MOSFET gate and drain can be shorted in some design?

[RHeTTRoNiCS]

Am troubleshooting a power supply...I saw one mosfet that having the gate and drain shorted and connected to S-Ground...The mosfet pins were not short when I removed and measured...So am sure the mosfet is ok.....The part number of mosfet is FR9120N...So is it my pcb having internal short or this can be like some of power supply design...Really appreciates any inputs...thanks a lot.

 

[betwixt]

Are you sure it is the drain pin connected to ground and not the source pin? There are often low resistance components between the gate and source but not normally between drian and gate. If you are certain of the connections and the MOSFET itself is OK, look for shorted snubber components and possibly shorted driver circuits.

Brian.

 

[RHeTTRoNiCS]

Hello Betwixt...thanks to you so much for the tips....follow your suggestion and found a high freq transformer connected in between. So the shorted reading is valid I guess. The power supply still got the same problem...The fan is not spinning when power-up. But I can read the vcc (13Vdc) from pwm ic pin (UC3842). Not sure what is the best troubleshooting method with this power supply, the model is Vega 650 (product code V60532M). Hope anyone help me...thanks.

 

[betwixt]

Apart from the obvious signs of explosion or burning, the most common cause of SMPS not starting is dried out capacitors. Look for signs of bulging tops on electrolytic capacitors or the can lifting off the PCB and exposing the plastic 'bung' underneath them. If all look OK, try changing the smaller value ones near the input end of the unit, often they are related to the 'kick-start' circuit that initially provides power to start the first few cycles of output before the main supplies take over. Their values are typically small, usually < 22uF but beware of their voltage ratings which are often high. Always replace them with ones rated to 105C temperatures, it gets hot in thos PSUs!

You could also try contacting the manufacturer TDK for a service manual.

Brian.