I need to design a +/-1.5A, 3kHz small-signal-bandwidth (absolute min, for 3° max phase and 1% max error on amplitude) current controller for a 20 Ohm 0.1H load with a +/-100V supply rail (the current setpoint profile I'm planning to request should make the load swing +/-80V).
I want to make that controller based on a high power op-amp (because I need very good smoothness so no H-bridge) from PowerAmp Design, the PAD129. It has in particular a GBW of 1MHz and a correct SOA for this application. The opamp controls the voltage across a sense resistor capable of dissipating the heat.
Here is the best of my many attempts using the manufacturer's SPICE model of the opamp, and yet it still does not work. I've also tried placing the RC between the output of the opamp and the negative input, with similar results.
I have calculated the transfer function and plotted it considering the opamp to have a finite gain equal to a first order with DC gain of 1 million and rolloff frequency of 5Hz, and I have noticed that the feedback network has very little impact on this resonance:
So it seems the inductor is resonating with something, but I don't see how to damp it enough to increase the gain-limited bandwidth (the phase-limited bandwidth is more or less what I need).
And finally, I thought the wobble on the waveform above was due to that resonance, but how can a pure sine input result in an output waveform with a different frequency, since the opamp is supposed to be linear (the output voltage is not clipped)?
Do you have an idea of how to modify the circuit to make it work? I'm out of ideas.