PWM, H-Bridge and NCO - PIC16F18446

[Ioannis]

The main task is to drive a load (this is a big coil of around 19mH of thick 2mm wire at 60cm diameter) with:

1. Unipolar, 0-20 volts, pulsating power of 0-50Hz

2. Bipolar 0-20 Volts, pulsating power of 0-50Hz

Because of 2. a full bridge is needed.

But because of the high currents at low frequency (below say 30Hz) the MosFets of the Bridge are not in the SOA and decided to use PWM to adjust the mean voltage driven to load along with the On/Off period just to cool the MosFets.

That PWM signal will drive the one of the upper transistors of the bridge while the NCO low frequency of 0-50Hz will drive one of the lower transistor of the same bridge.

In other words, say I want to drive with unipolar signal the load:

1. Upper right MosFet is modulated with PWM 1KHz 30% DC signal
2. Lower left Mosfet is modulated with NCO of say 10Hz


If I want to drive bipolar signal to the load:

1. Upper right MosFet is modulated with PWM 1KHz 30% DC signal
2. Lower left Mosfet is ON for x msec.
3. Lower left MosFet is OFF
4. Upper right MosFet is OFF
5. Upper left Mosfet is modulated with PWM 1KHz 30% DC signal
6. Lower right MosFet is ON for x msec
7. Lower right MosFet is OFF
8. Upper left MosFet is OFF
9. Loop from step 1

Alternatively, as the simulator shows:

1. Upper right MosFet and lower left MosFet is modulated with PWM 1KHz 30% DC signal for 1/2 of NCO signal
2. Upper left Mosfet and lower right MosFet is modulated with PWM 1KHz 30% DC signal for 1/2 of NCO signal
3. Loop from step 1

Simple, right? Well no. Cannot figure how this can be done...

On the Simulator file this is shown on two cycles of the 1Hz signal modulating the PWM of 1KHz signal.

I do understand that there is great danger of shoot through current and have to be very carefull.

Ioannis

P.S. Maybe add a 5th MosFet on the lower side of the H-Bridge and modulate that one with the PWM signal? Then how will the pairs in the bridge change according to the NCO signal?