Hi, I was learning the other day about MOSFETs (applied to repair things) and I noticed many times they go short circuit. If this happens, I thought about this, and I don't know if it's right or wrong:

Since the MOSFET is shorted, replacing it simply won't repair the problem of the device, as that short circuit more than probably created a bigger problem (heat and failure) in other part/parts of the PCB, creating an open circuit this time, right?

For example a resistor or cap gets too much current and fails, creating an open circuit. So, case 1, an open circuit will be enough to break a device, and case 2, a short will always create a failure of a component that will end up open circuit, hence again case 1.

Is this a correct line of thinking as a general rule, or not really?

 

A short can expose other components to excess current, an open circuit can expose them to excess voltage. Both can cause other components to fail.

Bob

 

Most MOSFETs fail without causing any collateral damage, just drain to source short and blow a fuse. This is often the case in switched mode power supplies. Replace the MOSFET and it functions again. Just caused by a designer being over optimistic about the heat dissipation.
However, a really catastrophic failure can melt the source bond wire, which causes a failure of the gate oxide as the gate-drain insulation is now exposed to the full drain voltage. This usually ends with the drain voltage in the control circuitry and a lot of dead components.

 

A MOSFET in the "on" condition creates by definition a "short" circuit. This is not necessarily a fault condition if the amount of current that can flow is limited in some way. I have seen relatively few failures of MOSFETs used as an isolated switch. It is more common in half bridge and full bridge circuits for two primary reasons:

1. No provision for dead-time between switching the high side and low side
2. Insufficient drive current to charge and discharge the gate capacitance