I am attempting to use a N-Channel Logic Level MOSFET as a switch for a phone charger that runs on a 42V battery.
I have a resistor circuit with an LDR that currently outputs 3.3v when LIT with a torch and 0v when completely dark. Daylight outputs around 0.7V. The circuit works as expected for a few uses and then the MOSFET fails short and is now always on permanently (even when removed).
simulate this circuit – Schematic created using CircuitLab
My Guess:
Could the load (5V Step Down Regulator/Phone Charger) be inductive, similar to a relay and need a flyback diode somewhere? Placing one on +/- of the charger has no effect on longevity, as well as one on the main +/- of the battery.
The current of the charger should be less than 200ma at 42V for a 5W charger. Could it be higher at startup (it's rated for 30A).
They seem to fail when it is switched on/off more frequently.
MOSFETs can be placed in parallel to split the load, would this help?
I have killed more than 10 MOSFETs so far trying different combinations
The MOSFET is FQP30N06L with this datasheet.
The Phone Charger is this 5V Regulator
Best Answer
Your LDR is a very slow device that responds to daylight that also varies slowly .Sunrise and sunset can take an hour or more depending on season and lattitude .This means that your power mosfet will spend lots of time half on or half off .Under these conditions the power wasted and hence device heating will be bad . Cheap modern powermos devices are specified for switching .They can current crowd in analog mode .This internal current crowding can cause early failure due to internal hotspots.The gate should be close to 5VDC for charger on or close to 0VDC for charger off .This is also better for the charger because it gets clean 42V for on and zero for off .You can employ schmitt action by whatever means to switch clean and stop blowing fets.