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

[not1xor1]

Depending on the results of your simulation, I might even go back to the pre-regulator approach.

the requirement to keep the component list short is a serious limitation 
otherwise you might use both the center tap and a preregulator

regarding your new circuit proposal I'll have a look at it tomorrow

Turns out switching between the secondary windings is not that easy. The transformer has (now, after Europe went from 220V to 230V) some very "odd" output voltages (considering mains AC is +/-10%):

• 14.3Veff to 17.4Veff, 20.2Vp to 24.6Vp
• 20.5Veff to 25.0Veff, 29.0Vp to 35.4Vp

To keep worst case losses in the linear regulator at a reasonable level (below 150W - I have two TO-3 BJT on big heat sinks on the backside) I'll have to set the switch point depending on the current AC voltage. After trying to come up with a design using transistors I went very fast for an opamp in schmitt trigger configuration Unfortunately that opamp needs some input protection on the feedback from the linear regulator (max. 30V) when I just use the 29.0Vp (below 30V) to 35Vp winding to supply the switcher. That's two more parts and it messes with the nice high input impedance Or I run the opamp from the combined windings. But then I'll need 60V capable opamp and relay as well as a second rectifier/filter to measure the AC level on my "lower" winding

Anyway, maybe I attach my current design later to this message ...

I have an old transformer as well... I bought it when I was a kid... it is probably 45 years old now

I used it for a while with a 2N3055s audio amplifier I built when I was 15 (until it died in a cloud of stinking smoke  )
It is centre tapped with a big "150VA" printed on the case, but one winding i much thicker than the other one so I guess it is something like 24V 6.25A / 48V 3.12A. For that reason I was already thinking about possible centre tap switching solutions. (I wonder if I should rather spell it "centre"

Here is what I simulated so far, just a quick sketch which you might hopefully find useful.

The transformer used in the simulation is centre tapped (15+15V - I modified the previous 30V spice model).
An LT1013 is connected as Schmitt trigger and compares Vout/6 with a 2.5V reference.
As soon as the voltage gets above about 15V, the LT1013 output goes to about 18V (its supply voltage is just 20V in this simulation) switching M1 on via Q1 so the input of the linear regulator is supplied by an higher voltage.

This circuit might work even with much higher voltage by just changing Q1 and M1 and modifying the rate of the voltage divider at the input of the Schmitt trigger.

R9 and C5 reduce a bit the output spike when the input voltage is switched (I might try to slow down M1 commutation as well).
The adjust pin of the linear regulator is driven by a delayed ramp to test the input voltage switching.

BTW the green trace in the top pane is the power loss of the linear regulator.

[not1xor1]

Anyway, the idea of my last design was to avoid switch-off spikes completely (by wasting power in the series transistor of course). The opamp works as an error amplifier or proportional regulator (voltage of C1 being the "process variable" and the desired voltage being the "setpoint"), not as a Schmitt trigger or comparator. It doesn't need a capacity at the gate of the MOSFET.

• As the voltage of C1 approaches the target voltage the MOSFET enters its linear region
• The current to C1 is decreased, the voltage of C1 rises slower
• As the voltage of C1 rises further the MOSFET is driven further into its linear region
• The current to C1 is further decreased, the voltage of C1 rises even slower

Your circuit just provides a gate voltage proportional to the difference between the linear regulator input and its output (and so to ripple) so I do not think it can help in reducing power dissipation.
I tried by changing various component values and output voltages and could not see any difference in power usage between your circuit and a plain linear regulator.

Anyway I've here an old NSC application note, AN1, november1967, about the LM100 IC (the first linear regulator IC?).
Among the various examples there is a switching PSU.
So what about a switching preregulator? Isn't 1967 enough vintage for you? 
Of course I do not suggest to use an LM100 

[robert67]

the requirement to keep the component list short is a serious limitation 

Yes, it is

otherwise you might use both the center tap and a preregulator

I'm actually thinking about that too. Making the linear regulator itself 60V capable is not that easy.

BTW the green trace in the top pane is the power loss of the linear regulator.

Wow, just 70W power loss is not too shabby!

And here's the circuit I've come up to switch between the transformer windings (the original innards of the power supply used a 16A relay for that). The hysteresis of the Schmitt trigger needs to take the ripple voltage at the filter capacitor into account, otherwise you might end up switching that relay at 100Hz The relays I've been looking at just draw 17mA at 24V. So you don't need a big capacitor to keep the ripple down, and you don't need a high power opamp. If the output voltage of the linear regulator is below the minimum voltage of the capacitor you won't need that zener diode and resistor to protect the opamp input.

[robert67]

Anyway, the idea of my last design was to avoid switch-off spikes completely (by wasting power in the series transistor of course). The opamp works as an error amplifier or proportional regulator (voltage of C1 being the "process variable" and the desired voltage being the "setpoint"), not as a Schmitt trigger or comparator. It doesn't need a capacity at the gate of the MOSFET.

• As the voltage of C1 approaches the target voltage the MOSFET enters its linear region
• The current to C1 is decreased, the voltage of C1 rises slower
• As the voltage of C1 rises further the MOSFET is driven further into its linear region
• The current to C1 is further decreased, the voltage of C1 rises even slower

Your circuit just provides a gate voltage proportional to the difference between the linear regulator input and its output (and so to ripple) so I do not think it can help in reducing power dissipation.
I tried by changing various component values and output voltages and could not see any difference in power usage between your circuit and a plain linear regulator.

That's not what I've intended it to do I have to revisit that circuit ...

Anyway I've here an old NSC application note, AN1, november1967, about the LM100 IC (the first linear regulator IC?).
Among the various examples there is a switching PSU.
So what about a switching preregulator? Isn't 1967 enough vintage for you? 
Of course I do not suggest to use an LM100 

That's definitely vintage enough

[hli]

One question about the thyristor-based pre-regulator: when I understand this correctly then the series transistor charges the bulk capacitor until it reaches a certain voltage threshold above the output voltage. Then the thyristors gets triggered and turns off the series transistor until the next half-wave from the rectifier.
If so, then it would mean that the real regulator cannot suddenly increase the output voltage (which might be needed in a constant-current-scenario when the load-resistance is reduced), since the regulator just does not have enough input voltage for that, and the pre-regulator cannot charge the bulk capacitor until the next input half-wave (all depending on the exact timing).
Do I understand this right? Because this would kind of disqualify it for any serious lab power supply

[blackdog]

Hi hli,

How fast do you think a "normal" power Supply is?
Most power supply's have a 220uF to 1000uF capacitor over the output connectors.
Wat wil the dynamic output impedance be?
How much energy is in this output capacitor, how fast is the current loop, how much time before the next half sine is there to charge the buffer capacitor.
Normaly about 3V over the regulator transistor is enough for my linear regulator design for almost al dynamic situations.

Try the circuit, experiment!, and I do not mean with "Spice", but with Solder! 
It is not a complicated circuit.

Kind regards,
Blackdog

[hli]

How fast do you think a "normal" power Supply is?

I have never calculated that, but I would expect a transient response below 1ms (my LM317, although not qualifying as "proper" lab supply does follow 1A transient in about 10µs). But whatever it is - the time from the pre-regulator will _add_ to it. And on average, depending on the load situation, it might be like 5ms.

Try the circuit, experiment!, and I do not mean with "Spice", but with Solder! 

Will do! (when I find out which of all the many version of it I should try But thats why I was asking for experiences.

Thanks,
hli

[not1xor1]

One question about the thyristor-based pre-regulator: when I understand this correctly then the series transistor charges the bulk capacitor until it reaches a certain voltage threshold above the output voltage. Then the thyristors gets triggered and turns off the series transistor until the next half-wave from the rectifier.
If so, then it would mean that the real regulator cannot suddenly increase the output voltage (which might be needed in a constant-current-scenario when the load-resistance is reduced), since the regulator just does not have enough input voltage for that, and the pre-regulator cannot charge the bulk capacitor until the next input half-wave (all depending on the exact timing).
Do I understand this right? Because this would kind of disqualify it for any serious lab power supply

Do not forget that the same thing happens without a pre-regulator: the levelling capacitor is charged every 10ms (full wave rectifier).

So to make that kind of pre-regulator work properly you have to switch off the mosfet when the capacitor has enough charge to to provide an high enough voltage to the linear regulator for 10ms.

You have to consider the worst case, i.e. the maximum load. In that case the linear regulator, from the pre-regulator POV works like a constant current sink and we know that the voltage of a charged capacitor "C" discharged at a current "I" after time "t" decreases by I*t/C volts.

So suppose you have a 10,000uF capacitor and 5A as maximum load and a full wave rectifier: the capacitor voltage would drop by 5*10e-3/10e-3 = 5V.
Now if the linear regulator requires at least 2V (drop-out) to work properly at maximum load, to ensure the circuit works properly in every load condition, you have to switch off the mosfet as soon as the preregulator capacitor voltage gets 2+5=7V above the output voltage.

P.S. it would be misleading to call this kind of pre-regulator an SCR pre-regulator. Real SCR preregulator switches are off by default and are switched on just to provide enough charge to the capacitor before the rectified wave decrease switching off the SCRs (or a mosfet with a proper control circuit). Here the switch is on by default and is switched off as soon as the voltage gets above a given threshold.

[not1xor1]

Hi hli,

How fast do you think a "normal" power Supply is?
Most power supply's have a 220uF to 1000uF capacitor over the output connectors.
Wat wil the dynamic output impedance be?
How much energy is in this output capacitor, how fast is the current loop, how much time before the next half sine is there to charge the buffer capacitor.
Normaly about 3V over the regulator transistor is enough for my linear regulator design for almost al dynamic situations.

Try the circuit, experiment!, and I do not mean with "Spice", but with Solder! 
It is not a complicated circuit.

Kind regards,
Blackdog

But you can save a lot of  time and tin if you first understand how it works and why. 

And simulations let you see immediately the drawbacks of various kinds of circuits, when they do not go haywire and break the conservation of energy law.

Your pre-regulator is nothing new, it has been proposed in many different variations many times in various magazines during the last decades, but AFAIK no manufacturer uses that kind of circuit for their bench PSU, while there are various devices based on SCRs (or mosfet working like SCR switches) or switching pre-regulators.

Besides the very low frequency (fraction of Hz to few Hz) noise at low load, I already wrote about in past,, that kind of circuit has a really poor efficiency at low output voltages and high load.
The reason is that you switch off the current when the transformer secondary winding has already stored a lot of energy and that energy is then just wasted (apart a little stored in the input capacitor).

Even with a tighter threshold control (like that in reply 46) you can save just few tenths of watts when at low output voltage and with 3-5A load.

[hli]

Do not forget that the same thing happens without a pre-regulator: the levelling capacitor is charged every 10ms (full wave rectifier).

But for a 'normal' power supply we assume that the output voltage is always below (max. input voltage - ripple voltage - dropout voltage). So it should _never_ run into the situation where the regulator runs out of input voltage. Yes, the capacitor is charged only every 10ms, but we accommodate for that with high enough input voltage.

I was specifically referring to a low-output-voltage scenario, where the output voltage needs to rise suddenly (e.g. because you programmed the PSU that way, or because the load changes in CC setting. This should never be a problem without pre-regulator, it should be fine with a switching one (since they run at high frequency), but this one here has just no chance to react fast enough (depending on the timing of the required voltage change).

hli

[blackdog]

Hi not1xor1,

And how about experience...
Spice tells you its OK, and then you wil build it, "what te Fu.." a lot off time you put in Spice is going out off the window 

I can only say, do not use Spice as a shortcut to learn electronics, its a tool.
And yes, i use all kinds of electronic tools to make my live easier.

You have to learn bij real bulding that a resistor has also a inductens and also behave as a capacitor all depending on witch frequency you are using.
And then we also have all de paracitics of the way you build the circuit, yes! electronic is coplicated.
Try to see it as this, you can study all you want on say Youtube, about skateboarding...
The first time you wil step on the skateboard you realise that you didn't build/learn any muscle memory 

Take my power supply design, it is FAST!!!! and because of that, i have to follow the the rules of HF design for the wiring...
A lot of wiring is twisted to keep the induction and EMC down.
The same with probing the electronics, are you doing it the wright way, is the nasti spike you see realy in the circuit, or is it the wire to the power section that is radiating its field to the probing point.
I can go on the whole morning like this 

Iám absolutely NOT against Spice, but wath i see on this forum and the Dutch forum circuitsonline don't make's me happy.
Most user can't even draw a schematic and trust there Spice, because Computers don't lie...
The Spice user forgets they are lying to them self *grin*

Sorry for the lousy Ingles...

Kind regards,
Bram

 

[not1xor1]

Do not forget that the same thing happens without a pre-regulator: the levelling capacitor is charged every 10ms (full wave rectifier).

But for a 'normal' power supply we assume that the output voltage is always below (max. input voltage - ripple voltage - dropout voltage). So it should _never_ run into the situation where the regulator runs out of input voltage. Yes, the capacitor is charged only every 10ms, but we accommodate for that with high enough input voltage.

I was specifically referring to a low-output-voltage scenario, where the output voltage needs to rise suddenly (e.g. because you programmed the PSU that way, or because the load changes in CC setting. This should never be a problem without pre-regulator, it should be fine with a switching one (since they run at high frequency), but this one here has just no chance to react fast enough (depending on the timing of the required voltage change).

hli

if you read more carefully my reply you will understand that you have to adjust the switch threshold, independently from the current real load, to the worst case load. That is the pre-regulator switch has to stop charging the capacitor as soon as the capacitor voltage gets to Vout+maxVdropout+maxVripple, where as maxVripple I mean the voltage decrease of the capacitor when in the worst case load.

For instance if Vout is 1V, the worst case drop-out of the linear regulator is 2V, and you have a 20,000uF capacitor with a worst case load of 5A, you have to stop charging the capacitor as soon as the voltage gets to 1+2+2.5=5.5V.

That is a 20.000uF capacitor charged at 5.5V cannot get below 3V (1V + 2V drop-out) in 10ms even if you step load it from 1mA up to 5A (of course a real electrolytic capacitor would lose a really small bit due to self discharge).

[not1xor1]

Hi not1xor1,

And how about experience...
Spice tells you its OK, and then you wil build it, "what te Fu.." a lot off time you put in Spice is going out off the window 

well I built several variations of that kind of pre-regulators a few years ago, so I'm not referring just to spice simulations.
... and in any case I do think no real circuit can break the law of physics... 

[prasimix]

Power supply is at the end as slow, as the slowest section of it. At the beginning I'm used mosfet pre-regulator for charging bulk capacitor (thanks Blackdog once again) and it's reasonable fast but not faster then SMPS pre-regulator for simple reason that later works with few order of magnitude higher frequency and is powered by DC not low-freq AC. When mosfet (or lets call it mains frequency switcher) pre-regulator has to follow rapid changes on the output (i.e. something that is in range of mains frequency or faster) then I noticed a sort of "hiccup" phenomenon. Here is how it looks like then output is programmed with 50 Hz, 50% duty (10 ms ON, 10 ms OFF), cyan trace is pre-regulator output, yellow is post-regulator output, blue is programming signal from DAC:



Nothing suspicious on the screenshot above, but now over longer period of time:



... and zoomed in:



In practice such extreme programming is rarely existing in DIY environment and I didn't gave up from blackdog's solution due to that phenomenon but in that time for another more practical reason: it's practically very bad match with toroidal transformer (that someone already addressed in this thread). AFAIK there is at least one attempt to do something with such pre-regulator and toroidal transformer that is described here.

Regarding fast output programming, any output capacitance with affect speed. That could be especially problematic when lower programmed value followed a higher one (e.g. 5 V after 30 V). But for that situation many commercial power supplies are using so-called down-programmer circuit that is used for power sinking.

[hli]

if you read more carefully my reply you will understand that you have to adjust the switch threshold, independently from the current real load, to the worst case load. That is the pre-regulator switch has to stop charging the capacitor as soon as the capacitor voltage gets to Vout+maxVdropout+maxVripple, where as maxVripple I mean the voltage decrease of the capacitor when in the worst case load.

Well, in my scenario the worst-case load is a switch to full output voltage. Under that assumption, the pre-regulator neds to provide the full input voltage.
I'm not worrying about a constant load - I fully understand how the pre-regulator works for that, and how it needs to be configured. What I was looking at is how the pre-regulaotr affects the response to load transients that result in an increase of the output voltage

For instance if Vout is 1V, the worst case drop-out of the linear regulator is 2V, and you have a 20,000uF capacitor with a worst case load of 5A, you have to stop charging the capacitor as soon as the voltage gets to 1+2+2.5=5.5V.

What happens when, after the MOSFET has turned off, in the same 10ms cycle, the output voltage is request to increase to 10V (independent of the current that delivered)? AFAICS the output voltage would rise to (5.5V-Vdrop), so maybe 4V (depending on the regulator), and stay there until the next  charge cycle starts and the voltage at the capacitor rises above the 5.5V.

[not1xor1]

Power supply is at the end as slow, as the slowest section of it. At the beginning I'm used mosfet pre-regulator for charging bulk capacitor (thanks Blackdog once again) and it's reasonable fast but not faster then SMPS pre-regulator for simple reason that later works with few order of magnitude higher frequency and is powered by DC not low-freq AC. When mosfet (or lets call it mains frequency switcher) pre-regulator has to follow rapid changes on the output (i.e. something that is in range of mains frequency or faster) then I noticed a sort of "hiccup" phenomenon.

You are referring to sudden changes in programmed output voltage, don't you?
Well, probably an audio amplifier would be better  , anyway that kind of pre-regulator could still work in such cases if you provide a further mosfet connecting a capacitor at full voltage (via a low Q inductor + parallel resistor) just for those "emergency" situations... I mean class-G audio amplifiers style.

Regarding switching pre-regulators I think you can get quite low output noise (I've not built anything real yet) if you keep the frequency low, like in those old Jim Williams circuits.
If you have a look at those linear regulators IC datasheets, you may notice that even the humble LM317 has quite a good ripple rejection up to several kHz.

[not1xor1]

What happens when, after the MOSFET has turned off, in the same 10ms cycle, the output voltage is request to increase to 10V (independent of the current that delivered)? AFAICS the output voltage would rise to (5.5V-Vdrop), so maybe 4V (depending on the regulator), and stay there until the next  charge cycle starts and the voltage at the capacitor rises above the 5.5V.

such kind of pre-regulator is just not suitable for programmable PSUs (unless you add a capacitor backup like I hinted in the other reply).

On the other hand, if you just want to build an ordinary PSU you usually do not mind if it takes a few hundredths of seconds to get the output from 5V to 20V.

Apart from testing the response of a circuit to supply voltage transients (and you could find other methods to achieve that) I do not see any usefulness in fast variations of output voltage.

[prasimix]

What happens when, after the MOSFET has turned off, in the same 10ms cycle, the output voltage is request to increase to 10V (independent of the current that delivered)? AFAICS the output voltage would rise to (5.5V-Vdrop), so maybe 4V (depending on the regulator), and stay there until the next  charge cycle starts and the voltage at the capacitor rises above the 5.5V.

such kind of pre-regulator is just not suitable for programmable PSUs (unless you add a capacitor backup like I hinted in the other reply).

On the other hand, if you just want to build an ordinary PSU you usually do not mind if it takes a few hundredths of seconds to get the output from 5V to 20V.

Apart from testing the response of a circuit to supply voltage transients (and you could find other methods to achieve that) I do not see any usefulness in fast variations of output voltage.

You're right, good to write that down, just for the record.

[hli]

You are referring to sudden changes in programmed output voltage, don't you?

Thats one of the cases where this happens. I was more thinking of a lab supply in constant current mode. But there, the worst case is that the supplied current is too low for some milliseconds (not more than 10ms, obviously). Which is something one could live with.

[hli]

such kind of pre-regulator is just not suitable for programmable PSUs (unless you add a capacitor backup like I hinted in the other reply).

OK, so I understood the behavior correctly. Thanks for the explanations. For a programmable PSU, one might also sync these changes to the beginning of a half-wave, when the FET is turned on anyway.

hli

[blackdog]

Hi hli,

If you want a realy fast programmable power supply, you have to buy/design a 4 quadrant type.
Most of the programmable power supply you can buy are "slow" sometime they have a "down programmer" to push down the output if you change the output to a lower voltage.
This to make them more quickly respons in light load conditions.

If you want a quick reacting current source, buy or design one.
You wil never have a "one size fits all" power supply.

The design i showd is for "normal bench" use, but extreme low noise, fast  and stable, if i want a fast current source with a high compiance, i will use another device.

Kind regards,
Blackdog

[robert67]

First of all, sorry that I was not attending my thread for a while. I was under the weather and now I have to catch up with work. So my contributions will be sparsely in the near future. But it seems you're all quite OK without me 

A thought or two about regulation speed

Even some high priced linear lab power supplies use a relay to switch between different taps of the transformer secondary. Those relays, that is the good ones, have a switching times of 8ms (6ms release). Add to that a few ms of bounce and any considerations about your pre-regulator being able to charge up your filter cap in time when a rising voltage transient comes along is mute.

The only way to respond to rising voltage transients when using a linear regulator in considerably less than 10ms is not to pre-regulate and to keep your filter cap at maximum voltage the whole time. But then of course you're burning a whole lot of power at lower output voltages.

Update on my brain farts regarding the rebuild of my vintage power supply

• Transformer tap switch to avoid insanely high power losses since the old thing has two windings anyway
• Rectifier and filter cap
• Linear pre-regulator that will burn up to 150W just because I can I mean I have two massive TO-3 heat sinks at the back of my case and that pre-regulator will of course eliminate the ripple at the filter cap - so a "no noise" design (take that Blackdog )
• Linear regulator that just burns its own dropout voltage, so about 15W at 5A to dissipate inside the case (which has a lot of ventilation).

[not1xor1]

You are referring to sudden changes in programmed output voltage, don't you?

Thats one of the cases where this happens. I was more thinking of a lab supply in constant current mode. But there, the worst case is that the supplied current is too low for some milliseconds (not more than 10ms, obviously). Which is something one could live with.

What do you need constant current for? I can think about measuring very low resistances, or charging a battery.
In both cases there is no problem if the voltage doesn't change so fast.

Of course there are probably many other cases where a tight current regulation with rapid voltage changes might be essential which I just do not know and cannot even imagine about.

I will be glad to learn about some more usage examples, I'm just an hobbyist, not an engineer.