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Preregulation of a linear bench PSU
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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:

--- Quote from: prasimix on January 02, 2019, 09:16:26 am ---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.

--- End quote ---

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 :box:)

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.



--- Quote from: Atom on January 01, 2019, 05:54:19 pm ---
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  :-DD 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.


--- End quote ---

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:

--- Quote from: Atom on January 02, 2019, 05:36:38 pm ---the project of the power supply that you linked (the one on the diy audio forum is really intresting)

--- End quote ---
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...


--- Quote from: Atom on January 02, 2019, 05:36:38 pm ---that said i think a full smps prereg is far out of my capabilities (for now :box:)

--- End quote ---
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.


--- Quote from: Atom on January 02, 2019, 05:36:38 pm ---
--- Quote from: Atom on January 01, 2019, 05:54:19 pm ---
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  :-DD 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.


--- End quote ---

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.


--- End quote ---
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:

--- Quote from: prasimix on January 02, 2019, 09:16:26 am ---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).

--- End quote ---

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
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