so good, I hooked it to a lm324 based linear reg and it works as expected.

Just for kicks I swapped the mosfet for a pair of pnp power transistors connected in darlington config and with a few tweaks it works though with some heat wasted as expected.

There is only minor problem, my transformer has so far, refused to thump, over heat or self destruct, I must be doing something wrong 

Joking aside, this works pretty good, I have not scoped it fully yet as its only bread boarded together but it is very promising.

Back to using mosfets and more testing 

As a side note, a brand of UK tv set used thyristors in what used to be known as a syclops circuit, synchronous line output I think, This came in to service during my early training and ever since just seeing the word thyristor fills me with fear !

If anyone building this follows the design Blackdog has set out then it should give good results, for those who wish to use the old venerable 106c thyristor then I have tested it and without modding, it works.

One point worth noting, the base resistor for Q3 is high enough to allow correct function but will also allow a linear reg based on an opamp to go to zero output, some designs I have tried that use a pnp transistor have the base resistor low enough in value as to prevent a true zero output.


--
EP

--
EP

[JohnnyBerg]

@Electroplated: nice!

@Blackdog: does the thyristor need to be a MCR106 (high sensitive gate logic type) ?

One problem I see, is that when using this in a pre-regulated PSU and the output needs to jump up, you could be out of luck.
Worst case it could take up to 10mS (on a 50Hz net) before getting a response

[Electroplated]

@Electroplated: nice!

Still testing it but so far I pulled just on 2.4A, I could have got more but I would need to dig out a higher rated transformer.


@Blackdog: does the thyristor need to be a MCR106 (high sensitive gate logic type) ?

One problem I see, is that when using this in a pre-regulated PSU and the output needs to jump up, you could be out of luck.
Worst case it could take up to 10mS (on a 50Hz net) before getting a response

I may be wrong but the large smoothing cap could go a long way to solve any lag caused by loading, I probably would have explored this but I spent more time than I wanted digging out heat sinks and a transformer, because I committed the worst crime, I cleaned my workbench yesterday, under duress of course.

--
EP

[JohnnyBerg]

The smoothing cap now has a much lower voltage then in the unregulated version. Worst case you have to wait for the next sine rise, to get the smoothing cap filled to a higher level.

[Liv]

Mains frequency preregulator used in Agilent U8002A and DIY PSA2 (preregulator controlled by a microcontroller). But if you need a good dynamics, you can use the output stage with multilevel supply, then will not have to wait for charging capacitors.

[Kalvin]

Good information here!

I have also run few simulations in the past on the design similar to blackdog's design. The idea is to create an "controllable rectifier diode" using a transistor or mosfet which will be turned on when the rectified transformer voltage goes to zero and the transistor/mosfet will be turned off when the voltage of the smoothing capacitor exceeds the desired tracking voltage. As blackdog stated, the circuitry will create inductive spike at the moment when the transistor/mosfet will be turned off. The inductive spike could be reduced or practically eliminated by slowing the turn-off time but this will of course increase heat loss in the transistor/mosfet.

[blackdog]

Hi Group,

Choose the size of the 10.000uF capacitor so that it works well for the current that the power supply has to deliver.
Fast programming, low noise, low dissipation, and again it is searching for the "holy grail"...

Fast programming and low noise, you need at least en 2 quadrant powersupply.
That will be a super opamp, or a DC coupled Audio Amplifier, and high efficiency, i do not think so.
It is much too complex, you can better take two different power supplies, that of course depends on what you're testing.

Kind regarts,
Blackdog

[Liv]

Turn on the transistor at zero crossing and off when the desired voltage across the capacitor provides a very sharp shape of the current and a large voltage spike due to the leakage inductance.

Turn on the transistor to the middle of the half-period provides a smooth current waveform but provides current spike in the beginning.

Slowing opening of the transistor eliminates the current spike.

Therefore, the transistor need to turn on in the middle of the half-period and turn off at zero crossing.

[timofonic]

Damn, I feel like an idiot reading your super geek comments. I'm amazed and a little jealous. I wish I would be able to understand all this and play with the design. And it's great to be able to have a nice oscilloscope and such.

What about importing the PCB design to KiCad and modify it in Github (open hardware? Please, I would love to see Open Hardware high qiality design of a linear adjustable bench lab power supply) I can try it, but not sure about my fails.

[Liv]

If you simulate the circuit from this branch, we can see that the capacitor at Base-Collector is able to slow down the turning off of the transistor and reduce the voltage overshoot. But the power dissipation in the transistor is too big.

Changing the transistor control when turning on in the middle of the half-period and turning off at zero crossing, to reduce the capacity of the B-C without the risk of spike. In this case power dissipation in the MOSFET is much lower.

What about importing the PCB design to KiCad and modify it in Github (open hardware? Please, I would love to see Open Hardware high qiality design of a linear adjustable bench lab power supply) I can try it, but not sure about my fails.

If you mean my project of linear power supply PSL-3604, it is a little different from the discussion here, since it does not have a pre-regulator. Instead, it has a multilevel supply. Unfortunately, neither KiCad nor Github not know.

[timofonic]

If you simulate the circuit from this branch, we can see that the capacitor at Base-Collector is able to slow down the turning off of the transistor and reduce the voltage overshoot. But the power dissipation in the transistor is too big.

Changing the transistor control when turning on in the middle of the half-period and turning off at zero crossing, to reduce the capacity of the B-C without the risk of spike. In this case power dissipation in the MOSFET is much lower.

What about importing the PCB design to KiCad and modify it in Github (open hardware? Please, I would love to see Open Hardware high qiality design of a linear adjustable bench lab power supply) I can try it, but not sure about my fails.

If you mean my project of linear power supply PSL-3604, it is a little different from the discussion here, since it does not have a pre-regulator. Instead, it has a multilevel supply. Unfortunately, neither KiCad nor Github not know.

KiCad is an open source EDA for PCB and schematic capture. These days CERN is investing on it and it's progressing faster than ever. It may be an Eagle killer in one year or less.

What software do you use? Are those SPICE simulations? Maybe it can be imported to KiCad.

GitHub is used commonly for Open Hardware and projects modified by a team with branches and final designs, similar to open software development methodology. Some propietary EDAs use a similar approach integrated in their software package, Open Hardware commonly uses Git for it and Github is the defacto standard.

[Liv]

Maybe it can be imported to KiCad.

Maybe Gerbers?

[cellularmitosis]

I've been interested in this topic lately (using a sine wave approach for low noise).

I just wanted to point out the TI SN6501 chip, which does exactly what you are proposing.  However, it is only good for a few hundred milliamps.

I've ordered a few and I'll throw them in my Dutch oven faraday cage and report back with some measurements.  Hopefully the results will be better than my ICL7660 efforts . https://www.eevblog.com/forum/projects/an-evening-with-the-icl7660/

[Liv]

sorry i just have to ask, how does the LM317 work for this arrangement?

LM317 works here as a current source and provides a constant current load. Not a good idea, it is better to use the output of the voltage error amplifier and turn on the load current only when the output voltage must be reduced.

[blackdog]

Hi 3roomlab,

The rectifier for the negative power supply will be changed, also the transformer in this schematic wil change.
This because i wil use the Fet pre-reg.
I need a negatif supply for the LM317, so that if the uitputvoltage is say 2V, the LM317 current source is stil working.
I will NOT use a swiching unit to maken a negatif voltage, its a low noise powersupply (at 5 amps, I was measuring less than 5uV noise at 20Khz bandwidth, yes Micro Volts)

The circuit with the LM317 current source remains in the circuit, these help to improve the dynamic behavior.
It always draws 57mA from the output and helpt at light loads to quickly settle the ouput voltage.


Kind regarts,
Blackdog

[blackdog]

Hi Liv,

LM317 works here as a current source and provides a constant current load. Not a good idea, it is better to use the output of the voltage error amplifier and turn on the load current only when the output voltage must be reduced.

I disagree, my extensive measurements show that this is a good way to get the response time to very low values.
The only negative thing I can say is this, it takes some power.


Kind regarts,
Blackdog

[Liv]

Yes, additional power dissipation - a disadvantage. And yet - when you turn off the output necessary to turn off the current source, otherwise the output will be a negative voltage. In his scheme, I use MOSFET current source, which eliminates the need for a negative supply. And it turned on only when needed. Problems with the dynamics is not observed, it looks like a 2-quadrant PSU.

[Electroplated]

HI,

I played around with the design trying different mosfets then changed the design to use npn transistors to explore the possibilities of using alternative pass transistors for my own reasons and is only at this stage a proof of concept, the design needs more work. I also had n channel mosfets in place of the npn darlingtons, only because I needed the two used in the original circuit on another project and will have to wait to restock.

I had this circuit feeding a LM317 regulator and it preformed reasonably good inside the constraints of the 317.

Attached diagram and scope shot, lower yellow trace, drive to Q3/Q4 upper blue trace, Thyristor gate waveform, neither are perfect but the waveforms are spike and aberration free across the full range of the LM317.

--
EP

[JohnnyBerg]

the load is no longer there, there is some "window" of hysteresis there. maybe a more active "pull" circuit?

Well, a good PSU has a current source on the output, that forces a minimum current through the series element, to ensure that the output is quickly going down when needed.

With a large capacitve load that is still not good enough, so my PSU has some other neat stuff to do that

[Liv]

i am starting to think the output side not only needs a push, it also needs a pull

True, the PSU must not only source a current, but sink it. In the simplest case this is current source, which is able to sink a certain current (e.g., 1/10 of the maximum output current). Such a scheme Agilent calls "Down Programmer". The next step - is to control both source and sink current. It may, for example, Agilent 6633A which has a push-pull output stage and two current control loops - both positive and negative.

[T3sl4co1l]

Last bench supply I built, I just made a beef-ass audio amplifier.  The 2N6059(?) outputs (so, it's quasi-complementary) are capable of dumping over 40A.  It would be a fine (sub)woofer amp in the 1-2 ohm range, and has a BNC input for analog use.  Most of the time, it's set to an internal TL431 reference and 10 turn pot, for 0-20V DC operation.  A consequence of the bidirectional output stage is, if you plug in a bypass capacitor to your breadboard, and it was still charged from a previous use, whether above or below the supply voltage, it will be charged or discharged in just the same manner.  Quickly.

Tim

[prasimix]

If someone is willing to play with blackdog pre-regulator presented in this thread, here is the LTspice model...

[Thor-Arne]

Thanks, was planning to play with this in LTspice. 

[Liv]

The main advantage of this preregulator is simple control. I do not understand what interest to do simple design today. PSU can not be imagined without an integrated microcontroller which can implement the software control of preregulator, for example, using the PI-algorithm.

[Thor-Arne]

The reason for using a pre-regulator like this one is that it is not under software control.

IMO, microcontrollers is fine for it's use, but it should be limited to providing a set point for the regulator. All control loops should be analog to ensure that it's not affected by software lock-ups.

[blackdog]

Hi Liv


Que! >>  I do not understand what interest to do simple design today...

One thing i learnt is this, do not make things more complex than necessary!
CPU controled Power Supply's, almost never i use them for developement.
I use most the Dutch DELTA Linear power supply's (some 40 years old), and multiple power supplies of HP/Agilent.
And multiple power supplies of my own design.
All linear, no switchers and no CPU control, give me two multiturn potmeters for U and i, and i'am happy 
All are analog, and there is one exception, and that is the "DELTA ES-30-5", a nice low noise switcher.

A cpu controled Power Supply can be usefull, i know, but fore development only sometimes necessary. (i know, i know there are exceptions)

These characteristics I expect from a Power Supply
Simple to use, all my attention should go to test the circuit and not with the operation of the Power Supply.
The power supply may not have power On or Power Off aberrations.
The power supply must have a mute without On/Off aberrations.
The power supply must be kind to itself and the load.
Low Noise under all conditions <0.1mV 200Khz BW.
Fast acting no oscilation current limiting.

I can make the list longer, but i think you understand my point :-)

Just 2 cent's of low noise remaks 

Kind regarts,
Blackdog

[Liv]

One thing i learnt is this, do not make things more complex than necessary!

Amateurs make devices for fun. What a pleasure from simple circuits?

[blackdog]

Hi Liv,

Thank you that you cal me a amateur...
You are intelligent enough to know that your comment is not true.
I like this one from Einstein: Make everything as simple as possible but not simpler
Som time's design's wil by complex, but complexity should not be a starting point in my point of view.

I have no problems with it, if you make your design's as complex as possible, because it give's you pleasure.


Kind regarts,
Blackdog

[Liv]

your comment is not true

This is not true for commercial projects where the cost should be minimal. But I do not understand why for fun for the hundredth time doing primitive PSUs using LM317. Modern PSU contains a microcontroller, has an informative display, advanced control functions. Even better - Intelligent Power Supply (IPS), where the feedback is closed software. Such projects are interesting to implement.

[schopi68]

Well... it is always easy to make things complex up to the boundary of human comprehension.
But really genious work is to make these complex things simple without sacrifying the result. 

On the other side in my opinion a microcontroller-controlled PSU is a comprehensive, but not a complicated design.

Do you think of building just the preregulator or the whole PSU microcontroller based?

[Liv]

I already gave the link here on DIY PSU (schematic) where preregulator controlled by a microcontroller, and it simplifies the circuit. The same microcontroller controls the entire PSU, although feedback in the analogue. This is an example of modern DIY PSU.

[schopi68]

Do you have any measurement values for this PSU?
I could not find information about items like Load regulation, rippe, noise, drift, transient recovery times, accuracy. Will a best of it's class "modern" PSU be able to compete in this parameters with a "classical" one?

[Liv]

Will a best of it's class "modern" PSU be able to compete in this parameters with a "classical" one?

These parameters are determined exclusively by analog circuitry and "modern" PSUs are the same as for the "classical". There is no need to improve them, in the older PSU generations has been reached a sufficient level. Development goes the other way - easy control, enhanced display and advanced feature set. As an example - Rigol DP 800 series. Analog circuits are the same as in the old series. But there was a beautiful high-resolution display, which is served by a powerful processor. This is the current trend until DIY PSU, unfortunately, still behind.

[schopi68]

These parameters are determined exclusively by analog circuitry and "modern" PSUs are the same as for the "classical". There is no need to improve them, in the older PSU generations has been reached a sufficient level. Development goes the other way - easy control, enhanced display and advanced feature set. As an example - Rigol DP 800 series. Analog circuits are the same as in the old series. But there was a beautiful high-resolution display, which is served by a powerful processor. This is the current trend until DIY PSU, unfortunately, still behind.

Maybe we are not talking about the same. I am interested in the possibilities of PSUs controlled completely by an microcontroller. This would also include the regulation path. If it is just about programming the output voltage i could easily use the remote programming feature of an 30 year old HP PSU and connect it to a microcontroller with a fancy front-end (resulting in a PSU with better data than most of todays devices).

[Liv]

In this example the microcontroller only programs the output voltage. This is much better than doing a PSU with analog controls. Of course, you can take a 30 year old HP PSU, but talk about DIY PSUs. I also gave an example where the microcontroller still controls the pre-regulator. But the main parameters there are provided conventional analog post-regulator. I've never seen examples of linear PSU, where the microcontroller closed the feedback. Typically, the IPS - is switch mode PSUs. I have plans to try to implement a high-voltage power supply as IPS.

[timofonic]

In this example the microcontroller only programs the output voltage. This is much better than doing a PSU with analog controls. Of course, you can take a 30 year old HP PSU, but talk about DIY PSUs. I also gave an example where the microcontroller still controls the pre-regulator. But the main parameters there are provided conventional analog post-regulator. I've never seen examples of linear PSU, where the microcontroller closed the feedback. Typically, the IPS - is switch mode PSUs. I have plans to try to implement a high-voltage power supply as IPS.

That seems very interesting! The low noise of a linear adjustable power supply plus the advantages of digital.

Do you have plans to make ripple tests and such? Do you have an oscilloscope?

[Liv]

The low noise of a linear adjustable power supply plus the advantages of digital.

Such a structure have most modern linear PSU. Only very simple models do not have a microcontroller.

Do you have plans to make ripple tests and such?

Which PSU do you mean? High-voltage IPS? It plans for the future. I do not have time for all projects.

Do you have an oscilloscope

Yes.

[schopi68]

In this example the microcontroller only programs the output voltage. This is much better than doing a PSU with analog controls. Of course, you can take a 30 year old HP PSU, but talk about DIY PSUs. I also gave an example where the microcontroller still controls the pre-regulator. But the main parameters there are provided conventional analog post-regulator. I've never seen examples of linear PSU, where the microcontroller closed the feedback. Typically, the IPS - is switch mode PSUs. I have plans to try to implement a high-voltage power supply as IPS.

Okay... i see i was thinking that you were thinking of about something totally different. So i think now, we are mainly in sync in our thoughts about what is right or should be a useful design. 

[exe]

sorry i just have to ask, how does the LM317 work for this arrangement?

LM317 works here as a current source and provides a constant current load. Not a good idea, it is better to use the output of the voltage error amplifier and turn on the load current only when the output voltage must be reduced.

I tried a trivial downprogrammer and it didn't work. It was just an opamp-based comparator that switched current sink on during overshoots. In theory. But LTSpice showed it was always on. I think this was caused by opamp's offset voltage. So... how did you overcome this problem?

[Liv]

Downprogrammer must be controlled by the error amplifier instead of a separate comparator.

[Kleinstein]

Many output stages also like to have a minimum current, because transistors usually get much slower al low currents. So the downprogrammer is not just for bringing the volatge back down fast, but also to give a minimum current, even at low output voltages.  A minimum load alows a much faster reaction of the regulating loop as the properties of the output stage does not change that much with load current.

[Liv]

In theory, you're absolutely right. But as practice shows, and modeling for the output stage need not load more than giving feedback divider. Improvements in performance are not visible. You can certainly do some quiescent current, but its value should be much lower than the downprogrammer current for reasons of heating.

[ludo3232]

Hi,

Very interesting schematic

It's possible to use NMOS in place of PMOS with this pre-regulator? I have a lot of NMOS and they are cheaper and are high current rated then PMOS

Thanks

[Boschi]

hi,
yes, it would be possible to use a NMOS, but it would be a  messy.
 you should charge a capacitor at least 5/10V more than the source and with some transistors switch on and off the gate, but you lose like 5/10V and the pre regulator would become useless.

sorry for the bad english, but i think you get the point 

[Marco]

You need a small boost/switched capacitor power supply to get some extra voltage headroom to drive the NMOS efficiently.

On the other hand, the NMOS will attenuate some more noise so there is value in it.

[David Hess]

in a typical opamp type positive drive BJT configuration, i tried in a simulation where in a "weak" load situation (voltage tending to overshoot), if the op amp also "drives" a current limited PNP/PMOS setup to drain the output (maybe 100mA drain), it seems to help "stabilize" the output, yet to try in practical

Many output stages also like to have a minimum current, because transistors usually get much slower al low currents. So the downprogrammer is not just for bringing the volatge back down fast, but also to give a minimum current, even at low output voltages.  A minimum load alows a much faster reaction of the regulating loop as the properties of the output stage does not change that much with load current.

Often when I see this problem it is caused by lack of a base-emitter shunt resistor to remove charge from the base.  It helps a lot to lower its value below that which is necessary to absorb collector-base leakage so that charge is removed more quickly and the faster responding drive transistor takes up more of the load.  Alternatively make the output from the error amplifier stiffer and allow it to pull charge out of the base of the pass transistor directly.

I suspect quasi-saturation of the output transistor at high currents and low voltage drops may explain mysterious transient response behavior I have seen in some designs.

Test the transient response over the entire output current and voltage range and then compensate it for where it is worst.  If the transient response varies a lot, then find out why and fix it.

Referring back to the original discussion post, 1 amp at up to 24 volts is well within the power handling capability of a single output transistor linear design.  Attention will need to be paid to the reference and error amplifier to keep noise low.  If necessary, maximum power dissipation can be cut in half without adding noise by automatically selecting the secondary tap with hysteresis.

[xavier60]

Turn on the transistor at zero crossing and off when the desired voltage across the capacitor provides a very sharp shape of the current and a large voltage spike due to the leakage inductance.

Turn on the transistor to the middle of the half-period provides a smooth current waveform but provides current spike in the beginning.

Slowing opening of the transistor eliminates the current spike.

Therefore, the transistor need to turn on in the middle of the half-period and turn off at zero crossing.

I think it would be even better if the transistor was turned on for some variable time period during the rising part and then during the falling part of the rectified half cycle. It is likely to need a micro-controller to produce the drive timing.

[not1xor1]

Turn on the transistor at zero crossing and off when the desired voltage across the capacitor provides a very sharp shape of the current and a large voltage spike due to the leakage inductance.

Turn on the transistor to the middle of the half-period provides a smooth current waveform but provides current spike in the beginning.

Slowing opening of the transistor eliminates the current spike.

Therefore, the transistor need to turn on in the middle of the half-period and turn off at zero crossing.

I think it would be even better if the transistor was turned on for some variable time period during the rising part and then during the falling part of the rectified half cycle. It is likely to need a micro-controller to produce the drive timing.

I guess you mean switching the transistor at 200/240Hz (depending on country AC line frequency).

I cannot see any advantage in that.
If you switch-off an inductor (in this case the secondary winding of a transformer) while current is flowing, you always get huge voltage spikes.

The advantage of SCR-style circuits is that current decreases naturally, thanks to the decreasing value of the rectified sine wave.
You do not have to waste any power in a "lazy" mosfet switch, working in its linear region to reduce just a bit  transformer voltage spikes.

You just need a large (it has to withstand high current without saturating) inductor after the SCRs (or mosfet switch) to slow down the current charging the levelling capacitor.

The tricky part is synch-ing the switch to turn on at the right time, according to output voltage and load, every 100-120ms.

Bad SCR-style circuits do not care of synch-ing and just switch-on the SCR as soon as the capacitor voltage gets too low, so are quite noisy and inefficient since  the dropout voltage varies a lot and needs a higher threshold to ensure proper regulation.

[xavier60]

So the best is to turn on during the falling side and not have to worry about turn off voltage spikes. High transformer leakage inductance would help soften the current pulse also.
I have been trying to figure out the schematics for the Agilent U8002A. I see that it has a 900uh inductor between the secondary and the bridge. I would like to think that it turns on the MOSFET during the falling side also.

[not1xor1]

So the best is to turn on during the falling side and not have to worry about turn off voltage spikes. High transformer leakage inductance would help soften the current pulse also.
I have been trying to figure out the schematics for the Agilent U8002A. I see that it has a 900uh inductor between the secondary and the bridge. I would like to think that it turns on the MOSFET during the falling side also.

I downloaded that schematic a while ago, but so far have not fully understood how it works, but have not spent much time on it...

In any case I made some modifications to one of those J. Williams circuits to make it work with mosfets.
I did not make many tests, but I think it needs some further development to make it work with any value of output voltage and load and to be able to properly respond to transient load.

I'm attaching the asc file in case anybody is interested in playing with it.
BTW it wastes quite a lot of power in the transformer... it might be more efficient with a huge capacitor across the secondary winding...

It uses a quite rough transformer model I wrote about here: transformer.zip

[T3sl4co1l]

I can think of several reasons why this is a bad idea to begin with, and a few more reasons why it's simply, unconditionally, worse (in performance) than an alternative arrangement of the same parts, give or take... 

Tim

[xavier60]

It shouldn't be any worse than the SCR version. There is a risk of the MOSFET not getting proper Gate drive at low pre-regulation voltages. I'm not certain if D11 is needed.
I once saw the big bites taken out of the secondary waveform in an SCR controlled battery charger and wondered how efficient the idea really was with that large voltage drop being imposed on the secondary voltage.

[T3sl4co1l]

It shouldn't be any worse than the SCR version. There is a risk of the MOSFET not getting proper Gate drive at low pre-regulation voltages. I'm not certain if D11 is needed.
I once saw the big bites taken out of the secondary waveform in an SCR controlled battery charger and wondered how efficient the idea really was with that large voltage drop being imposed on the secondary voltage.

Well, that and the fusing rating of the MOSFET.  Among other things.

Or more to the point, what's the reason that SCRs have such a rating and MOSFETs never do?

Tim

[xavier60]

I wasn't aware of the fuse rating. It may not be important. I wish I had some time to experiment. I would like to put a large power choke into the pre-reg circuit to stretch out the current pulse.  This would be be kinder for the transformer and might reduce noise by reducing current transients.

[not1xor1]

I can think of several reasons why this is a bad idea to begin with, and a few more reasons why it's simply, unconditionally, worse (in performance) than an alternative arrangement of the same parts, give or take... 

Tim

I'm not suggesting to build that circuit, but to use that in simulations to see how an SCR-like circuit works.
BTW the mosfet doesn't suffer any stress at all. Its losses are just 3-5W, because, thanks to the inductor, there is not much current on switching on, and no current on switching off as the half sine wave is already below the capacitor voltage.

xavier60, you are right about the gate drive at low output voltage.
BTW I added D11 because the path from the levelling capacitor to ground, through the PMOS body diode, prevented the circuit to work in the first tests I ran.

It should work better with NMOS switch and control circuit bootstrapped by the positive output voltage.
A differential amplifier connected to Vpre-reg,Vout, with a much lower RC constant and with a reference changing according to the output voltage would also help to deal with large variations of that (output voltage), load and transient loads.

In simulations of a different circuit of this kind, I made last year, I found that a large capacitor, resonating at about AC-line frequency with the input inductor, had a huge impact on efficiency, bringing it almost at the level of switching pre-regulators.
If I get enough spare time, I'll try to find (or re-draw) it, in the next few days.

BTW the other kind of pre-regulator, that one that switches-off at a given output  voltage threshold, has a really poor efficiency at low output voltage.

[not1xor1]

I wasn't aware of the fuse rating. It may not be important. I wish I had some time to experiment. I would like to put a large power choke into the pre-reg circuit to stretch out the current pulse.  This would be be kinder for the transformer and might reduce noise by reducing current transients.

The inductor is already there, and even without inductor the peak current would not be so high to destroy a power mosfet.
The real problem might rather be the inductor itself.
In some cases the circuit might work better with an inductor as high as a few mH and able to withstand 10-20A without saturating.
One should probably build it with those E-I ... pieces... I can't recall the proper English term  but I guess you understand what I mean.

[xavier60]

Yes, the E and I pieces are stacked with their own kind rather than interleaved so that and air gap can be placed between the E stack and the I stack.

[jaycee]

On the subject of downprogrammers, I once heard them described as "something a little bit like a Class AB output stage". So I tried that in my bench PSU design... seen here as Q5 biased by Q6. Ordinarily it functions as a 30mA constant current load, but can either increase to pull the voltage down, or reduce to allow the voltage to ramp up quicker. If more grunt was needed then it would be trivial to add to Q5 with further NPN devices much like a quasi-complimentary output stage of an amplifier.

[David Hess]

On the subject of downprogrammers, I once heard them described as "something a little bit like a Class AB output stage".

That is about right although some may be class-A or class-B.  The variable power supplies I commonly use just have small constant current or constant resistance loads on the output.

Another way to do it is to drive the sink stage in current mode from the error amplifier as shown in National Semiconductor linear brief LB-28 shown below.  It struck me as an odd feature considering that this 10 amp design with careful frequency compensation only has 0.22uF of output capacitance but it is designed to drive any amount of additional load capacitance.

[jaycee]

That is about right although some may be class-A or class-B.  The variable power supplies I commonly use just have small constant current or constant resistance loads on the output.

Yep most of the cheaper supplies I've seen just have a dumb power resistor across the output. I figured I'd try something with a little more finesse in my own design I also designed the PCB so that an extra BC556 can be fitted across Q5 to form a simple constant current sink, in case the class AB biased downprogrammer didnt work out

[David Hess]

That is about right although some may be class-A or class-B.  The variable power supplies I commonly use just have small constant current or constant resistance loads on the output.

Yep most of the cheaper supplies I've seen just have a dumb power resistor across the output. I figured I'd try something with a little more finesse in my own design I also designed the PCB so that an extra BC556 can be fitted across Q5 to form a simple constant current sink, in case the class AB biased downprogrammer didnt work out.

It does not necessarily mean cheap; it may just mean that nothing else was required.  The Tektronix power supplies that I like draw a small current of 3 to 6 milliamps to a point below ground simply so that the output can reach ground without any ambiguity.  Due to a deliberately designed in offset, the outputs actually go a few millivolts below ground to make sure it is possible to reach zero.