Hi,

The MOSFET IRFZ44N is driving a external relay (Freewheeling diode might not be used across the relay coil. the coil will draw a current of 200 mA) which in turn will be used for driving START solenoid of generator or driving hooter or driving a contactor. The supply is 12 V / 24 Battery. I am driving the mosfet using a ULN (driving uln thru microcontroller) with pulse amplitude of 5 V DC(max).

Connection is as follows :

The output pin of ULN is pulled up to +5V dc thru 3.3k resistance. The drain of mosfet is connected to one side of external relay and other end is connected to Battery +Ve and source is connected to Battery -Ve.

I am getting frequent failure of this mosfet from the field. What could be the reason & what way i could protect the failure of mosfet. Pls suggest.

rgds,

 

you are getting inductive kick-back from the coil, use zener diode snubber (breakdown > supply) or commercial snubber circuit

common problem, you can also aleviate if you can control the "turn-off" rate of the mosfet i.e. rc filter on gate

have fun

 

You must use a free-wheeling diode whenever you drive a coil with a MOSFET.