PWM, H-Bridge and NCO - PIC16F18446

[Ioannis]

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

[HenrikOlsson]

Sorry, mode 2 still isn't clear to me.... "2. Bipolar voltage on the load with a rate of 0-50 Hz"
Do you mean...

A) That polarity of the voltage across to the load is switched at 0-50Hz (and power is regulated by the dutycycle of the 1kHz PWM signal) or
B) That polarity of the voltage across to the load is switched at 1kHz (the PWM frequency) and the 1kHz carrier is turned on/off at a rate of 0-50Hz or
C) None of the above meaning I still don't get it :-)

[richard]

Do not know if or how NCO can be steered PWM/CWG module.


Richard, also nice idea about CLC module. I suppose the low freq signal is external of the PIC. Or may be the NCO signal too.

you need to have a good look at the data sheet
valid cwg input's :-
the nco
a clc output
any of the ccp /pwm modules
the dsm output

valid clc input's:-
the nco
any of the ccp /pwm modules
the dsm output

there are many possible methods of modulating a LF osc source with a HF pwm signal with the clc or dsm modules
any of which can be turned into a full bridge signal source with the cwg module , try a few

[Ioannis]

Hope this draw clarifies a bit the problem.

Ioannis

[richard]

so where does the 1khz fit in ?


i expected more like this

[Ioannis]

You are right, I just didn't want to clutter the draw.

Ioannis

[amgen]

looking at your diagrams, seems like you could ..... set up the PWM module for 1KHZ as normal, for the 20HZ part, set up a timer interrupt for the low freq maybe with a couple of roll overs for lower freq.... then on tmr int, only need to switch the Q's and add a little dead time on each switchover.