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Half-Bridge 0-30V, 0-20A Feedback loop problem
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xavier60:

--- Quote from: paladyn on July 31, 2019, 07:07:57 am ---Does it run smoothly by controlling PWM by applying filtered DC to pin 2 of the SG3525's error amp?
pin 1 and pin 2 of the SG3525 system are not used and not connected. Pin 9 is connected to an external operational amplifier that realizes the loops.


I am using an external operational amplifier, but yesterday I did a test where I connected an adjustable 500kohm pin 5 resistor with pin 9 of the SG3525 chip

I must admit that I did not notice the difference in the operation of the inverter. At a small value of the resistor some soft anomalies already appeared in the noise at the inverter output and I stopped reducing the resistances.

Overall, I noticed that the problem occurs when SG3525 can no longer reduce PWM because it has almost reached zero. Perhaps the use of a huge choke would help?

Summing up the problem of squeezing the transformer occurs mainly at low resistance loads and low output voltages.

--- End quote ---
That's right,the pulse width can't go below 600ns so the SG3525 has to skip pulses and causing some transformer noise. I can't hear it with the covers fitted.
My choke is about 80µH at high load current.
Driving the MOSFETs too hard can cause extra interference.
mrjoda:
hello guys.

At the beginning : paladyn, very nice project, i will definitely watch it.

I am not able to help you with compensation of feedback, but ... we are working with smps and converters in work and we are using network analyzer for feedback loop. You can simply check compensator pole/zero locations according to nyquist criteria and tune the feedback loop to sweet spot. Without it, it is only shooting to dark, because you don't know the amount of phase margin. Is there any chance for you to borrow one or visit some lab with correct instruments and tune it correctly ? It will save you a LOT of time.


At first, i thought the smps are easy to design... now,when i know some background, i don't have balls for my own design :)
paladyn:
mrjoda
Thank you for the words of appreciation. Indeed, the design of impulse power supplies is a difficult matter. At first it looks like it's quite a simple thing, but the amount of detail determines how the power supply then works. As you mentioned, testing the loop without network analyzer is like working in the dark. I will try to look for a person or institution that would have such an analyzer. It is a pity that they are so expensive, because I would buy one.



There is certainly a lot to improve in the current project, but this is the purpose of the prototype. The inductor has too low inductance, current radiators are only suitable for testing also PCBs must be improved to reduce parasitic inductance. I also noticed that the input capacity is too small. With a power output of 750W, the capacitor voltage is only 280V instead of 320V. Of course, such a decrease can be taken into account in the amount of winding of the transformer itself, but the current one works at the limit of the possibilities.
mrjoda:
its comes to my mind... what about ADALM2000 ? it costs around 120€ and one of its features is network analyzer https://www.analog.com/en/design-center/evaluation-hardware-and-software/evaluation-boards-kits/adalm2000.html


i want to buy it soon. Of course it will be like half blind but maybe, just maybe it could help.
paladyn:
Honestly? Somehow I am not convinced that you can do something with this equipment. If you buy, please report, because maybe I am wrong.
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