Author Topic: Preregulation of a linear bench PSU  (Read 11238 times)

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Offline AtomTopic starter

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Preregulation of a linear bench PSU
« on: December 24, 2018, 01:04:05 pm »
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
 

 

Offline AtomTopic starter

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Re: Preregulation of a linear bench PSU
« Reply #1 on: December 24, 2018, 07:09:42 pm »
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
« Last Edit: December 24, 2018, 07:29:31 pm by Atom »
 

Offline not1xor1

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Re: Preregulation of a linear bench PSU
« Reply #2 on: December 25, 2018, 08:31:57 am »
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...
 

Online Kleinstein

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Re: Preregulation of a linear bench PSU
« Reply #3 on: December 25, 2018, 10:30:09 am »
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.
 

Offline AtomTopic starter

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Re: Preregulation of a linear bench PSU
« Reply #4 on: December 25, 2018, 01:22:45 pm »
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.
 

Offline AtomTopic starter

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Re: Preregulation of a linear bench PSU
« Reply #5 on: December 25, 2018, 01:45:00 pm »

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 :-DD).
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 ?
 

Online Kleinstein

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Re: Preregulation of a linear bench PSU
« Reply #6 on: December 25, 2018, 06:33:04 pm »
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
 

Offline AtomTopic starter

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Re: Preregulation of a linear bench PSU
« Reply #7 on: December 25, 2018, 06:33:58 pm »
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 |O
« Last Edit: December 25, 2018, 06:36:51 pm by Atom »
 

Online Kleinstein

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Re: Preregulation of a linear bench PSU
« Reply #8 on: December 25, 2018, 06:44:40 pm »
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.
 

Offline AtomTopic starter

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Re: Preregulation of a linear bench PSU
« Reply #9 on: December 25, 2018, 07:15:59 pm »
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?
 

Offline BravoV

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Re: Preregulation of a linear bench PSU
« Reply #10 on: December 25, 2018, 07:16:22 pm »
Not sure if this is suitable, another design idea posted while ago here on taps switcher -> HERE

Online Kleinstein

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Re: Preregulation of a linear bench PSU
« Reply #11 on: December 25, 2018, 08:55:28 pm »
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).
 

Offline AtomTopic starter

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Re: Preregulation of a linear bench PSU
« Reply #12 on: December 25, 2018, 10:35:04 pm »
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
 

Online Kleinstein

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Re: Preregulation of a linear bench PSU
« Reply #13 on: December 25, 2018, 10:43:18 pm »
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.
 

Offline AtomTopic starter

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Re: Preregulation of a linear bench PSU
« Reply #14 on: December 26, 2018, 09:57:53 am »
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.
 

Offline not1xor1

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Re: Preregulation of a linear bench PSU
« Reply #15 on: December 26, 2018, 02:17:25 pm »
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).
 

Offline not1xor1

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Re: Preregulation of a linear bench PSU
« Reply #16 on: December 26, 2018, 02:23:28 pm »
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 |O

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

Offline not1xor1

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Re: Preregulation of a linear bench PSU
« Reply #17 on: December 26, 2018, 04:18:05 pm »

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.
 

Offline duak

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Re: Preregulation of a linear bench PSU
« Reply #18 on: December 26, 2018, 07:56:29 pm »
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,
« Last Edit: December 26, 2018, 11:43:41 pm by duak »
 

Offline AtomTopic starter

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Re: Preregulation of a linear bench PSU
« Reply #19 on: December 26, 2018, 08:36:05 pm »
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.
« Last Edit: December 26, 2018, 08:46:27 pm by Atom »
 

Offline AtomTopic starter

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Re: Preregulation of a linear bench PSU
« Reply #20 on: December 26, 2018, 08:41:32 pm »
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.
 

Online Kleinstein

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Re: Preregulation of a linear bench PSU
« Reply #21 on: December 26, 2018, 08:48:19 pm »
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.
 

Offline soldar

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Re: Preregulation of a linear bench PSU
« Reply #22 on: December 26, 2018, 09:44:19 pm »
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.
All my posts are made with 100% recycled electrons and bare traces of grey matter.
 

Offline not1xor1

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Re: Preregulation of a linear bench PSU
« Reply #23 on: December 27, 2018, 08:25:01 am »
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.
 

Offline not1xor1

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Re: Preregulation of a linear bench PSU
« Reply #24 on: December 27, 2018, 08:30:34 am »
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.
 


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