Hi all!
I have a DC motor that I'm trying to control with a PIC and a FET, doing pwm. The motor is a two coil design. 24 V, consuming about 4 A with no load. In "real life" it takes about 15 A at full load. (the markings on the motor says 24 V / 20 A)
I've connected the coils in paralell, from +24V to drain on my FET. A IRF540 logic level type. Then a resistor between gate and the PIC output pin. (PIC16F870 with hardware pwm)
My problem is that the freewheeling diode for the motor keeps blowing up, or at least gets very hot. I first tried a 1N4007, and switching freq at approx 20 KHz. The literally exploded! Then tried a bigger diode, some 3 A type. It breaks in a couple of seconds.
Next try was to lower the switching freq to 244 Hz. Then it works pretty OK, but the diode still gets warm. And that's at 50% dc, 2,5 A and no load. The FET seems to be doing fine. Only a tiny bit warm. But a cold running FET doesn't help much, when my diodes are glowing...
So, how do I solve this problem? Bigger diode? Different circuit? I'd prefereably keep the switching freq above audible, but that seems impossible here. What is actually happening here? Wheen the FET close, the motor generates a negative spike, that the diode "kills", right? Or does the motor become a generator, sending the same amount of current trough the diode that it gets from the FET?
I'd might consider a h-bridge later, to change direction easy. How are the values for the FETs calculated? Low Rds is fine, but doesn't help much if the diode inside "scrubs off" 0,5 V when the FET isn't conducting. Please enlighten me!
Odin