Motor Drive Circuit - Stability Criterion

Thread Starter


Joined Jan 9, 2009
Hello All,

I am analyzing this circuit for work and I have it all simulating and am pleased with that. I have decided, just to be safe, to put it through some stability analysis and for reasons of phase margin am questioning what I am seeing.

Some key information:

1. Don't worry about the circuits to the left of the CONTROL line (they are just enable lines which for the AC analysis I have everything permanently enabled)
2. I have cut the feedback at R24 for the open loop analysis
3. Please note the inductor at the load has 8 Ohms internal resistance

My concerns/questions:

1. The phase starts off at > 0 dB and 180 degrees. My understanding was that this means guaranteed instability
2. Once the compensation etc.. takes off (i.e. > 10 Hz) the phase never gets near 180 degrees positive or negative so stability seems to be no concern

The circuit works fine but I believe there is some misunderstanding in my head which is making me worried about this given the 180 degrees phase at low frequencies. Perhaps I am just analyzing it wrong and any guidance is welcome.

I have attached an image of the open loop and closed loop transfer functions. The open loop is done to the cut point at R24 and the closed loop is from Vmotor+/Vcontrol (the points of actual interest during application).

Many thanks in advance!