Henrik, for a while forget the PWM.

Just think of an H-Bridge that drives a load in two modes:

1. Unipolar voltage on the load with a rate of 0-50 Hz

2. Bipolar voltage on the load with a rate of 0-50 Hz

The mode 2 obviously doubles the power consumed by the load.

Because of the low frequency, I ended to the NCO as Richard proposed. But have not found a way to drive this to an H-Bridge yet.

The second problem is that at very low frequencies (0 to say 20 Hz) the power loss on the bridge transistors is high enough to destroy them because of violation of the SOA. I have to either put in parallel more devices or use a PWM signal to lower the losses and give time to the transistors to cool.

So, the NCO signal needs to be modulated with PWM.

Easy and complicated at the same time...

Ioannis