polar pwm

[pokiko]

Hi

I have to drive full bridge thyristors with pic18f2520. i need two pwm which are opposite and variable duty cycle.. When T1 and T3 triggered at ton of pwm, the other pair(T2 and T4) must be toff. And when T1 and T3 is at toff then T2 and T4 have to be at tonn. How can i make such pwm over ccp1 and ccp2? Is there any risk to burn them?

[Ttelmah]

You'd be better off choosing a chip with the ECCP/Power PWM module. The 4520 (larger version of the 2520), has this, or in smaller packages, chips like the 1330.
The key is that you not only need to ensure the opposite 'pair' are off, but normally have a tiny period, when both pairs are off, or during the crossover, unacceptable currents may flow. You can do this by designing the power drivers, so that the switching times are assymetric, ensuring that the transistor going 'off', always switches faster than the one coming 'on', but it is a lot cheaper, safer, and easier, to have a programmable delay. The power PWM module in particular, is designed for exactly this type of application, with a programmable transition deadband period. If you use the two CCP's, One will always be started fractionally ahead of the other (depending on how fast you can program this part of the chip), and this provides the deadband in one direction, but then the other will involve you being very careful on the pulse widths allowed on each pair. It is not an easy thing to do safely...

Best Wishes

[pokiko]

I have already a circuit at present used 18f2520. The circuit is designed to drive one pair of thyristors with ccp1 and the other paır with ccp2. All i need is a variable pwm duty cycle with deadband delay. I searched for it in the forum but i couldn't be able to find any examples about this subject. Is there any examples?


Thanks a lot Ttelmah.