I'm designing a processor-controlled buck converter. I'm seeing some instabilities, which I suspect may be due to improper frequency selection at different points in my system. I'm trying to regulate output current, with a result that looks like a sawtooth. Current drops to a trough, I get some short pulses which seem to stop the drop, then I get one much longer pulse which kicks the current up to a peak. No pulses until the next through. Average current as measured both by my control system and by an external clamp are regulating to, say, within 20% of my setpoint. Not good enough for my spec.
- Switching frequency: 12 kHz
- A/D sampling frequency: 12 kHz
- Control loop update frequency: 12 kHz (observed)
- Anti-aliasing RC filter on feedback signals entering A/D converter: single pole, 1 kHz corner frequency
- Digital filtering: None
- Choke: ~2 mH
Should I perhaps be sampling faster? Updating my control loop slower? Using an anti-alias filter with a higher corner frequency? Is there some obvious flaw in this control scheme as I've described it? In general, how would one properly select these frequencies and values relative to each other?