SMPS with two control loops


I got playing around with some old ATX power supplies lately. I'm trying to make'm adjustable, and so far have managed to build the external control loop circuitry. First there is a inner loop that sets the TL494 PWM from the output inductor current. The set point to this inner loop sets it up as a constant current supply. Secondly, I have another control loop that programs the current set point from the output voltage. i.e your classic current mode control.

I would like to get some idea of the frequency response of the loop(s), but found I need a frequency response analyzer. I tried injecting a signal into the control loop as explained in: http://www.evaluationengineering.com/archive/articles/1201power2.gif
http://www.evaluationengineering.com/archive/articles/1201power.htm

But the switching noise makes it impossible to compare two tiny sine waves on my CRO. I can get some estimation of the crossover frequency by looking when the fuzzy sine waves are approximately equal in amplitude, meaning the frequency where the open loop gain magnitude is unity. BTW, I think this transformer injection trick is just brilliant.

Prices of such frequency response analyzers (FRA) are >= $10k , so obviously not every SMPS engineer had the luxury of utilizing an FRA to help build his/her design. I'm only a hobbyist.

The current loop is stable, 0 to 25A at short circuit to open circuit, and I measured crossover point at 10kHz with about 50~80 degree margin (using transformer injection method), giving 1/10th the switching frequency as recommended by the experts. The current error amplifier has a flat gain after a zero located at 1kHz. This gives good current tracking at lower frequencies. All is well...

The voltage loop is causing problems, I cannot get it stable at light/zero loads. Am I in a situation where I'm best to plug in various caps/resistor values around the voltage error amplifier until the thing just works ?

I've given up in vain trying to mathematically predict gain and phase margins of the two control loops using Scilab/Octave software. I'm guessing this is how all the real power supply designers do it. Ok, I know I'm asking a lot out of a SMPS capable of 0% to 100% load change, adjustable constant current and constant voltage. Would this be considered as a challenging task ? I'd like to know if I'm wasting my time.

Regards,

Adam Seychell
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