PWM, H-Bridge and NCO - PIC16F18446

[Ioannis]

The idea: Drive a MosFet H-Bridge with the upper transistors with a PWM output signal (freq of ~1KHz) and lower transistor with a low requency NCO output signal (0-50Hz).

Requirement is to have two modes of operation. One in full bridge, so the load accepts changing polarity according to NCO and the power controlled by the PWM signal.

The other mode drives the load with one polarity only, NCO drives the On/Off state of the load and again PWM the power.

Have setup the NCO correctly on the 16F18446 chip, along with the PWM so then one polarity signal can be produced.

But cannot thing how I can drive output for full bridge mode. It is not exactly as PWM driving fullbridge since I need also a low freq signal for the polarity change along with higher frequency PWM signal.

If waveforms are needed to clarify things I may draw them on paper and scan then.

Ioannis

[richard]

why not use the CWG module, with appropriate steering all modes are possible

[Ioannis]

Thanks Richard.

Have not used about that module yet. I will have a look at that to see what can do.

Here is a small simulation on LTSpice to describe what I want to do. Please delete the .txt at the end of the file name and run it on LTSpice.

The load is driven at 1Hz with a PWM signal of 1KHz.

Ioannis

[richard]

i can't see what you are trying to do. the spice sim does not help.

its easy enough to modulate a 1khz pwm signal @50hz , it's easy to lock a h-bridge one way or another.
i am having difficulty seeing how the top and bottom halves of a h-bridge can be different frequencies without shoot through destroying every thing

[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?

[HenrikOlsson]

I'm probably missing something...
Why not simply turn on the lower MOSFET in question, apply your 1kHz PWM signal to the (correct) upper MOSFET and then modulate it using your 0-50Hz signal. In other words, turn the 1kHz carrier on/off at whatever rate you want.

If that's possible or not at the hardware level, ie "route" the NCO output to the enable bit of the PWM carrier is beyond my knowledge at the moment but at such low frequency (0-50Hz) a software (perhaps interrupt driven) aproach sounds more than reasoanble.

[Acetronics2]

Hi, Ioannis

What drives your PWM ratio ??? Pot, control or ?

Alain