Preregulation of a linear bench PSU

[Atom]

So, first of all HI to everyone,

second...after building the blackdog psu i'm having trouble with the preregulation, let me explain, the preregulator works but is has some problems.
by the way the linear regulator is a beast and even with all the noise that is injected by the preregulator none is seen on the output.

problems of the preregulator.

1.the transformer is buzzing like crazy because of the high current that is being drawn.

probe across 10cm of 1.75mm^2 cable in the gnd return path  for measuring current drawn by the capacitors.

now making some calculations wire resistance 0.96mohm(taken from a calculator on the internet). the spikes that i see are 60mV at 3.5A load.(yellow trace)

the current that the caps are drawing is 60mV/0.96mohm = 62Amps.

the blue trace is the rectified voltage ..as you can see it drops quite a bit before returning up.

2.Loading the transformer even more 9/10 amps the spikes increase to 100mV... so 100A spikes.the yellow trace also seems to flatten towards the end (the transformers cant provide the current i think?, explain this to me ).

i tried to add an inductor as people suggested in other disscussion but you need a huge inductor (photo of mine below) i don't know the inductance either.

possible solution: my transformer has a center tapped winding 0-14-28Vac so i need a circuit that has to switch betweens the 2, if someone has an already made circuit that i could take ideas from please share it.

everything is in a drive folder i have a lot of images and the max size of the formu woudn't suffice.

note: switching preregulator are excluded because high freq noise is difficut to flatten out, maybe in a smaller bench psu i ll'try it.

https://drive.google.com/drive/folders/1aw1aTxQGjsbswE9cbhaWeTquFv80WQW1?usp=sharing
 

[Atom]

So going for the tap switching solution, this is the circuit i've come up with ...i wanted to know if there is something that can be improved.

considering the ripple of the capacitor at aload of 8A is 4pk-pk ,i'v added a volt so the pass transistors can regulate the voltage properly.

the ranges are :

vout 0-15V first tap (rectified 14vac= 20VDc)

vout 0-30(or 35) second tap (rectified 28vac ) 40VDc)

the circuit consist of a nonc relay and a comparator lm393.

i wanted to know if there is something that i can improve...

switching times won't be the best and i'm worried relay contacts could weld themselves if i have a load on the output (up to 8 amps). the ncno relay that i have now is made for switching 10A at 250v, i have also 17A relays but they are only no

i thought of using mosfet but how can you turn on a n mos passing an ac signal? you need a floaring dc voltage on top the ac?

i haven't tested the circuit and multisim wont simulate it  (stupid multisim uses only one core...pfff)

file of the circuit is on the drive(if you want to try and simulate): https://drive.google.com/drive/folders/1aw1aTxQGjsbswE9cbhaWeTquFv80WQW1?usp=sharing

[not1xor1]

So going for the tap switching solution, this is the circuit i've come up with ...i wanted to know if there is something that can be improved.

I'm in a hurry at the moment so have not yet checked carefully your circuit (I find some symbols quite odd).
But here is a proof of concept I simulated with LTspice a while ago.
That would work even with just a TL431 as a comparator to switch the rails, but it is for negative rail regulation. You have to reverse it and use P-MOS (with usually higher Rds-on) for positive rail...

[Kleinstein]

The type of pre-regulator shown, with switching at the line frequency can be tricky. If something is not working 100% right and symmetric, it could cause some DC current to the transformer, and thus can make the transformer to make quite some noise. The current spikes can also be quite high if the turn on is under power. The active rectifier is also a possible source of trouble. So I am not convinced it is a good idea to use active rectifier in combination with the switching for pre-regulation.
If everything is working fine it can be a good type of pre-regulator.  However filtering is difficult at the low frequency.

I can't understand the circuit shown for tap switching. It looks rather odd - somewhat missing the important part.
I think the planed turn over at 15 V  is a little optimistic, as one will need some reserve for ripple and the relay switching time. In addition Tap switching with a relay normally needs quite some hysteresis to prevent to frequent switching that wears out the contacts. So relativistic turn over would be more at 10 and 13 V.  Switching DC with a relay is tricky, switching AC causes quite some peak current from charging the capacitors. So it might be better to use more than 2 step switching - this is what many cheap linear supplies use.

With just 2 transformer taps there is also the alternative to use 2 transformer taps with linear turn over. It is a little more effective than switching 2 taps and no spike on turn over. As a downside there is a little more drop out (e.g. some 1 V) and one needs twice (compares to ideal switching) the number of power transistors, though only half of them will get hot at the same time. Ina addition it takes more filter caps - though with less peak current to them.

[Atom]

really nice circuit , i'm going to try it right now, i'm going to look on how the tl431 can be used as a comparator so i can reduce the part count.

[Atom]

I can't understand the circuit shown for tap switching. It looks rather odd - somewhat missing the important part.
I think the planed turn over at 15 V  is a little optimistic, as one will need some reserve for ripple and the relay switching time. In addition Tap switching with a relay normally needs quite some hysteresis to prevent to frequent switching that wears out the contacts. So relativistic turn over would be more at 10 and 13 V.  Switching DC with a relay is tricky, switching AC causes quite some peak current from charging the capacitors. So it might be better to use more than 2 step switching - this is what many cheap linear supplies use.

the reserve is already there because at 8amps the ripple pkpk on the bulk cap (13000uf) is 4V so adding 1 volt should be enough right, 2V would be better so 20-6= 14V treshold.

you are right about the treshold in fact i changed it to 13.5V

i already tested the circuit with a 5 Amp load and the switching is ok and i don't see any dips in the output voltage when switching from high to low and from low to high. i'v taken the voltage directly from the output and have done some modification to the original circuit that i'v posted ...mainly because if you short the output or have a high current load that makes the output dip(cc limit) the relay will switch to the lower tap and start to oscillate (already welded one ).
with the correction it didn't happen anymore...i'll still try the mos solution because faster switching ad less losses 

modified voltage sensing in the picture. s2 charges the capacitor that can't be discharged by the load on the output providing i spike free voltage to the voltage divider that goes to the comparator.

With just 2 transformer taps there is also the alternative to use 2 transformer taps with linear turn over. It is a little more effective than switching 2 taps and no spike on turn over. As a downside there is a little more drop out (e.g. some 1 V) and one needs twice (compares to ideal switching) the number of power transistors, though only half of them will get hot at the same time. Ina addition it takes more filter caps - though with less peak current to them.

what is linear turnover? i'm not familiar with it?

i also wanted to test the psu with more exotic load , what should i try to make the preregulation oscillate ?

[Kleinstein]

To avoid fast switching up and down, one should have quite some hysteresis and delay between turn on and turn off. Other wise there is a chance the variable loads in CC mode and thus variable voltage can lead to frequent switching of the relay. Some relays don't like this as is can lead to welded contacts. The critical test would the so called file test - so an fast intermittent short like the contact on a file. Just directly a load after the linear regulator to make the pre-regulator oscillate should not be possible. It is more like a load that is already oscillating so that the oscillation get through to the pre-regulator.

So a solution switching with MOSFETs could be a good idea, as this would have less problems with frequent switching.

The circuit from not1xor1  might work, though I would prefer a diode instead of the 2.nd MOSFET. The little extra loss at the lower voltage tap should not be such a big deal.

I had just shown an example of the linear use of 2 taps: https://www.eevblog.com/forum/projects/200v-200300ma-power-supply/msg2060992/#msg2060992

[Atom]

I'm in a hurry at the moment so have not yet checked carefully your circuit (I find some symbols quite odd).
But here is a proof of concept I simulated with LTspice a while ago.
That would work even with just a TL431 as a comparator to switch the rails, but it is for negative rail regulation. You have to reverse it and use P-MOS (with usually higher Rds-on) for positive rail...

So i tried it and it worked at first wit no load then i don't know what really happend but i blew up the mosfets ..you mentioned that this circuit was for a negative and that i have to swap the mosfet to a p channel.

what is the problem with the circuit and why isn't suitable as it is ? is it for the body diodes because i really have no idea of what else could it be...please explain

[Kleinstein]

There is slight chance the Mosfet for the common pin is not turning off fast enough. So there could be transient current flow through both MOSFETs, especially of the grid voltage is not a nice smooth sine, but with sharper slopes. So it might be a good idea to use a diode instead of the MOSFET at the COM pin.

[Atom]

There is slight chance the Mosfet for the common pin is not turning off fast enough. So there could be transient current flow through both MOSFETs, especially of the grid voltage is not a nice smooth sine, but with sharper slopes. So it might be a good idea to use a diode instead of the MOSFET at the COM pin.

so the problem with the circuit is that there isn't enough dead time and all the 2 mosfet could be conducting ad the same time shorting out COM with AC2 (and maybe themselves since there is a lot of current passing in a short)?

 is my understanding correct?

[BravoV]

Not sure if this is suitable, another design idea posted while ago here on taps switcher -> HERE

[Kleinstein]

There is slight chance the Mosfet for the common pin is not turning off fast enough. So there could be transient current flow through both MOSFETs, especially of the grid voltage is not a nice smooth sine, but with sharper slopes. So it might be a good idea to use a diode instead of the MOSFET at the COM pin.

so the problem with the circuit is that there isn't enough dead time and all the 2 mosfet could be conducting ad the same time shorting out COM with AC2 (and maybe themselves since there is a lot of current passing in a short)?

 is my understanding correct?

Yes having both MOSFETs turned on at the same time would be the thing I fear could happen. Maybe not with a clear sine, but possible with a distorted grid voltage. I would not take the chances and use a diode instead of the one of the MOSFETs.

Not sure if this is suitable, another design idea posted while ago here on taps switcher -> HERE

The the link shows a linear turn over with MOSFETs more multiple taps, similar to the linear version I suggested earlier for 2 taps with a BJT based circuit. It's different in the details but similar in the principle. It is especially suitable for the regulator with floating supply, as the drive of the gates / bases is easy in this case (fixed relative to auxiliary voltage).

[Atom]

Not sure if this is suitable, another design idea posted while ago here on taps switcher -> HERE

while i find the circuit pretty intresting i think it isn't worth it mainly because i would need to buy other bulk capacitor, increasing already the size and cost of this psu. i think i'm going use the mos switching circuit.

the losses on the diode if i use one instead of the mosfet are Vd*I right so 0.7*8Amp max = 5.6W that's quite a bit of power

[Kleinstein]

Not sure if this is suitable, another design idea posted while ago here on taps switcher -> HERE

while i find the circuit pretty intresting i think it isn't worth it mainly because i would need to buy other bulk capacitor, increasing already the size and cost of this psu. i think i'm going use the mos switching circuit.

the losses on the diode if i use one instead of the mosfet are Vd*I right so 0.7*8Amp max = 5.6W that's quite a bit of power

The loss at the output transistor is still much higher, more like 10 V drop on average.  If really needed one could have a Schottky diode - though not that easy with high peak current.

[Atom]

Well sure the losses on the power transistor are much higher ..higher tap 40 v  switching at 13v max current is 8 Amps

27×8 = 216 W  well yeah thas quite  a bit of power in fact i was thinking of limiting the current at 5/6 amps  in the higher tap mode.

[not1xor1]

To avoid fast switching up and down, one should have quite some hysteresis and delay between turn on and turn off. Other wise there is a chance the variable loads in CC mode and thus variable voltage can lead to frequent switching of the relay. Some relays don't like this as is can lead to welded contacts. The critical test would the so called file test - so an fast intermittent short like the contact on a file. Just directly a load after the linear regulator to make the pre-regulator oscillate should not be possible. It is more like a load that is already oscillating so that the oscillation get through to the pre-regulator.

So a solution switching with MOSFETs could be a good idea, as this would have less problems with frequent switching.

The circuit from not1xor1  might work, though I would prefer a diode instead of the 2.nd MOSFET. The little extra loss at the lower voltage tap should not be such a big deal.

I had just shown an example of the linear use of 2 taps: https://www.eevblog.com/forum/projects/200v-200300ma-power-supply/msg2060992/#msg2060992

To avoid faults due to intermittent short-circuit (file test) one might use an additional circuit to slow down CC/CV switching without affecting the CV/CC one.

One might use a sort of buffered peak detector connected to the output of the current control opamp, with the detector output connected via a 3rd diode (i.e. besides V control and C control) to the base/gate of the power device.

When the current control opamp switches off, the peak detector keeps the output voltage close to the previous value for a bit longer making it increase slowly to the set output voltage as the hold capacitor discharges.

In simulations the spikes on short recovery were greatly reduced or completely cancelled depending on the capacitor value/discharge time.

Unfortunately the discharge time needed to avoid any spike even when the CV is set to fraction of volts, makes the recovery time last several seconds when conversely the output voltage is set in the order of several tenths of volts.

In any case, I might be wrong, but while I see the advantage of having a fast CV/CC switch, I do not see any advantage from a fast CC/CV switch.

The disadvantage is that you have to use 2 additional opamps.
Although a complementary 2 BJTs buffer + electrolitic capacitor also make a coarse but yet fairly effective circuit (at least in simulations).

[not1xor1]

I'm in a hurry at the moment so have not yet checked carefully your circuit (I find some symbols quite odd).
But here is a proof of concept I simulated with LTspice a while ago.
That would work even with just a TL431 as a comparator to switch the rails, but it is for negative rail regulation. You have to reverse it and use P-MOS (with usually higher Rds-on) for positive rail...

So i tried it and it worked at first wit no load then i don't know what really happend but i blew up the mosfets ..you mentioned that this circuit was for a negative and that i have to swap the mosfet to a p channel.

what is the problem with the circuit and why isn't suitable as it is ? is it for the body diodes because i really have no idea of what else could it be...please explain

Did you noticed that the center tap mosfet is reversed so that the body diode prevents conduction when it is off?

[not1xor1]

so the problem with the circuit is that there isn't enough dead time and all the 2 mosfet could be conducting ad the same time shorting out COM with AC2 (and maybe themselves since there is a lot of current passing in a short)?

 is my understanding correct?

I made a more complete simulation with a 1Hz 0-30V triangle wave to simulate the variation of a PSU output voltage.
I found that even with a 1ms deadtime there may be cross conduction if the switch doesn't occur on zero-crossing.

So to work properly the circuit would need a zero-cross detector, a histeresis comparator and a flip-flop to be set/reset when both AC is 0V and voltage is over/below the given threshold.

[duak]

The first ranging power supply I encountered was an hp 6002A.  It used a state machine to select the best combination of transformer secondary taps for the output voltage and current.  The design was written up in the June 1977 hp Journal: http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1977-06.pdf

Figure 3 on page 4 shows the basic transformer and rectifier circuit.  The takeaway from this is that for the low voltage range, a standard bridge rectifier provides all the current and when a higher range is needed, a higher voltage tap is switched in and the LV rectifier doesn't conduct.  In this supply, thyristors were used for tap switching but I don't see any reason that a MOSFET can't be used.  The hp supply is complicated because it has four ranges and a 200 W maximum power limit.  I have one that I repaired but I don't use it much so I can't say if its complexity is worth the capability.  I can't even say how clean its output is compared to non-ranging or SCR supplies.

Cheers & Best Wishes,

[Atom]

I made a more complete simulation with a 1Hz 0-30V triangle wave to simulate the variation of a PSU output voltage.
I found that even with a 1ms deadtime there may be cross conduction if the switch doesn't occur on zero-crossing.

So to work properly the circuit would need a zero-cross detector, a histeresis comparator and a flip-flop to be set/reset when both AC is 0V and voltage is over/below the given threshold.

So you are telling me that switching should be done at the zero crossing point...and i would need some external circuitry to get everythig to work ... that adds substantial complexity to the circuit..i'll se what i can do.

stupid idea but if i can induce a proper delay and drive the mosfet gates from a stabilized dc supply(without any ripple) those problems would go away right? or i'm still missing something

since the trasformer is a toroidal one i can add an aux winding pretty easily for driving mosfet gates. Could you share the LTspice file i'm not familiar with it but since it's what everyone uses it's time to learn something new.

[Atom]

The first ranging power supply I encountered was an hp 6002A.  It used a state machine to select the best combination of transformer secondary taps for the output voltage and current.  The design was written up in the June 1977 hp Journal: www.hpl.hp.com%2Fhpjournal%2Fpdfs%2FIssuePDFs%2F1977-06.pdf&usg=AOvVaw18_7cLX_j2ET9_i8jT4dl8

Figure 3 on page 4 shows the basic transformer and rectifier circuit.  The takeaway from this is that for the low voltage range, a standard bridge rectifier provides all the current and when a higher range is needed, a higher voltage tap is switched in and the LV rectifier just doesn't conduct.  In this supply, thyristors were used for tap switching but I don't see any reason that a MOSFET can't be used.  The hp supply is complicated because it has four ranges and a 200 W maximum power limit.  I have one that I repaired but I don't use it much so I can't say if its complexity is worth the capability.  I can't even say if its output is clean compared to a non-ranging or SCR supply.

Cheers & Best Wishes,

since the voltage drop of an scr is 1V more or less even a relay soultion is better from that standpoint. without considering the power losses of it. nonetheless an intreasting reading.

the link seems broken here's one working:http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1977-06.pdf

sadly even if i wanted to replicate the circuit i coudn't because i don't have power scr on hands.

[Kleinstein]

An extra delay could avoid having both MOSFETs on at the same time. Howver there is a second possible problem, and that is a high peak current when switching from the lower tap to the high in a moment when die voltage is high. It gets less dramatic if the switching is done at a low voltage.

It is still the question if the switching should be done as hard switching between the taps or also with fast turn off if the required voltage (+ some overhead) is reached. This would give an added continuous pre-regualtion  similar (but other part of the wave)  to the SRC regulators.

The SCR could also replace a diode, to the 1 V drop is not that bad, though a little more than a diode.

[soldar]

I have a commercially purchased 0 ~30 Volt, 5 Amp, adjustable bench power supply.  It switches the taps of the secondary with two relays and it has three possible positions: 0-10, 10-20 and 20-30 volts sensed on the output.

The sensing and switching is done with a couple OpAmps which have no hysteresis designed and it has never been a problem. It would be easy though to add a couple resistors to add some hysteresis if I ever find it necessary.

The relays switch fast enough that it is not a problem.  Only one switches at a time.

The unit has given me many years of good service now. It cost me about $30 ten years ago.

[not1xor1]

The first ranging power supply I encountered was an hp 6002A.  It used a state machine to select the best combination of transformer secondary taps for the output voltage and current.  The design was written up in the June 1977 hp Journal: http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1977-06.pdf

Figure 3 on page 4 shows the basic transformer and rectifier circuit.  The takeaway from this is that for the low voltage range, a standard bridge rectifier provides all the current and when a higher range is needed, a higher voltage tap is switched in and the LV rectifier doesn't conduct.  In this supply, thyristors were used for tap switching but I don't see any reason that a MOSFET can't be used.  The hp supply is complicated because it has four ranges and a 200 W maximum power limit.  I have one that I repaired but I don't use it much so I can't say if its complexity is worth the capability.  I can't even say how clean its output is compared to non-ranging or SCR supplies.

Cheers & Best Wishes,

thanks for the link...
I'll have a look later, but as far as I remember HP uses triacs not scr, and they are switched on zero-crossing so a simple mosfet cannot replace them.
You need back-to-back mosfets, zero-crossing detector and insulated drivers.

[not1xor1]

I made a more complete simulation with a 1Hz 0-30V triangle wave to simulate the variation of a PSU output voltage.
I found that even with a 1ms deadtime there may be cross conduction if the switch doesn't occur on zero-crossing.

So to work properly the circuit would need a zero-cross detector, a histeresis comparator and a flip-flop to be set/reset when both AC is 0V and voltage is over/below the given threshold.

So you are telling me that switching should be done at the zero crossing point...and i would need some external circuitry to get everythig to work ... that adds substantial complexity to the circuit..i'll se what i can do.

stupid idea but if i can induce a proper delay and drive the mosfet gates from a stabilized dc supply(without any ripple) those problems would go away right? or i'm still missing something

since the trasformer is a toroidal one i can add an aux winding pretty easily for driving mosfet gates. Could you share the LTspice file i'm not familiar with it but since it's what everyone uses it's time to learn something new.

I think zero-crossing switching is safer.
Later I'll see if I can find a simple solution.

[Atom]

thanks for the link...
I'll have a look later, but as far as I remember HP uses triacs not scr, and they are switched on zero-crossing so a simple mosfet cannot replace them.
You need back-to-back mosfets, zero-crossing detector and insulated drivers.

Yes my bad, hp uses triacs and not scr since they are switching the ac portion and not a dv voltage. By the way beginner question here why spikes of current are harmful for mosfets? Mosfet rated for 60A continous current are for 200A spikes so even a large current spike of 60 amps or more shouldn't damage them.

For example the irf has a continuos current of 110amps and a peak current of 390A , i know that those are values that you can't even approach  but we aren't going anywere near them.

[Kleinstein]

If the MOSFET is turn on hard with capacitors on both sides, there can be quite some peak current spike. The thread started with the problem of high current spikes that looked like up to 100 A.
The current spike may be OK for the MOSFET if it does not get much lager. However with so large a current there is noise and also possible voltage spikes.  I would be more worried about loss at the capacitors and also some of those peaks coming through to the output. The audible noise is a second problem. On fast load changes (short to open, CC to CV) there could be also quite some current spike - though usually only at a low frequency.

For the switching there are several options: Triac before the transformer, relays before the rectifier, SCRs as part of the rectifier and a MOSFET behind the rectifier. The switching can be hard between taps or in some cases also with a continuous output. The circuit from the beginning is turning on in zero crossing and doing some continuous adjustment. The circuit does not look so bad - I would expect even more current spikes from the continuous adjusting  SCR version.

[Atom]

If the MOSFET is turn on hard with capacitors on both sides, there can be quite some peak current spike. The thread started with the problem of high current spikes that looked like up to 100 A.
The current spike may be OK for the MOSFET if it does not get much lager. However with so large a current there is noise and also possible voltage spikes.  I would be more worried about loss at the capacitors and also some of those peaks coming through to the output. The audible noise is a second problem. On fast load changes (short to open, CC to CV) there could be also quite some current spike - though usually only at a low frequency.

For the switching there are several options: Triac before the transformer, relays before the rectifier, SCRs as part of the rectifier and a MOSFET behind the rectifier. The switching can be hard between taps or in some cases also with a continuous output. The circuit from the beginning is turning on in zero crossing and doing some continuous adjustment. The circuit does not look so bad - I would expect even more current spikes from the continuous adjusting  SCR version.

i know that you are a really good EE and your knowledge is far greater than mine..i wish my professors could teach the things that i learn here. yes the preregulator proposed at the start has the problem that makes the transformer buzz and has a continuos adjustment creating high current spikes, but the circuit of not1xor1 switches only once in a time when i need to switch between taps so there would be a single current spikes at the mosfet turn on, what i'm trying to say it's that if the mosfet takes a current spike once there shouldn't be a problem since this circuit doesn't switches continuosly.

[Kleinstein]

For the circuit from  not1xor1 it still depends on how the switching is controlled. It could be switching with hysteresis and thus not so often or it could still be fast control within a line period.

If going from lop tap to high tap there could be quite some current spike, as the filter cap is charged fast. This could be more than the 60-100 A seen at start. Here switching near zero would still help, as the charging is no faster than the sine wave. Still some 10 V/ms and 10mF of filter capacitor would be 100 A in theory, if there is no other limit.

The other point would be rare cases where the switches are active both at the same time, if 2 Fets are used (using a diode instead of the 2nd FET (at COM) could avoid this). These peaks could be damaging.

Another point that could help a little against high current spikes could be a fuse before the rectifier - it's resistance also limits the current a little. As rectifiers sometimes fail short, it is usually a good idea to have a fuse to protect the transformer.

[not1xor1]

thanks for the link...
I'll have a look later, but as far as I remember HP uses triacs not scr, and they are switched on zero-crossing so a simple mosfet cannot replace them.
You need back-to-back mosfets, zero-crossing detector and insulated drivers.

Yes my bad, hp uses triacs and not scr since they are switching the ac portion and not a dv voltage. By the way beginner question here why spikes of current are harmful for mosfets? Mosfet rated for 60A continous current are for 200A spikes so even a large current spike of 60 amps or more shouldn't damage them.

For example the irf has a continuos current of 110amps and a peak current of 390A , i know that those are values that you can't even approach  but we aren't going anywere near them.

The problem of the previous circuit I showed is that in some instances both mosfet are on and that should be avoided.
The current spikes through the capacitor are less relevant as there are plenty of mosfets able to withstand that.

[not1xor1]

If the MOSFET is turn on hard with capacitors on both sides, there can be quite some peak current spike. The thread started with the problem of high current spikes that looked like up to 100 A.
The current spike may be OK for the MOSFET if it does not get much lager. However with so large a current there is noise and also possible voltage spikes.  I would be more worried about loss at the capacitors and also some of those peaks coming through to the output. The audible noise is a second problem. On fast load changes (short to open, CC to CV) there could be also quite some current spike - though usually only at a low frequency.

For the switching there are several options: Triac before the transformer, relays before the rectifier, SCRs as part of the rectifier and a MOSFET behind the rectifier. The switching can be hard between taps or in some cases also with a continuous output. The circuit from the beginning is turning on in zero crossing and doing some continuous adjustment. The circuit does not look so bad - I would expect even more current spikes from the continuous adjusting  SCR version.

The single tap switch uses just one capacitor. The current spikes are the usual current spikes you get when you switch on a transformer or when you switch transformer windings via relais. Nothing different, no noise, no overshots.
The only problem with my first draft was that in some cases both mosfets were on. That is easily solved by switching on zero crossing.
In the circuit I'll discuss tomorrow I also enforced fast switch off on both mosfets, just to make that even safer.

You get instead huge voltage spikes when you switch the mosfet off as soon as the capacitor gets to the required voltage, because the mosfet and the inductance of the transformer work like a stepup switching circuit.
You can only mitigate a bit the problem by slowly switching the mosfet off, but at the expense of more power dissipation.
And yet some more power has to be dissipated by the transformer and a bleeding resistor and or a TVS.

[not1xor1]

For the circuit from  not1xor1 it still depends on how the switching is controlled. It could be switching with hysteresis and thus not so often or it could still be fast control within a line period.

hysteresis plays no useful purpose here because even if the comparator would "misfire" at 50 Hz (i.e. every other cycle) the capacitor voltage would be higher than the center tap voltage and that alone would prevent the center tap mosfet conduction.
I'll try to show that tomorrow...

[Atom]

hysteresis plays no useful purpose here because even if the comparator would "misfire" at 50 Hz (i.e. every other cycle) the capacitor voltage would be higher than the center tap voltage and that alone would prevent the center tap mosfet conduction.
I'll try to show that tomorrow...

i'll look forward to it.

[not1xor1]

hysteresis plays no useful purpose here because even if the comparator would "misfire" at 50 Hz (i.e. every other cycle) the capacitor voltage would be higher than the center tap voltage and that alone would prevent the center tap mosfet conduction.
I'll try to show that tomorrow...

i'll look forward to it.

sorry... I've been too busy yesterday... anyway here it is



The circuit uses a couple of BJTs to detect when the 100Hz (or 120Hz) rectified AC gets to 0V. Then on the collector of Q6 you get a short (compared to the AC line period) positive pulse.

Each positive pulse then gets to one input of the two NAND gates U5 and U6.
Depending on the status of the voltage comparator U2 output (and inverted U1 one) the positive pulse becomes a negative one at the output of either U5 or U6.

If the flip-flop is not yet in the required status, the pulse changes that, switching off one of the mosfets and leaving the other get to the on state.

The circuit has been simulated with LT1013, LM358, LM393 (with pull-up resistors on the output) but should work with any rail-to-rail I/O opamp and probably with most single supply one. I also added some hysteresis as suggested by Kleinstein to reduce unneeded tap switching.

It looks like the peak current on switching from the low tap to the high one gets at once about 4 times higher than the full load peaks and then gets back to normal in 3-4 cycles. The output voltage ramps up at a speed of few milliVolts per µs so shouldn't cause too much trouble for the voltage regulator stage.

Possible improvements might include usage of N-MOSFET (greater choice, low cost and low Rds-on) and switching them off via an optocoupler.

With few modifications, including back-to-back MOSFETs switches and a bit more complex logic, two transformers and bridges (or a single one with dual split winding secondaries) might be stacked in series to get 4 voltage levels.

I'm also attaching the .asc file (see below). Apart from CD4000_v.lib and logic gates symbols it uses just LTspice stock components.

The following screenshots show the logic signals:





These show the MOSFETs current and source-gate voltage:











These last two screenshots show the peak current and voltage spike when switching from low to high voltage.
The simulation has been run with a huge 47.000µF capacitor and a load of 10Amps.

[blackdog]

Hi,

If you only need one switchpoint than take a look at this schematic, it uses a Schottky diode and a PowerMOSfet.
www.bramcam.nl/Diversen/CO-2016-PSU.pdf

The whole Power Supply Page on Circuitsonline: https://www.circuitsonline.net/forum/view/130041/1#highlight=voeding+co+0

And this is a page on Circuitsonline with my customized version of this power supply: https://www.circuitsonline.net/forum/view/131554/1#highlight=voeding+co%200

I don't think it's too difficult to adjust the circuit around the transformer, the extra Schottky diode and the PowerMOSfet for 5 to 10 Amps.

But beware, the peak currents at large electrolytics can rise considerably, which is normal with large capacitances and large output currents.

Kind regards,
Bram

[Atom]

These last two screenshots show the peak current and voltage spike when switching from low to high voltage.
The simulation has been run with a huge 47.000µF capacitor and a load of 10Amps.

Really nice circuit i'll try it, i have already on hand some low rdson Pmos
I was a little worried about the peak current 200A but then i saw that the measurments were done with 47000uf @ 10 amps

[Atom]

If you only need one switchpoint than take a look at this schematic, it uses a Schottky diode and a PowerMOSfet.
www.bramcam.nl/Diversen/CO-2016-PSU.pdf

thanks i'll try also this out

[Kleinstein]

Hi,

If you only need one switchpoint than take a look at this schematic, it uses a Schottky diode and a PowerMOSfet.
www.bramcam.nl/Diversen/CO-2016-PSU.pdf
...

That version of tap switching looks nice and simple.

The nice point with the diode is that, there can be no cross conduction. The voltage loss only happens at in the lower tap case, where is does not hurt much. The relatively slow turn on of the MOSFET limits peak currents, when going from the center tap to the full voltage. There would be some heat loss at the MOSFET, but not that much (e.g. comparable to 1/4 the energy in the filter caps) and more of a transient mode.
Moving the switching from low to high tap close to the zero crossing could further reduce the current peak a little.

[not1xor1]

That version of tap switching looks nice and simple.

The nice point with the diode is that, there can be no cross conduction. The voltage loss only happens at in the lower tap case, where is does not hurt much. The relatively slow turn on of the MOSFET limits peak currents, when going from the center tap to the full voltage. There would be some heat loss at the MOSFET, but not that much (e.g. comparable to 1/4 the energy in the filter caps) and more of a transient mode.
Moving the switching from low to high tap close to the zero crossing could further reduce the current peak a little.

I agree that Blackdog's circuit is much simpler (I made something even simpler as it was for regulation on the negative rail).

When converting from AC to DC you have a fixed voltage loss (max peak-to-peak ripple voltage + max dropout of regulator) plus diode bridge and transformer loss.

In the case of center tap transformer the diode bridge and transformer losses are halved, but the other aren't so saving that fraction of volt wouldn't be so bad. 

In any case such a complicated circuit is obviously an overkill for a single tap. I designed that just for the puzzle solving fun.
But in future I would start from there to replace the triacs in the HP 3 taps switching design.

[Kevin.D]

A full wave voltage doubler could be added on to that to give two extra extended ranges. There is one disadvantage I can see
though as it's halving the output capacitance (since 2 caps in series) on the lower voltage (higher current) ranges where its needed most.
Notice on the third range how the top caps gets charged upto double the voltage of the bottom cap to get us the third range (sw1 closed sw2 open).
The available outputs currents and voltages I give on the schematic are what should be theoretically available from the
transformer and don't include rectification losses.

Happy New Year . 

[Mechatrommer]

why dont use one chip solution? like 50¢ 34063? i once try to build my own discrete pre regulator from some comparator and/or schmitt trigger and/or mcu pwm. but with all the hassle, on another circuit, i just use the one chip solution, all i have to take care of is the noises spike and ground loop. in fact i'm building another one right now. ymmv.

[Kleinstein]

A switched mode converter is in deed a real option. However not an easy one, as it takes care to keep higher frequency noise out. The slow tap switching has a chance to produce less higher frequency noise.

However the MC34063 is not a good choice for higher power - if at all it is acceptable when the internal switch is sufficient and efficiency is not important. There are more modern chips available and also older standard ones drive an external MOSFET switch. A cheap old one would be UC2842 and similar.

It is in dead a little odd to look at the low noise tap switching at high power (e.g. 8-10 A current). Especially the slow phase angle like switching versions to get a continuous regulation can also produce quite some ripple / noise problems. Even with just a normal rectifier, at 10 A load current the peak currents at the filter cap can be significant.

[Mechatrommer]

However the MC34063 is not a good choice for higher power - if at all it is acceptable when the internal switch is sufficient and efficiency is not important.

it can drive external mosfet, pnp, npn etc. there are circuits examples in the datasheet for buck, boost, invert etc with or without external switcher (power element). the nice thing is comparator, hysterisis, PWM, logic are all taken cared of in single chip, there is also Timing Capacitor (TC) pin i believe to control switching frequency (but i dont care i just use recommended value) and current limit (Ipk sense) pin as well (which i also disabled in my current circuit). the output is open collector-open emiter bjt (much like optocoupler) so the limit is your imagination.

currently i'm upgrading my earlier version long serving but now dead dual tracking psu to add few features, one of it is smps pre regulation. in the attached picture (bottom part) you can see i have to arrange 34063 on -ve rail to mirror the 34063 on +ve rail (top part), its Vcc is PSU's GND, and its GND is PSU's -ve input voltage (VCC-, -20V in my case), feedback is uA741 full swing +ve to -ve rail supply, i know what fanboys are going to say, but nevermind i made them to work for my need, and dont mind the Vrfs (voltage reference for smps, slightly higher but adjustable than  Vref for linear regulation output) its not good enough i should've add buffer to it since i use it in voltage division, i overlooked that until i populate them on pcb.

i dont know what specificity of the OP to use diy version, sorry if its not suitable, just thought its related. if say we want to limit current spike maybe we can slow down the mosfet driver to managable level, in my circuit attached, the driver is maxed to about an amp in and out of the gate, maybe i overstress the 3906/3904 pair but o well. for power element you can put anything you like (AO3400/3401 rated 5A in my case although my psu only rated to 0.5 - 1A) fwiw...

[Atom]

it seems we have new people that have joined the discussion

I agree that Blackdog's circuit is much simpler (I made something even simpler as it was for regulation on the negative rail).

When converting from AC to DC you have a fixed voltage loss (max peak-to-peak ripple voltage + max dropout of regulator) plus diode bridge and transformer loss.

In the case of center tap transformer the diode bridge and transformer losses are halved, but the other aren't so saving that fraction of volt wouldn't be so bad. 

In any case such a complicated circuit is obviously an overkill for a single tap. I designed that just for the puzzle solving fun.
But in future I would start from there to replace the triacs in the HP 3 taps switching design.

today i had time to tes blackdog circuit so i used a 12mohm nmos in a to252 package the current of the load was 6/7amps at 25V ....the mosfet died after the first switch, i think the peak current killed it, no big deal it was a chinese mosfet

2nd try .....i don't have low voltage high current mos on hand....i used a 14nk50z to 247 on heatsink .....0.380ohm of rdson 500vds 14A continuos

https://www.st.com/resource/en/datasheet/stb14nk50z.pdf

i know that's not the right mosfet for the job but i wanted to try it, it works, but there is a problem the voltage measured from drain to source is 11V pkpk so 5.5v. @ 6.6A load

power disspiantion is Rdson = 5.5/6.6 = 0.83ohm not good  power disspiation is 0.83*36= 30W (the mos was on a heatsink).

yes that's not the right mosfet but it works, i'm waiting for low voltage high current mos.

for anyone intrested the diode used was byw99w200 : https://www.st.com/resource/en/datasheet/CD00000692.pdf

Happy new year to everyone!!

[Atom]

A full wave voltage doubler could be added on to that to give two extra extended ranges. There is one disadvantage I can see
though as it's halving the output capacitance (since 2 caps in series) on the lower voltage (higher current) ranges where its needed most.
Notice on the third range how the top caps gets charged upto double the voltage of the bottom cap to get us the third range (sw1 closed sw2 open).
The available outputs currents and voltages I give on the schematic are what should be theoretically available from the
transformer and don't include rectification losses.

Happy New Year . 

really intresting i was taught that voltage doublers are used in low current application and for generating supply rails if you have an ac signal nearby withouth using overcoplex circuits...never thought to use it in a power supply, it could be used in a more hv / low current psu.

why dont use one chip solution? like 50¢ 34063? i once try to build my own discrete pre regulator from some comparator and/or schmitt trigger and/or mcu pwm. but with all the hassle, on another circuit, i just use the one chip solution, all i have to take care of is the noises spike and ground loop. in fact i'm building another one right now. ymmv.

yes the mc34063 i have some on hand..but still i'm trying to keep the noise as down as possible so a switchmode prereg is excluded...btw that doesn't mean that i'm not intrested in switching prereg...maybe in a smaller and more compact unit than the one that i'm trying right now.
i've now ordered boards from jlcpcb the 100*100 mm pcb incudes a tpa3116 class d amp (but that's irrelevant, props to the author https://github.com/kurozz/kamp_tpa3116d2)
 
the other 2 boards are one with the LM2679 simple switcher, main candidate to a smaller psu prereg because it's really simple and can deliver quite a bit of power
the second one is based around the LM5116 Synchronous Buck Controller, the board were generated by webench designer and tweaked to make them smaller and better by me.
planning to play around with them and see what i can do, another option would be the xlsemi 4005 or similar but the 32v max input is a bottle neck

LM2679 : http://www.ti.com/lit/ds/symlink/lm2679.pdf
LM5116 : http://www.ti.com/lit/ds/symlink/lm5116.pdf
xl4005   : http://www.xlsemi.com/datasheet/xl4005%20datasheet.pdf

then i discovered that some seller on aliexpress is selling those dcdc industrial modules (3.4mm thick pcb) made from DELTA and MURATA, i'm going to take some of those since they are extremely cheap (4E). maybe the feedback of those one can me modified so we can control the voltage of the output.

MURATA  https://it.aliexpress.com/item/Lungo-staffa-con-la-dissipazione-di-calore-piatto-Murata-RBQ-isolamento-ad-alta-potenza-DC-DC/32957694449.html?spm=a2g0s.13010208.99999999.262.42653c00ZZeXhp

DELTA     https://it.aliexpress.com/item/Con-dissipazione-di-calore-Delta-Isolato-500-w-ad-alta-potenza-DC-DC-modulo-di-conversione/32955585915.html?spm=a2g0s.13010208.99999999.259.42653c00ZZeXhp

[prasimix]

Hi Atom, seems that you are on the similar journey as myself, searching for power pre-regulator solution. I've started with "blackdog" type of stuff, and gave up (possibly too early, since at least @tombi from this forum reported good results independently from @blackdog). I spent some time after that with LM5118, LM5088, and finally end up with LTC3864 for EEZ H24005 power supply. I didn't stop on DC-DC converter as the final solution since it still require a) huge mains transformer or b) an AC-DC coverter to get "DCbus" voltage for pre-regulator (48 Vdc in my case). Therefore I finally decide to make an AC-DC converter that can works with fixed or widely adjustable output voltage as pre-regulation. Progress is reported here.
I'd like to mention once again what was said here, if you are in the phase to try DC-DC converter as pre-regulator first.

Finally not completely on-topic (sorry for that): It's interesting that people when someone mention bench power supply start to talk about low noise and ripple. That could be indeed beneficial, but question is for what kind of applications? Does everyone is working with sensitive RF circuits, have ultra-precise voltage reference, or whatever? I'm still didn't find a good article or post about noise levels/categories with explanation when low PARD become of paramount importance and present a possible show stopper or not.

Keep up with good work, and all the best in New (Western counting) Year to you, and all members of the forum.

[Atom]

Hi Atom, seems that you are on the similar journey as myself, searching for power pre-regulator solution. I've started with "blackdog" type of stuff, and gave up (possibly too early, since at least @tombi from this forum reported good results independently from @blackdog). I spent some time after that with LM5118, LM5088, and finally end up with LTC3864 for EEZ H24005 power supply. I didn't stop on DC-DC converter as the final solution since it still require a) huge mains transformer or b) an AC-DC coverter to get "DCbus" voltage for pre-regulator (48 Vdc in my case). Therefore I finally decide to make an AC-DC converter that can works with fixed or widely adjustable output voltage as pre-regulation. Progress is reported here.
I'd like to mention once again what was said here, if you are in the phase to try DC-DC converter as pre-regulator first.

Finally not completely on-topic (sorry for that): It's interesting that people when someone mention bench power supply start to talk about low noise and ripple. That could be indeed beneficial, but question is for what kind of applications? Does everyone is working with sensitive RF circuits, have ultra-precise voltage reference, or whatever? I'm still didn't find a good article or post about noise levels/categories with explanation when low PARD become of paramount importance and present a possible show stopper or not.

Keep up with good work, and all the best in New (Western counting) Year to you, and all members of the forum.

the project of the power supply that you linked (the one on the diy audio forum is really intresting), and i was already following the development of your smps preregulator. that said i think a full smps prereg is far out of my capabilities (for now )

and for the noise thing personally (form a student stanpoint) i want to exclude any kind of noise(of the psu) from the pcb under test so i already know that the supply isn't the problem, so if you have to troubleshoot something you have already the certainty that the psu isn't the problem, and then when you're moving into more complicated stuff you have a nice low noise psu to help you.

i know that's not the right mosfet for the job but i wanted to try it, it works, but there is a problem the voltage measured from drain to source is 11V pkpk so 5.5v. @ 6.6A load

power disspiantion is Rdson = 5.5/6.6 = 0.83ohm not good  power disspiation is 0.83*36= 30W (the mos was on a heatsink).

yes that's not the right mosfet but it works, i'm waiting for low voltage high current mos.

mesuring the output of the psu there is a 100mvpkpk 1mhz wave (ac copuling).. i think is the mosfet oscillating ? probably because it isn't the right type of mos for this application, that would explain the large power dissipation and the fact that i wasn't able to go to the full 30vdc on the output, anyway i have to wait or the better mos to arrive.

[Mechatrommer]

the project of the power supply that you linked (the one on the diy audio forum is really intresting)

maybe not really hitting it but audio amps that i usually fixed last time are using TL494 running full banana (50% PWM 30+ KHz) into 1:1 & 1:-1 toroidal transformer to create dual rail. fwiw...

that said i think a full smps prereg is far out of my capabilities (for now )

if you want to learn offline smps, there are good people explaining it out there https://www.eevblog.com/forum/projects/offline-smps-psu-explained/msg1459572/#msg1459572 i myself dont have time for such project since no urgent need and yes its much more involved (and dangerous) project.

i know that's not the right mosfet for the job but i wanted to try it, it works, but there is a problem the voltage measured from drain to source is 11V pkpk so 5.5v. @ 6.6A load

power disspiantion is Rdson = 5.5/6.6 = 0.83ohm not good  power disspiation is 0.83*36= 30W (the mos was on a heatsink).

yes that's not the right mosfet but it works, i'm waiting for low voltage high current mos.

mesuring the output of the psu there is a 100mvpkpk 1mhz wave (ac copuling).. i think is the mosfet oscillating ? probably because it isn't the right type of mos for this application, that would explain the large power dissipation and the fact that i wasn't able to go to the full 30vdc on the output, anyway i have to wait or the better mos to arrive.

from the way you explain it i think your mosfet is running in saturation (current limit) mode (linear region in bjt term) its not fully off and not fully on, not enough hysterisis? the datasheet you linked is a beefy mosfet capable of 14A (48A pulsed). the fact that it only run at 6A and burn is that, running at current limit mode, its not about its a not good china knock off mosfet.

[not1xor1]

Hi Atom, seems that you are on the similar journey as myself, searching for power pre-regulator solution. I've started with "blackdog" type of stuff, and gave up (possibly too early, since at least @tombi from this forum reported good results independently from @blackdog).

So far I've not yet seen any data regarding power efficiency at various output voltages and currents.

In that kind of pre-regulator the MOSFET and the transformer secondary behave like a powerful step-up circuit.

And the problem is not just the huge voltage spikes, but all the energy stored in the transformer secondary which has to be wasted in some way.

Blackdog reduced a bit both the spikes and the energy by reducing the speed of the switch-off at the expenses of more dissipation in the MOSFET(s) switch.

But the doubt I got from multiple simulations of lot of variations of that kind of pre-regulator is that the efficiency might be beaten even by a simpe center tapped transformer.
I can't be sure of that as the transformer model I use is just too rough.

Besides that the screenshots posted in the blog you linked tell us nothing about the spikes.
It would be much more interesting to see in finer details what happens when the MOSFET goes off and that would require different time-base settings on the scope.

[tombi]

Hello,

Sorry - I've been away - I didn't see great spikes at switch-off of the pre-regulator and I also didn't have issues with the transformers getting hot or humming.

Could be the transformers I used were over-kill. I used  160VA ones from RS but never draw more than 4.8A from the output of the supply (at up to 15V 3A at 30V)
https://au.rs-online.com/web/p/toroidal-transformers/6719012/?relevancy-data=636F3D3126696E3D4931384E525353746F636B4E756D626572266C753D656E266D6D3D6D61746368616C6C26706D3D5E2828282872737C5253295B205D3F293F285C647B337D5B5C2D5C735D3F5C647B332C347D5B705061415D3F29297C283235285C647B387D7C5C647B317D5C2D5C647B377D2929292426706F3D3126736E3D592673723D2673743D52535F53544F434B5F4E554D4245522677633D4E4F4E45267573743D36373139303132267374613D3637313930313226&searchHistory=%7B%22enabled%22%3Atrue%7D

I slightly modified the classic circuit and used a comparator. I added a little hysterisis and I made the circuit slow to reduce the pre-regulator voltage but quick to ramp it up. This stops the PSU getting caught out if it briefly goes into current limit.

The biggest issue I had was the bounce in the ground line when the pre-regulator MOSFET turns on. I used a differential amplifier to measure the output voltage fed back to the analog control loop though so this minimized the impact.

I don't have a current probe but if you want me to capture any voltage waveforms I'm happy to crack the unit open and poke a probe into it.

Tom

[not1xor1]

Hello,

Sorry - I've been away - I didn't see great spikes at switch-off of the pre-regulator and I also didn't have issues with the transformers getting hot or humming.

Could be the transformers I used were over-kill. I used  160VA ones from RS but never draw more than 4.8A from the output of the supply (at up to 15V 3A at 30V)
https://au.rs-online.com/web/p/toroidal-transformers/6719012/?relevancy-data=636F3D3126696E3D4931384E525353746F636B4E756D626572266C753D656E266D6D3D6D61746368616C6C26706D3D5E2828282872737C5253295B205D3F293F285C647B337D5B5C2D5C735D3F5C647B332C347D5B705061415D3F29297C283235285C647B387D7C5C647B317D5C2D5C647B377D2929292426706F3D3126736E3D592673723D2673743D52535F53544F434B5F4E554D4245522677633D4E4F4E45267573743D36373139303132267374613D3637313930313226&searchHistory=%7B%22enabled%22%3Atrue%7D

I slightly modified the classic circuit and used a comparator. I added a little hysterisis and I made the circuit slow to reduce the pre-regulator voltage but quick to ramp it up. This stops the PSU getting caught out if it briefly goes into current limit.

The biggest issue I had was the bounce in the ground line when the pre-regulator MOSFET turns on. I used a differential amplifier to measure the output voltage fed back to the analog control loop though so this minimized the impact.

I don't have a current probe but if you want me to capture any voltage waveforms I'm happy to crack the unit open and poke a probe into it.

Tom

BTW I disagree with you about the need for fast recovery from current limit. A slow recovery is not a problem and might even be useful to avoid overshots.

Apart that, I tested similar circuits 5-6 years ago and am sure that unless you slow down the MOSFET (with further power dissipation) and add a capacitor (a low ESR one) before the MOSFET (and/or use a relatively large capacitor + resistor across the MOSFET) you get huge voltage spikes (and noise) from the transformer.
It is a classical step-up circuit. You suddenly switch off the current through an inductance (in this case the transformer secondary) and the voltage increases.

But what I did not measure then (and can't do now because my lab needs masonry works) is power efficiency.
I ran lots of simulations on variations of that kind of preregulator (i.e. where the capacitor charge is interrupted as soon as the voltage gets to the desired value) and all of them showed a quite poor efficiency.

I mean I suspect that even a humble center tapped transformer PSU in the worst case (lowest voltage before switching to low voltage tap and maximum load) would provide similar if not better efficiency without all the hassle.
And that may be one of the reasons why no commercial PSU preregulator works this way, but rather starts to charge the capacitor after the AC voltage peak.

So could you please provide any information about that? 
thanks

[tombi]

Hmm I can try. Again a current clamp/probe would be handy. Let me think about it a bit. Maybe a resistor in series that i could measure with my diff probe.

I wasn't aiming for efficiency - it's a linear power supply after all. All I wanted was to reduce the amount of heat-sink I would need on the pass element.

Just for my understanding - and please note I am not an EE but this is a hobby for me - if the transformer voltage spiked after the MOSFET cuts off, why does this matter? I would think resistance of the MOSFET is pretty high in the off state so this will not go anywhere.

Tom

[not1xor1]

Hmm I can try. Again a current clamp/probe would be handy. Let me think about it a bit. Maybe a resistor in series that i could measure with my diff probe.

I wasn't aiming for efficiency - it's a linear power supply after all. All I wanted was to reduce the amount of heat-sink I would need on the pass element.

Just for my understanding - and please note I am not an EE but this is a hobby for me - if the transformer voltage spiked after the MOSFET cuts off, why does this matter? I would think resistance of the MOSFET is pretty high in the off state so this will not go anywhere.

Tom

No doubt that even this kind of pre-regulator saves some energy, but IMHO that is probably much less than one might expect as the transformer and rectifier bridge have to work harder.

In any case MOSFETs are affected by breakdown voltage, and as far as I can remember from the tests I made a few years ago, if you do not take any counter-measure to dump the spiikes, they would get as high as that breakdown value. Then the MOSFET acts like a sort of power zener dumping them.
BTW I did not dare to check how long they can stand that. 

Besides that, energy cannot just disappear like magic so, given that energy of an inductance is 1/2*L*I² where I is usually tenths of Amps (on charging capacitors), those voltage spikes at the end will make something ... hotter  .

BTW to have an idea of the efficiency you would need a power analyzer or at least one of those energy meters (although most of the cheap ones aren't probably able to deal with odd current waveforms). You cannot just multiply RMS voltage * RMS current, you must integrate the instantaneous product of V * I...

[Kleinstein]

Even the cheap power meters are usually not that bad. Their limitation is more in limited BW and resolution at low currents.

For a lab supply it's usually less about the efficiency but more about keeping the heating low - it is kind of related, as energy is conserved. Still loss distributed over more parts can be less of a problem.

[not1xor1]

Even the cheap power meters are usually not that bad. Their limitation is more in limited BW and resolution at low currents.

For a lab supply it's usually less about the efficiency but more about keeping the heating low - it is kind of related, as energy is conserved. Still loss distributed over more parts can be less of a problem.

Yes, but the question is (3rd time I write that  ) : is it really any better than a center tapped PSU?

To have at least a quite rough idea one would need:
- at least one of those cheap energy meter
- a pre-regulated PSU
- a constant current load
- beeing disposed to unmount (desolder/unconnect the wires) the pre-regulator + post-regulator for the test sake 

Since an ordinary linear regulator is just a variable electronic load from the perspective of the transformer+bridge+capacitor circuit, to get a good approximation of the power wasted at various output voltages and maximum load one needs just one measure with an electronic load set to the proper value.
The power wasted at a given voltage by a linear regulator would then be just : total_measured_power - (given_output_voltage * I_load).

Once known that, one could just measure the power wasted by the pre-regulated PSU at various output voltages and maximum load current.

Of course for power measurement I mean measurement taken at the 230VAC supply.

[tombi]

I actually did both tap switching (parallel vs series) and a pre-regulator.

I have a cheap lab PSU that I bought from a local electronics shop (Jaycar) and if you set it to say 25V then when it goes into current limit (and the output voltage drops) it will switch transformer taps.

This is fine unless the load is transient in which case it makes a horrible tap switching racket as it flicks between the taps.

I didn't want that. So if you set the target voltage on my homebrew PSU to 25V it keeps the transformer tap configuration the same if it goes into current limit. Instead the pre-regulator reduces the capacitor voltage to prevent the pass element going nuclear.

Apart from this the pre-regulator does help a bit but not a lot. You need a few volts of headroom above the output depending on the size of your capacitors/current so it never goes that low even if you set the output to say 1.8V.

Not sure if this helps but I also reduce the overhead if the output current is low. That is the pre-regulator voltage is a function of both the output voltage AND current. If the current increase then the pre-regulator increase the capacitor voltage.

Tom

[Atom]

Hi seems there is an intresting discussion here  , by the way i manged to find some power mos 80V 75A 15mohm rdson to220 package.

the circuit from blackdog the tap switcher works flawlessly and i would reccomend it to anyone that needs a similar solution, the to220 package didn't even get hot, it was extremly cool to the touch.

Thanks to every one that helped!!

do you have any suggestions when it comes to books? on my to buy list there are:

The art of electronics 3rd ed
jim williams books
Power Supply Cookbook by Marty Brown (i've read online that it's good and i have to get into switchmode as fast as possible (5th year thesis).

In the past i've tryied to get into smps but was limited by the lack of various other things, now i feel ready in the end the basics are easy to understand, the calcs could be more challenging but i have professors that could help me.

 

[not1xor1]

I actually did both tap switching (parallel vs series) and a pre-regulator.

Then it is not bad.

Actually I've been really dumb. 
Obviously, in any case Blackdog's style pre-regulator would be more efficient than a center tapped one from about half max Vout up to 90% where the efficiency should be about the same.

It is when the output voltage is below half of maximum that the pre-regulator offers little if any advantage (vs. center tapped one) according to my simulations.

Anyway, since you used it both with the top and the mid tap of the transformer then that is not your case.

So in case somebody needs to use a single winding transformer, e.g. a salvaged one, a Blackdog's style pre-regulator might still be useful although much less efficient than a buck pre-regulator.

I think anyway there is still much room for improvement, especially regarding efficiency at low output voltages.