SMPS feedback

[v-bro]

I'm looking for a good way to regulate a power supply. It's a car audio PWM step-up power supply with SG3525 and under load the voltage drops quite a bit.

How is this best done?

Reference:
Switchmode Power Supply For Car Audio

[mastodon]

By how much does the voltage drop at the output, and how much power are you sinking from it at that moment?

Have you been able to verify if the controller adjusts the PWM duty cycle when under load? If not, there might be a problem with the feedback signal to the error comparator. Can you successfully adjust the output voltage by adjusting VR1 (assuming you implemented the schematic you link to)?

[v-bro]

I have a prototype now and am now at the point where everything works unregulated (open loop) and I intend to adjust the final voltage to +/-55Vdc.

What I have now mostly resembles the bottom (simplified) schematic on the page there, which doesn't feature the voltage adjustment.

The voltage drops more than 10V at the output while the input voltage at my lab supply stays stable.

So I understand this is the best way to do it, only adding feedback on the positive rail and that's it? Maybe I will add another comparator/inverter to monitor the negative rail as well.... Or what?

How about TL494 and UC3825? The latter does seem much more capable, seems like I can build a serious monster around that one. Or should I just add a pair of fet drivers to the 3525 to soup it up a bit?

[luka]

2 way would be, first is op amp and make PID, second is similar, but instead of op amp, you use tl431 with components for compensation (pid again).
You can add feedback were you want to, were you can,..
it can be +, it can be - or you could go crazy and even do for both (which ever wants to be higher will be regulated)

I would say, you monitor + (which is almost the same as -).

What IC you use, pretty much doesn't matter, you can regulate them all, since this is "outside" circuit, that you add. You don't have fet drivers? well I wouldn't go without them, for either of those IC's


Do you have any pictures to show, schematic? since it is unregulated, that still doesn't mean it will sag a lot... with my, it stays pretty much at the same voltage

[v-bro]

This is pretty much what I've built, only adjusted gate drive resistances to the fets I am using, and adjusted the zobels, dead time and frequency to the transformer/fets I am using:

Waveforms look superb and it does deliver about 200-300 watt on a dummyload without dropping more than a few volts. Only it whould be able to do twice as much with the core I am using, now it drops about 10V.... I would like to see that improved...

Do you have any schematic of that PID implementation?

The 3525 is well capable of driving fets directly, so why use a fet driver? Then the 3825 can do about ten times as high output load... so that should make a difference in the end....

[luka]

Well you can do it like that, but SG is not the best fet driver itself, only to a point (I'm not saying you should change anything)(for fet driver, I mean simple BC and BD transistors, really cheap... but on 300w+ on dummy load supply I didn't have them)
remove that 15v diode, it doesn't do you any good really, I would also look into not using input inductor

Also I forgot, you could just do zener feedback, circuit is the same up to opto, if you use PID or zener, I find that zener is very simple was stabile, while PID was rock solid, almost no change in output

check -=[ FirstClass :: Home Page ]=-
at the end, in schematic, look for top left, where TL is, look also at SG, how pins are connected, coz its not the same as your has.
All this, that is marked around TL, that says board for feedback, this can be your zener diode

[v-bro]

I already removed that 15V zener, got mi own design for that.... 75A diode after a big 40A polyswitch array, 4x 10A parallel so if the polarity is swapped or the circuit is overloaded the polyswitches will break contact....

Thanks for the link and info Luka!

[luka]

try and report back if it works as it should or if you have more questions