Simplistic Pre-Regulator (Tracking, Mains Zero Crossing Detecting)

[robert67]

Rebuilding a vintage lab power supply (you can see how I gutted it here https://youtu.be/P1JIkpT76wY) I found myself in need of a tracking pre-regulator. Staying with the vintage theme I didn’t wanted a switch-mode type and I wanted it to be as simple as possible.
 
During my search I stumbled on the EEVblog forum upon a design that fascinated me because of its use of a thyristor for main zero crossing detection: https://www.eevblog.com/forum/projects/very-low-noise-preregulator-for-benchtop-power-supply/. So I took it and simplified it (maybe to a fault, but it’s my thing to use as few parts as possible) …

Advantages of the simplified design:

A) Of course much simpler (10 instead of 21 parts – not counting rectifier, capacitor and linear regulator)
B) Can pre-regulate down to lower voltages (using a BJT instead of MOSFETs)
C) Injects two orders of magnitude less current into the output of the linear regulator
 
Drawbacks of the simplified design:

A) Considerably less efficient (using a BJT instead of MOSFETs)
B) More expensive parts (I really maxed out on the parametric searches of the distributors)
C) Creates more noise (the pass transistor is switched off hard)

I haven’t ordered the parts for it yet, because some of them are quite expensive (e.g. $10 for a MJ11032G TO-3 BJT). Instead I wanted to put the design under your esteemed scrutiny first. So if you spot an error, please stick it to me. Or, even better, if you think it can be done with even fewer parts, please let me know. Here it goes …
 
DESIGN CONSTRAINTS

Not really technical constraints, but anyway …

1) The switching power transistor has to come in a TO-3 package (the vintage power supply has two external TO-3 heat sinks – one will be used for the pre-regulator and the other for the linear regulator – so a purely “aesthetical” constraint)

2) Can’t do anything about the AC input voltage and its wide swing (in Europe AC mains is specified as 230V +/-10%, and I’m reusing the transformer)

3) The input voltage range of the linear regulator (7V - 37V) and the maximum current (5A) is a given (OK, these are actually technical constraints)

CIRCUIT DIAGRAM

see attachment

PRINCIPLE OF OPERATION

Unless otherwise noted “current voltage” references the output of the bridge rectifier D1 at it cathodes (rectified sine wave), its anodes being the ground (0V).

The footnotes contain information in relation to the actual choice of values and parts.

A) Charging Operation

A constant current source (U1, R1) delivers a current to any point low enough below the current voltage (1).

Assuming D3 is “off” – see B) Voltage Feedback and C) Zero Crossing Detection – that current flows to the base of Q1, if the base is pulled low enough below the current voltage by the voltage drop across the collector and emitter of Q1 (2). This voltage drop includes the base-emitter forward voltage of Q1 (3).

As Q1 becomes conductive, current flows through D2 and charges C1, if the voltage drop across D2 (4) plus the voltage of C1 is below the current voltage minus the voltage drop across the collector and emitter of Q1 (3). 

(1) 2.7V
(2) 4.2V - 6.2V
(3) 1.5V - 3.5V
(4) 0.2V - 0.8V

B)  Voltage Feedback

A voltage divider (R2, R3, and R4) delivers a part of the voltage differential between C1 and the output of the linear regulator (1) to the base of Q2. If it is low enough below the voltage of C1 (2), a current flows through the base of Q2.

As Q2 becomes conductive, a current flows through its emitter and collector via R5 to the gate of D3, and switches it on.

The current from the constant current source (U1, R1) is then flowing through D3 to ground, instead to the base of Q1. The charging of C1 stops.

(1) nom. 7V
(2) 0.63V

C) Zero Crossing Detection

As long the current from the constant current source (U1, R1) flows through D3 to ground, D3 stays conductive, even if its gate no longer receives any current from Q2 through R5.

The constant current source (U1, R1) will be able to maintain that current as long as the current voltage doesn’t fall too low (2). So the charging of C1 can only resume after the current voltage has fallen to that low point.

That low point (2) is below the required voltage drop across the collector and emitter of Q2 and D2 to resume charging C1 (3). In effect the charging of C1 is further delayed until the current voltage has risen again beyond that required voltage drop, that is after its zero point.

(1) 2mA
(2) 3.6V
(3) 4.4V

CHOICE OF PARTS

U1: LM317HVT (http://www.ti.com/lit/ds/symlink/lm117hv.pdf) – At first glance a no brainer for a simple constant current source working at a maximum of 60V. I choose one in a TO220 package to keep it cool (I don’t like to burn my fingers on my breadboard). However, there are constant current LED drivers available that would do the job with just one part (e.g. AL5809-20P1-7), not needing a resistor. Alas, they are only available in SMD packages (doesn’t fit into the vintage theme at all). I also had a classical Zener (ZPY1)/transistor/resistors 4-part solution designed. Its minimum operating voltage was much lower, shaving off 12W from Q1’s worst case power loss. But its two parts more and “Winter ins Coming” (12W will help to fight the cold).

Q1: MJ11032G (http://www.onsemi.com/pub/Collateral/MJ11028-D.PDF) – Expecting high peak currents I choose the beefiest transistor in a TO-3 package I could find. It had to be a Darlington in order to keep the rest of the circuitry lean and simple.

D2: FERD40H100S (http://www.st.com/content/ccc/resource/technical/document/datasheet/group3/4e/b5/b8/47/b6/af/41/d6/DM00281757/files/DM00281757.pdf/jcr:content/translations/en.DM00281757.pdf) – A strong enough Schottky diode with an acceptable voltage drop. For various reasons I restricted myself to types coming in a TO220 package (mainly because I have about two dozen TO220 heatsinks lying around).

Q2: BC560C (https://www.fairchildsemi.com/datasheets/BC/BC556.pdf) – Nothing special here, but for the C models providing a high current amplification, thus enabling the voltage divider being high impedance. Its maximum collector-emitter voltage of 45V should be enough here, because it’s only seeing the already pre-regulated voltage.

D3: EC103D1 (http://www.ween-semi.com/documents/EC103D1.pdf) – I choose this one for its low gate trigger current. Again this allows the voltage divider in the voltage feedback part to be high impedance, because it enables Q2 to be driven with low base currents.

Thank you so much for your time!

Again, if I made some mistakes – stick it to me.
And if you think it can be done with fewer parts – please let me know.

[jaycee]

Linear Technology App Note 32 is worth a read.. theres examples of using SCR pre regulation there.
App Note 30 also covers the subject

[robert67]

Thanks for the pointers Jaycee! Both papers are very interesting reads and you can get a lot of ideas from them.

Unfortunately the BOM of the SCR pre-regulator (fig. 5 in app note 32 http://cds.linear.com/docs/en/application-note/an32f.pdf) doesn't fit my design goal of minimizing the parts count - it has about 25 And it requires an transformer with a middle tap which my vintage power supply doesn't have

In the old days SCR pre-regulators were quite common (http://powersupply.blogs.keysight.com/2013/05/more-on-early-power-supply-preregulator.html). But the simpler ones create a whole lot of noise, the SCRs being switched on somewhere within a half wave.

Linear Technology App Note 32 is worth a read.. theres examples of using SCR pre regulation there.
App Note 30 also covers the subject

[jaycee]

You can make that circuit work with the transformer you have, you would just require 4 SCR's instead of 2, and make a full wave bridge rectifier with them. Understandably though it's more complex.

I adapted the circuit in Figure 8a/b for use with an AC input - C1B and it's associated parts can go away if you have an AC input, because you can generate the higher Gate voltage using a Cockroft multiplier.

You could probably simplify it even more by using a P-channel MOSFET instead, as the concerns the designer had in 1989 dont so much apply now in 2017 - P MOSFETs with good power handling are readily available. This is exactly what Blackdog did with the pre-regulator design you linked to.

[jaycee]

heh just fully watched your video rather than skimming through it. The design seems pretty typical of the time. 10% resistors used in the current shunt is not too surprising - they would have struggled to do better than that in parts tolerance back then, at least for a sensible cost. These days, you can get 1% accurate metal strip resistors and there's no problem

It looks like they were using an RCA clone of the LM723, again, not too surprising. That chip does have its issues, though.

2x2N3055 seems a bit skimpy for 5A of output with the drop across the regulator you'd typically see. I bet it got hot.

Funnily enough I am working on a 0-30V 0-5A design myself. As a "starter for ten" I prototyped a 0-18V 0-1A design using all discrete parts, as going up in voltage/current is simply scaling this design up. I'm using nothing exotic so far - TL431 as the voltage reference, TL082's as the opamps, and TIP35C for the outputs on the small PSU. The two TIP35C's are being driven by an MJE15032, but to be honest a BD139 would probably do for just 1A.

The design is quite similar to how the Mastech power supplies work - I have another, small (probably 2-3VA) transformer providing +5/-5V supplies for the opamp. The centre tap of this floats around the main regulator's output, a neat trick. As an example of how that works, look up the ELV-22532 power supply schematic (you've probably already seen it if you've followed Blackdog's thread )

[schmitt trigger]

Pre-regulators for linear supplies have been around for a long time. People realized that sometimes the dissipated heat would be simply too much.

The first design I saw was in the late-1970s in a Popular Electronics Magazine article. I don't remember the details other than it used a pair of 723s as the controllers; one operating in its usual fashion, but the other as a comparator to trigger the SCR at a certain level.

Good luck on your project! I also love working on retro projects, and this certainly qualifies as one. Using TO3 power transistors is a must for authenticity. Hope you can get the metal-can 723s for the same reason.

[robert67]

You can make that circuit work with the transformer you have, you would just require 4 SCR's instead of 2, and make a full wave bridge rectifier with them. Understandably though it's more complex.

You're absolutely right. A transformer without a center tap together with a full bridge rectifier made of SCRs works just fine. In fact you can make the full bridge rectifier out of two diodes (from the AC inputs to one DC output) and two SCRs (from the two AC inputs to the other DC output).

[robert67]

heh just fully watched your video rather than skimming through it. The design seems pretty typical of the time. 10% resistors used in the current shunt is not too surprising - they would have struggled to do better than that in parts tolerance back then, at least for a sensible cost. These days, you can get 1% accurate metal strip resistors and there's no problem

Yeah, the current limiting was pretty awful, using a 3-position rotary switch to change between different shunts for a 0.5A, a 1.5A and a 5A limit 

2x2N3055 seems a bit skimpy for 5A of output with the drop across the regulator you'd typically see. I bet it got hot.

The transformer has actually two secondary windings: ~20Veff and ~15Veff. Their "pre-regulator" (the small board) switched between using only one winding (I guess it was the 20Veff) for lower output voltages and both windings in series for higher output voltages. So the maximum power loss must have been about 100W (20V*5A).

[jaycee]

Yeah, the current limiting was pretty awful, using a 3-position rotary switch to change between different shunts for a 0.5A, a 1.5A and a 5A limit 

Seems a bit odd. Would have been just as easy to use a pot!

The transformer has actually two secondary windings: ~20Veff and ~15Veff. Their "pre-regulator" (the small board) switched between using only one winding (I guess it was the 20Veff) for lower output voltages and both windings in series for higher output voltages. So the maximum power loss must have been about 100W (20V*5A).

Yeah, even allowing for that I'd say it was too skimpy. If you're looking for a good pass transistor to replace them, the MJ21194 is pretty hard to beat. Use an MJE15032 to drive them as darlingtons. Remember to add small (say 0.22 ohm) emitter resistors so that they current share - also do this with your 2N3055's actually!

[blackdog]

Hi Robert67,

It is good to see that you are using "my" prereg 

One "but", some of the items you left out, were not in the schematic for "fun"
There is know no limit on the "switch off" time, and you wil hate the transformer noise and high EMK pulses because of that.
Only trowing away components is almost never better, less components and stil have the same performance, thats OK

For everybody else "This is not a Thyristor pre regulator!!!!!"
Almost al Thyristor pre regulators are noisy as Hell!
This pre regulator, if build wel, with a controled "switch off" time, is low noise.

Kind regards,
Blackdog

[jaycee]

Hi Blackdog

Your articles on the other forum were great, even if reading them was a bit challenging (Google Translate can be quite imaginitive with technical terms)

I'm currently experimenting with using an LM2576 as a pre-reg, but with an extra stage of filtering. While it looks OK in LTSpice, i expect real life will be another matter!

[blackdog]

Hi jaycee,

Thank for the compliment!
It takes me a lot of effort to write those articles, i'am a dyslectic monkey. 
And if Google translates it into English, I can understand that it looks like it's written in Klingon ... 

Kind regards,
Bram

[robert67]

Hello Blackdog,

One "but", some of the items you left out, were not in the schematic for "fun"
There is know no limit on the "switch off" time, and you wil hate the transformer noise and high EMK pulses because of that.
Only trowing away components is almost never better, less components and stil have the same performance, thats OK

You're absolutely right. My design creates some nasty noise when the pass transistor switches off hard. Up to now I was totally focused on switching the pass transistor on during the zero point of a half wave. So thank you for the hint I will edit my topic and add that as drawback of my (current) design

Maybe I can somehow come up with a soft switch off with a few parts ...

For everybody else "This is not a Thyristor pre regulator!!!!!"
Almost al Thyristor pre regulators are noisy as Hell!

Yes, up to now the thread wasn't really about (flaws in) my design but SCR pre-regulators Which is also an interesting (vintage) topic though

Best Regards
Robert

[robert67]

Yeah, even allowing for that I'd say it was too skimpy. If you're looking for a good pass transistor to replace them, the MJ21194 is pretty hard to beat. Use an MJE15032 to drive them as darlingtons. Remember to add small (say 0.22 ohm) emitter resistors so that they current share - also do this with your 2N3055's actually!

In my pre-regulator design I'm using a MJ11032G: 300W, 50A (100A pulsed), NPN, darlington, TO-3. It's probably too big for the job, but I wanted to be on the save side. Anyway, it's going to replace one of the old 2N3055s on the back side.

For the linear regulator itself (which is not part of this thread) I'm currently using a single 2N5883: 200W, 25A (50A pulsed), PNP, TO-3. Yeah, it's a PNP, beefing up a reference L200 design. In between I used two of them with 0.1Ohm emitter resistors. But again, that belongs into another thread.

[not1xor1]

For everybody else "This is not a Thyristor pre regulator!!!!!"
Almost al Thyristor pre regulators are noisy as Hell!
This pre regulator, if build wel, with a controled "switch off" time, is low noise.

have you checked the RMS current through the transformer secondary?

several months ago I simulated your circuit via LTspice and (if I remember correctly) noticed that the transformer current is more than double the DC load current, so I suspect that the power (or at least great part of it) saved in the linear regulator is wasted by the transformer

of course even with a simple non linear load, like a diode bridge followed by a capacitor, 50-60% more (RMS) current goes through the transformer than through the load, but with your regulator I think that the transformer has to dissipate some 30-50% more power over a circuit with just a linear regulator

[robert67]

For everybody else "This is not a Thyristor pre regulator!!!!!"
Almost al Thyristor pre regulators are noisy as Hell!
This pre regulator, if build wel, with a controled "switch off" time, is low noise.

have you checked the RMS current through the transformer secondary?

several months ago I simulated your circuit via LTspice and (if I remember correctly) noticed that the transformer current is more than double the DC load current, so I suspect that the power (or at least great part of it) saved in the linear regulator is wasted by the transformer

of course even with a simple non linear load, like a diode bridge followed by a capacitor, 50-60% more (RMS) current goes through the transformer than through the load, but with your regulator I think that the transformer has to dissipate some 30-50% more power over a circuit with just a linear regulator

Interesting question (to Blackdog I guess). I have not given that any thought up to now. And I have to admit that I'm not a LTspice guy.

That said, your LTspice simulation results are a bit surprising. Using any kind of switching (at 50/60Hz in our case) pre-regulator instead of just a linear regulator REDUCES the active power (W) drawn from the transformer. Of the  course the apparent power (VA) will be higher, current and voltage being out of phase.

Anyway, your LTspice results suggests that the efficiency of the transformer decreases at a significantly higher rate than the active (W) power drawn from the transformer is decreased by the pre-regulator. And that is compared to the filter capacitor directly connected to the rectifier, where voltage and current are already considerably out of phase.

But the only difference here is that these pre-regulators (Blackdog's and mine) stop the current flow to the filter capacitor when it has reached the desired voltage. Blackdog's does that soft, avoiding any transients (@Blackdog: and thus noise  ). Mine does it hard, creating a transient that has somehow to be dissipated in the (magnetic field of the) transformer.

Could you tell us for which operating point of the regulator (max/min output current, max/min output voltage) you are getting those LTspice results?

[not1xor1]

For everybody else "This is not a Thyristor pre regulator!!!!!"
Almost al Thyristor pre regulators are noisy as Hell!
This pre regulator, if build wel, with a controled "switch off" time, is low noise.

have you checked the RMS current through the transformer secondary?

several months ago I simulated your circuit via LTspice and (if I remember correctly) noticed that the transformer current is more than double the DC load current, so I suspect that the power (or at least great part of it) saved in the linear regulator is wasted by the transformer

of course even with a simple non linear load, like a diode bridge followed by a capacitor, 50-60% more (RMS) current goes through the transformer than through the load, but with your regulator I think that the transformer has to dissipate some 30-50% more power over a circuit with just a linear regulator

Interesting question (to Blackdog I guess). I have not given that any thought up to now. And I have to admit that I'm not a LTspice guy.

yes, I was asking Blackdog...
I tested this kind of preregulator (but I used an opamp as a sort of flipflop) a few years ago and did not think then about checking the current through the transformer
so now I'm not sure what happens in a real circuit, i.e. I wonder if the spice model of the transformer I'm using is realistic

That said, your LTspice simulation results are a bit surprising. Using any kind of switching (at 50/60Hz in our case) pre-regulator instead of just a linear regulator REDUCES the active power (W) drawn from the transformer. Of the  course the apparent power (VA) will be higher, current and voltage being out of phase.

you lose something more in the transformer... probably just due to the Joule effect (i.e. caused by the current increase on the windings resistances)

Anyway, your LTspice results suggests that the efficiency of the transformer decreases at a significantly higher rate than the active (W) power drawn from the transformer is decreased by the pre-regulator. And that is compared to the filter capacitor directly connected to the rectifier, where voltage and current are already considerably out of phase.

But the only difference here is that these pre-regulators (Blackdog's and mine) stop the current flow to the filter capacitor when it has reached the desired voltage. Blackdog's does that soft, avoiding any transients (@Blackdog: and thus noise  ). Mine does it hard, creating a transient that has somehow to be dissipated in the (magnetic field of the) transformer.

Could you tell us for which operating point of the regulator (max/min output current, max/min output voltage) you are getting those LTspice results?

That is the same method I tested (real circuits) a few years ago... but a few months ago I made many simulations of different circuits... so now I have something more than 100 .asc files in that directory ... I just don't remember which is the right one 
but I think the problem is more noticeable at low output voltage and maximum load

in the next few days I'll see if I can find some more details

[not1xor1]

Hi, Robert67

OK, I read more carefully your introductory post
your circuit will be a sort of parody of the Vulcanian "live long and prosper"... something more like "live short and die in a puff of stinking smoke" 

just kidding... please do not take it as an offence...

the first part to die will probably be the LM317 used as constant current regulator
it may work for a short while like a zener dumping the huge voltage spikes caused by the transformer secondary winding acting like the inductor in a stepup switching supply

I ran a few simulations in LTspice comparing 3 circuits with as little as possible variations, implementing:
1) just a linear regulator
2) your schematic
3) Blackdog preregulator

of course no device runs the risk of exploding when running in software 
so I could realize that Blackdog preregulator, besides avoiding destruction of real hardware, at the cost of few more devices is even more efficient than your circuit

Regarding the Blackdog circuit, I could also check the RMS current through the secondary and noticed that while much higher than that of the plain linear regulator circuit, the transformer losses increase just about 15% (at least in that particular test: 1.3V/3.2A output)

on the other hand the preregulation is a bit lose and especially at low output voltage both the mosfets and the linear regulator waste a lot of power

Tomorrow I'll give more details with the shots of the circuits and of the measures and some more comments.
Later on I'll provide some more schematic I tested in past...

[robert67]

Hi Not1xor1 (1? Not (1 xor 1)?),

OK, I read more carefully your introductory post
your circuit will be a sort of parody of the Vulcanian "live long and prosper"... something more like "live short and die in a puff of stinking smoke" 

just kidding... please do not take it as an offence...

Good one, the Vulcan parody that is. And no offense taken! That's why I put the design out there ... to be shot down In some circles that process is called "review"  And I'm very grateful that you take your time to have a look at my Klingon abomination ("Today is a good day to die!")   

the first part to die will probably be the LM317 used as constant current regulator
it may work for a short while like a zener dumping the huge voltage spikes caused by the transformer secondary winding acting like the inductor in a stepup switching supply

Yes, I remember thinking at one point about that. But somehow I forgot about it in my quest to reduce the parts count. However, as Blackdog brought up the switch-off noise problem its was coming to me again.

I'm currently looking at the following solutions:

A) RC snubber (2 parts)
B) Zener diode (1 part) - not using the LM317 for that 
C) Soft switch-off like Blackdogs (at least a RC element and a diode, so 3 parts)
D) Any combination of A), B) and C)

I do not like the soft switch-off for two reasons: (1) It increases the power loss in the series transistor (not that this would matter in my circuit) and (2) it messes with the regulation (that's the killer for me: if +/- a few V is OK it's fine, but I want to stay within a few 100mV).

so I could realize that Blackdog preregulator, besides avoiding destruction of real hardware, at the cost of few more devices is even more efficient than your circuit

Regarding the Blackdog circuit, I could also check the RMS current through the secondary and noticed that while much higher than that of the plain linear regulator circuit, the transformer losses increase just about 15% (at least in that particular test: 1.3V/3.2A output)

on the other hand the preregulation is a bit lose and especially at low output voltage both the mosfets and the linear regulator waste a lot of power

I noticed that too about Blackdog's circuit: The MOSFETs are operated under certain (low voltage)  conditions in their linear regions. And they are always going through their linear regions during the soft switch-off. And the whole soft switch-off throws off the regulation a bit (see my dislike for it above). However, his design goal was a real quite pre-regulator, so the drawbacks in efficiency and regulation accuracy are acceptable.

My design goal was the simplest possible pre-regulator of this sort. So some noise and decreased efficiency is acceptable to me. Self-destruction of course not   

Tomorrow I'll give more details with the shots of the circuits and of the measures and some more comments.
Later on I'll provide some more schematic I tested in past...

I'm looking forward to it!

[not1xor1]

Hi Not1xor1 (1? Not (1 xor 1)?),

yes 

the first part to die will probably be the LM317 used as constant current regulator
it may work for a short while like a zener dumping the huge voltage spikes caused by the transformer secondary winding acting like the inductor in a stepup switching supply

Yes, I remember thinking at one point about that. But somehow I forgot about it in my quest to reduce the parts count. However, as Blackdog brought up the switch-off noise problem its was coming to me again.

I'm currently looking at the following solutions:

A) RC snubber (2 parts)
B) Zener diode (1 part) - not using the LM317 for that 
C) Soft switch-off like Blackdogs (at least a RC element and a diode, so 3 parts)
D) Any combination of A), B) and C)

I do not like the soft switch-off for two reasons: (1) It increases the power loss in the series transistor (not that this would matter in my circuit) and (2) it messes with the regulation (that's the killer for me: if +/- a few V is OK it's fine, but I want to stay within a few 100mV).

A) an RC snubber may help, but not so much, as far as I remember
B) a TVS diode is more appropriate (and usually powerful) than an ordinary zener, but its purpose should be that to protect from occasional spikes rather than dissipate them at all times
C) soft switch-off does work and greatly reduces the spikes and improve the overall efficiency... and you are right, you just need 3 components
D) so yes IMHO you need all of these solutions

in any case any reduction in the peak current reduces both output noise, spikes and transformer stress

if you reduce the current too much the PSU may fail to respond properly to transient load and you may decrease the overall efficiency, but there is a sweet spot where you can get the optimal result

possible solutions to reduce voltage spikes (and noise) and improve the overall efficiency while reducing the transformer stress are:
- a low value resistor (100-220m\$\Omega\$) between the bridge and Q1 collector
- rather than using a big capacitor, make a pi filter with 2 capacitors and a low value resistor with lower value capacitors

LTspice showed that these solutions do work with low voltage and a fixed load
I have to test more voltage variations and the behaviour with a fast changing load

I'll post more informations later
bye

[robert67]

Hi not1xor1,

A) an RC snubber may help, but not so much, as far as I remember
B) a TVS diode is more appropriate (and usually powerful) than an ordinary zener, but its purpose should be that to protect from occasional spikes rather than dissipate them at all times
C) soft switch-off does work and greatly reduces the spikes and improve the overall efficiency... and you are right, you just need 3 components
D) so yes IMHO you need all of these solutions

A) That's still my first choice so far. I would place the RC snubber directly across the transformer secondary, no need to let the switch-off spike travel through the rectifier first. As the pre-regulator switches off at max 37V the RC snubber needs to keep the spike just below 23V (at 60V the LM317HVT starts to fry).

B) I've ruled out zener and TVS diodes. There are simply no parts available that cut off the voltage safely at 60 volts without being "shorts" at 58.8V (made a parametric search at digikey). And MOVs would die a quick death being used 100 times per second.

C) Yes, soft switch-off works, but I still don't like that it also "softens" the regulation. Maybe I just have to get over that ...

D) So far I would go for a combination of A) and (if I can bring myself to it) C).

possible solutions to reduce voltage spikes (and noise) and improve the overall efficiency while reducing the transformer stress are:
- a low value resistor (100-220m\$\Omega\$) between the bridge and Q1 collector
- rather than using a big capacitor, make a pi filter with 2 capacitors and a low value resistor with lower value capacitors

Another three parts ...

This 100Hz switch pre-regulator is getting more complex by the minute. Kudos to Blackdog for his design!

Anyway, I'll try to calculate the values for the soft switch-off RC element.

I also explore a little the feasibility of a linear pre-regulator (something you mentioned in one of your earlier posts). But such a pre-regulator burns a lot of power in the series transistor(s) (worst case - drawing 5A at almost 0V - about 275W). Thankfully my transformer has two secondary windings (not the same voltage, that is not middle-tapped), so maybe the power loss can be reduced by "vintage" winding switching to an acceptable value.

I'll post more informations later

As always looking forward to it.

[not1xor1]

Hi not1xor1,

A) an RC snubber may help, but not so much, as far as I remember
B) a TVS diode is more appropriate (and usually powerful) than an ordinary zener, but its purpose should be that to protect from occasional spikes rather than dissipate them at all times
C) soft switch-off does work and greatly reduces the spikes and improve the overall efficiency... and you are right, you just need 3 components
D) so yes IMHO you need all of these solutions

A) That's still my first choice so far. I would place the RC snubber directly across the transformer secondary, no need to let the switch-off spike travel through the rectifier first. As the pre-regulator switches off at max 37V the RC snubber needs to keep the spike just below 23V (at 60V the LM317HVT starts to fry).

unfortunately you would need a large snubber capacitor to reduces the spikes to a reasonable value (I had to use 220µF in the simulation) and as a side effect that increases both transformer losses and overall power consumption

in the screenshots below you will notice that without snubber the spikes get to 600V which is unrealistic since the BJTs would reduce that to a much lower value (until they die)

these are the data I got (3A load):

snubber	| input power	| transf. RMS current	| transf. power loss (% rel. to lin. reg)
none	106W	7.5A	39.6W (+28%)
220µF+1ohm	117W	8.9A	47.5W (+60%)

B) I've ruled out zener and TVS diodes. There are simply no parts available that cut off the voltage safely at 60 volts without being "shorts" at 58.8V (made a parametric search at digikey). And MOVs would die a quick death being used 100 times per second.

you should use just a resistor or a 2 BJT constant current source (like I did in the simulation) to get an higher voltage tolerance

C) Yes, soft switch-off works, but I still don't like that it also "softens" the regulation. Maybe I just have to get over that ...

unfortunately with the emitter follower configuration you can't just use an RC + diode to slow down the switch-off.. just few mVs of base voltage decrease put the darlington off and a large capacitor value would take too long to be charged by the current source

D) So far I would go for a combination of A) and (if I can bring myself to it) C).

possible solutions to reduce voltage spikes (and noise) and improve the overall efficiency while reducing the transformer stress are:
- a low value resistor (100-220m\$\Omega\$) between the bridge and Q1 collector
- rather than using a big capacitor, make a pi filter with 2 capacitors and a low value resistor with lower value capacitors

Another three parts ...

This 100Hz switch pre-regulator is getting more complex by the minute. Kudos to Blackdog for his design!

Anyway, I'll try to calculate the values for the soft switch-off RC element.

I also explore a little the feasibility of a linear pre-regulator (something you mentioned in one of your earlier posts). But such a pre-regulator burns a lot of power in the series transistor(s) (worst case - drawing 5A at almost 0V - about 275W). Thankfully my transformer has two secondary windings (not the same voltage, that is not middle-tapped), so maybe the power loss can be reduced by "vintage" winding switching to an acceptable value.

I suggest you to use Blackdog's circuit
according to my simulations you just have to derate the transformer by 50%, i.e. if your transformer is rated for 5A on a resistive load you should limit it to 2.5A
BTW I noticed that the maximum stress on the transformer is at about 50% of max output voltage

Radj:                   1m      315    550    1k    1.5k    2k    2.7k
output voltage          1.2V    5.2    8.1   13.7   19.9   26.2   34.9
transf. current (RMS)   6.8A    =      =      6.7    6.5    6.3    5
input power           104.2W  112.7  120    133.6  147.5  159.2  162.4
transformer loss       32.8W   33.4   33.8   34.6   35.2    =     29.8
rect. bridge loss      12.3W    =     12.2   =      12     12.6   10.5
prereg. switch loss    30.8W   29.3   27.6   23.9   18.9   12.6    0.5
linear regulator loss  21.7W   19.5   19.3   18.9   18.4   17.9   15.6
load power              3.7W   15.5   24.3   41.1   59.8   78.5  104.7




the other circuit I mentioned in a previous post works like Blackdog's one, but keeps a tighter control on the delta Vprereg-out and manages to save a few Ws of power, but requires an additional transformer (or winding)

more efficiency may be achieved by a circuit which switches off on zero-cross, like the SCR ones, but it requires a large inductor and an awfully complicated circuit to properly manage output voltage, load and AC line variations...

the best solution IMHO would be to just use a buck converter IC as preregulator

[blackdog]

Hi,

Some remarks...
Normaly its about 60 to 65% from the transformer current you can use if there is a bridge and buffer capacitor and not 50% (but most of the time i use 50% to be on the save side)
In my design there is 2x 10000uF and i went back to 10000 a 12000 uF, this wil dubble the ripple but the active rectifier with the LT4320 keep my losses down so there wil be no drupout problems.
So there is more voltage on the buffer capacitor and of course extra ripple, maybe a 33V transformer is a better choice for 5-A at 30V...
This has al to do with the variation of the Power Line you are willing to accept.
A higher transformer voltage will give you a 5A-30V output by lower Powerline voltages, but also higher dissipation.

Because now the peak currents go down because the buffer elco is smaller, the dissipation in the transformer becomes lower, but also in the pre-regulator it becomes smaller.
The reference section is so good, this together with the high gain of the opamps means that you do not find the extra ripple on the buffer capacitor at the output of the power supply.

On the other forum (Circuitsonline.net) you cant find the latest version of the schematic, i'am stil working on it.
There is now a new candidate for the opamps, and dat is the OPA140 series.

This design of the preregulator is made to get the Power dissipation down, not to be the best in efficiency, and do it with low noise.

Hi not1xor1
the best solution IMHO would be to just use a buck converter IC as preregulator
You must be kidding...

I design a extreem Low Noise Lineair power supply an you start to talk about Buck converters.
Switching preregulator in low noise design is like Water and Fire... :-)
And yes a know, with high kost and a long design time, and a lot of testing you can do it.

But what i have seen from the professionals, no..., always high noise levels if they use switchers in front a analog regulator.
If you build my design wel, the 22KHz bandwith noise wil be smaler than 5uV and the Ri extreem low til at least 100KHz.

Everybody who thinks he can cut corners in my design will throw away performance, and spice is nice, but building and measuring is better.
You have to tune the preregulator like a stated for the transformer you are using.

Also the TVS diode for your transformer and line voltage and also the snubber, maybe you need to change the 2.2Ohm and 3.3uF...
If you are in a 60Hz country, you can use 20% smaler value for the buffer capacitors.
In a 50HZ country you can use this rule of thumb: 1A and 10000uF wil give you 1V TopTop Ripple.
Another rule of thumb is you need 2000 tot 3000uF per Ampere buffer capacitor, most designers use the 2000uF in there design.

But...
Even if you do the electronics the good way, and make a mess of the wiring, you wil trow the performance of this design out of the window 

Kind regards
Blackdog

PS
Most simpel solution is a 32V transformer with tabs, 8V, 16V, 24V and 32V.
Or more simpel, use 2 transformers of 2x9V, 150VA a 160VA and use some relais for switching or triacs

[robert67]

Hi not1xor1, Hi Blackdog,

please don't forget that my design goal is "as simple as possible" and that I'm willing to sacrifice some (a whole lot) of efficiency for it.

@Blackdog: Of course my (not obviously ) not working design sacrificed some performance compared to your original. I've stated that and I was willing to do so to save a few components. Anyway, I might go back to an older version of my design that was a lot closer to yours (still a few parts less  ).

@not1xor1: Thank you so much for the simulations! The results are most educating (if not a bit sobering). I was also busy myself and added kind of an "active" RC snubber and the soft-switch off:

See attachment Pre-Regulator 4.1.jpg (I really have to learn how to include attachments as images in the HTML)

The soft switch-off (D5, R8, R3) actually works as long as there is a load, that is current is drawn from the filter cap C1. And if there's no current drawn from C1 there's also no switch-off spike. In addition it doesn't interfere with the regulation. The soft switch-off can only supply base current when the voltage of C1 is decreasing, so no overshoot. However, if there is a load transient at switch off time from, e.g. 5A to 0A, the soft switch-off can not supply any current to the base and there is a spike 

The "active" RC snubber (D4, R6, R7, C2) works as follows: D4 just prevents the base of getting any current from the snubber. R7 discharges C2 while D3 is off, so it's nice and empty to absorb the switch-off spike. R6 is just a security measurement (that thyristor can take up to 16A pulses). Please ignore the values of the components, I had not the time to calculate them properly.

I also doodled together a simple linear pre-regulator (you suggested in a previous post it would be the best for the transformer - don't try to simulate that  ):

See attachment: Pre-regulator 6.jpg

That thing burns a whole lot of power. Probably more than the heat sink on my vintage power supply can take. Anyway, I mainly did it to compare the required number of parts.

Anyway, best regards to both of you

[not1xor1]

Hello Blackdog,

Normaly its about 60 to 65% from the transformer current you can use if there is a bridge and buffer capacitor and not 50% (but most of the time i use 50% to be on the save side)
In my design there is 2x 10000uF and i went back to 10000 a 12000 uF, this wil dubble the ripple but the active rectifier with the LT4320 keep my losses down so there wil be no drupout problems.
So there is more voltage on the buffer capacitor and of course extra ripple, maybe a 33V transformer is a better choice for 5-A at 30V...

Transformers are rated in VA, that is under a plain resistive load.
When you connect a rectifier bridge, a levelling capacitor and a resistive (or constant current load), both the phase and the RMS value of the transformer current change, so, in order to provide the nominal current to the load, the transformer has to withstand more power losses.

I made a rough model of a 30V 150VA transformer and simulated the behaviour with a pure resistive load compared to rect. bridge + capacitor and got the following results, that while might not have an absolute value,  IMHO have a meaningful comparative one:

load type   |        load values      |   transf.values         
            | V (RMS) I (RMS)  power  |  I (RMS)  power loss
AC resistor | 33.1V    5A     165.8W  |   5  A    27.3W
DC 3A       | 40.7V    3A     122  W  |   4.8A    30.3W
DC 5A       | 36.8V    5A     184  W  |   8.2A    55.7W

This shows that even derating the transformer to 60% of the nominal current the power losses increase by about 10%. That usually is not a problem, especially with large transformers in well ventilated cases.

But now, your preregulator circuit adds even more distortion to the transformer current and with a 3A constant current load and depending on the output voltage, the RMS current through the transformer secondary varies between 5A and 6.8A and the power losses go up to 35.2W (pls. notice that I tested just 7 different output voltages).

So due to the increase in transformer stresses, when using such a preregulator, I think it would be wise to limit the load current to 50% or less of the nominal transformer current.

Because now the peak currents go down because the buffer elco is smaller, the dissipation in the transformer becomes lower, but also in the pre-regulator it becomes smaller.

The capacitor value seems to have little effect on the transformer power losses, at least according to LTspice (transf. + brdige + cap + 5A constant current load, 10-20 seconds interval averages)

capacitor   transf. RMS current / power losses
 3300µF     7.6359A / 55.2  W
 6600µF     7.541 A / 55.585W
13300µF     7.4929A / 55.826W

That is: a 400% increase in capacitor value produces just  1% increase in transformer power losses.

Hi not1xor1
the best solution IMHO would be to just use a buck converter IC as preregulator
You must be kidding...

I design a extreem Low Noise Lineair power supply an you start to talk about Buck converters.
Switching preregulator in low noise design is like Water and Fire... :-)
And yes a know, with high kost and a long design time, and a lot of testing you can do it.

well... there are plenty of low noise IC switching regulators which may serve well the main goal stated by Robert67, i.e. minimum components count, and it is easy to filter out the high frequency noise with the aid of one or more LC filter stages between the preregulator and the postregulator

But what i have seen from the professionals, no..., always high noise levels if they use switchers in front a analog regulator.
If you build my design wel, the 22KHz bandwith noise wil be smaler than 5uV and the Ri extreem low til at least 100KHz.

Apart that the PSU ripple and noise is usually specified for a 20Hz-20MHz bandwidth, your preregulator design, while low noise, still is affected by two little defects:
- high frequency noise due to the charge of the 100µF input capacitor quickly transferred on each zero-cross
- on low load you also have very low frequency noise since the output capacitor is mainly charged via the input one and just once in a while by the transformer

See the the screenshots below with your preregulator low load noise compared to an SCR-style (using mosfets) preregulator at low load noise (which I left on the virtual world of LTspice due to the overcomplicated circuit and unrealistic value of some components)




bye