Very Low Noise Preregulator for Benchtop Power Supply

[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

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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.