Hi,
Great job! I've read all the posts, and it's an amazing project.
I have a question related to mosfet pre-regulator, because I was planning to do a psu use the pre-regulator circuit.
from the site, http://www.envox.hr/eez/bench-power-supply/psu-pre-regulator.html, it's said
Status Completed, not recommended for toroidal main transformer.
in your post before, there is some big inrush current if a toroidal main transformer is used.
and also there is a soft-start circuit in main 220AC, here: http://www.envox.hr/eez/bench-power-supply/psu-auxiliary-supply.html.
I was wandering if this big inrush current is the only problem?
is there any other drawback for the mosfet pre-regulator used with toroidal main transformer?
Thanks!
I tried to add a soft-start to the pre-regulator, by monitoring the mosfet current, using the mosfet Rds-on as the sense resistor.
when the current is big enough, it turn on the SCR, and like the other part, turn on until next cycle.
attachment is the LTspice and some screen shot.
no real test in breadboard or PCB, maybe it can solve the inrush problem?
Thanks funshine for your comments. Notice/disclaimer about pre-regulator is valid, and I didn't have a chance yet to elaborate that on this forum but now it's a time. The issue with such kind of pre-regulator is huge current peaks that resulting in excessive heating of main transformer. That is much more obvious in case of toroidal transformer for two reasons: it's primary and secondary windings are more hardly coupled, and core is inside (covered with windings) what make its cooling more difficult. I realized that in latest stage when everything was completed. As you probably know that is not my design, nor I tried to make any chances to eventually improve it. There was no warning that one should at least avoid toroidal type of transformer or at least I didn't remember that blackdog who presented this circuit
here mentioned anything like that. Unfortunately, I missed opportunity to ask a right question. And yes, as it's usually a case right questions usually moves us in right direction but asking right question require right level of experience and knowledge
.
BTW, blackdog presented this pre-regulator on the thread with right title started by Kalvin: Very Low Noise Preregulator for Benchtop Power Supply. I didn't ask if he eventually find something or not. There is another attempt started by tombi (Tom) based on the same "blackdog"'s pre-regulator (put into quotation since blackdog said that he is not author). Tom's progress could be follow
here, and I warned him about possible issue with main transformer (over)heating. On another (not English language) forum was discussion about pre-regulator and I can say some real experts told me such behavior is expected and that Agilent uses special designed transformers when is using SCR or mosfet phase controlled pre-regulation. Interesting thing is also that people on that forum find few errors in schematic of Agilent model E3640 where pre-regulator with
SG3525 PWM controller! We don't know if that errors are intentional or not but PWM controller cannot work with presented values. Some of the member from the forum succeed to finally run it and answer to my question how hot is transformer was unfortunately the same: its start to be hot pretty soon with higher currents.
To summarize I'd like to say what is possible with current solution: on the transformer I can measure 48oC when current is 2 A. But, with the 5 A SMPS pre-regulator (shown below) temperature rises to 46oC on the same transformer with 4 A! Please take a note that no forced cooling is deployed, and I didn't tested such combination since I'd like to stick with initial goal to not use (noisy) fans what is btw a standard.
For higher currents sourced on both channel transformer temperature could easily rise over 70oC. Not nice at all, so NTC is added to transformer to avoid possibly catastrophic results. So, in general this solution WITH toroidal transformer and WITHOUT any forced cooling is pretty usable for output currents of up to 2 A.
Regarding added inrush current limiter and soft-switch: it was not added to resolve issue with continuous work of pre-regulator but only address initial in-rush current and serves as convenient MCU driven main switch.
It's interesting that so many attention and energy is spent of power supplies without pre-regulation (actually that is post-regulator or final stage in our design) and many people is possibly not aware of the problem when someone presented e.g. linear 0-30 V / 2 A without disclaimer (or awareness) or that it cannot deliver 2 A on 3.3 or 5 V
continuously since that requires ~50 W of dissipation on heatsink which is too small or that is sometimes outside pass-element's SOA. From other side many people believes that SMPS (e.g. buck) pre-regulation could solve that problem but didn't think or want to talk about huge output noise that easily bypass linear post-regulator and is still present on the output. Again I'm talking about currents of 2 or more amps. The latest and (first?) resort is main transformer with multiple secondary windings that are switched on sequentially in old-fashioned way with power relays or in some more advanced designs with SCRs or mosfets. If multiple windings transformer is acceptable solution then I'd rather like to recommend Liv's
design that is superior to mentioned relays, SCRs or mosfets but is not so simple.
The story about pre-regulator and complete power supply is not finished yet. It's a work in progress but now I can say for the first time that I have a real platform that can be used on daily basis without fear that it will kill the connected load/device due to some error in hardware and/or
firmware. A real achievement since our primary "expertise" is software not hardware and we learned a lot about different aspect of building and hopefully all that is presented could help other people to make even better solution that is
O P E N and suitable for DIY.
Stay tuned ... and I'd like to thanks in advance for all your further inputs.