I designed my first P/N FET H-bridge. Gif attached (Olin warning: you
will have to zoom in). It sort of works. The GP4/5 pins are used to
select backward or forward (switch the P-fet on, release the lock on the
opposite N-fet), the GP2 pin (PWM out) is used to switch the N-fet. The
resistor and R-zener dividers (I don't have the zeners yet, so the 15V
zeners are now 4k7 resistors) are required to keep the gate voltage
below 20V with the input voltage of 24V. 

So far so good, but while testing with just a pair as LEDs as load to my
surprise the P-fet that is supposed to stay off gets moderately hot!
When its feed resistor is disconnected from the ULN2003 it stays hot, so
it is not ULN leakage. 

Changing the P-FET gate divider to 2k2 seems to solve the problem, but I
don't feel comfortable when a problem goes away for an unknown reason.
Can someone explain what's going on?

Wouter van Ooijen

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Van Ooijen Technische Informatica: www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: www.voti.nl/hvu
 

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