PWM with dead band delay

aderogba · Joined: 03 Jun 2005 Posts: 8

Please I need some guidance on setting up pwm with dead band delay in half bridge mode. The pic I'm using is 18F6722.

PCM programmer · Joined: 06 Sep 2003 Posts: 21708

The dead time should be set to a little bit more than the amount
of time it takes for your FETs to turn off. In your FET data sheet,
you should see values for the "turn off delay time" and "fall time".
You could add those together, but I think it's better if you measure
the turn off time with an oscilloscope. That's because you normally
have a series resistor going to the gate of your FET, and the FET has
a lot of gate capacitance, so this acts as a RC circuit and slows down
the edge of the gate drive waveform.

You didn't give the FET part number, so let's just pick a large initial
value for the dead time, say 1 us. The 18F6722 data sheet says
the dead time is controlled by the ECCPxDEL registers. The lower
7 bits set the dead time, in terms of instruction cycles. So if your
PIC is running at 20 MHz, one instruction cycle is 200 ns. So your
dead time value has a 200 ns resolution. So if you want the dead
time to be 1 us, you would set the ECCP1DEL register = 0x05.
(Bit 7 of that register is used to control the auto-shutdown feature,
but I'm not going to worry about that now.)

aderogba · Joined: 03 Jun 2005 Posts: 8

Thanks for the response. Please do you know if there is ccs sample code or code snippet for this application? I'm still having problem setting the program up.

kender · Joined: 09 Aug 2004 Posts: 768 Location: Silicon Valley

Is it possible to have PIC�s CCP generate 2 square waves out of phase? I had to generate 2 out of phase square waves with deadband at 500kHz. I ended up doing it with GAL (it was really easy).

valemike · Guest

valemike · Guest