High-side PMOSFET failing (can't turn it off after a few days) 3

[blcpro]

Hey all,

I'm having some trouble with a PMOSFET which I use to supply power to a thermal printhead. The problem is, after several days, or several power cycles, the PMOSFET seems to become a short circuit, and can't be turned off.

In the schematic provided, Q1 controls the turn-on and turn-off of the PMOSFET T1. There is a power sequencer that enables T1 after the system is ready. This seems to work fine for a while, but then the PMOSFET won't turn off, even when its gate voltage is 24V (Q1 off, R84 pulls gate voltage up to +24V).

The specs for the transistor T1: Alpha and Omega Semiconductor AOD4189: 40V, 40A, Vgs(th) ~ 3V @250uA.

http://www.aosmd.com/res/data_sheets/AOD4189.pdf

I know there is a lot of capacitance here, and I added D7 to help conduct when the power +24V is turned off, with those big capacitors fully charged. I thought maybe the body diode was failing, but adding D7 has not helped. What I have noticed, is that the resistance measured between the source and drain is very low after the "failure". I measured some new pmosfets and they seemed more normal (high resistance). Also, did a diode check on the PMOSFET body diode, and also on D7. D7 is still functional, but the Q1 body diode seems like a short.

Any thoughts on what might be happening here?

B

 

[blcpro]

I do tend to stare at my circuits too long. Sometimes it helps. Sometimes it don't.