Electronic – Burnt MOSFET in the following circuit. Why

Consider the equivalent BJT circuit, which may be more familiar:

^{simulate this circuit – Schematic created using CircuitLab}

This holds provided the input voltage is >0.65V, with of course some variation based on temperature, output current, manufacturing variation, etc. However, as a first approximation this circuit outputs a constant 0.65V.

The MOSFET circuit is no different, but instead of the 0.65V from a forward-biased silicon PN junction, we get the threshold voltage of the MOSFET. This parameter varies between models of MOSFETs, but is usually some volts. If the output voltage, which is also the gate voltage, is above the threshold voltage, the MOSFET turns on more, shunting more current to ground, increasing the current through the resistor, lowering the ouput/gate voltage such that an equilibrium is reached:

$$ V_{GS} = V_{out} \approx V_{GS(th)} $$

This sort of circuit would be useful as a reference voltage, for example, to implement a voltage regulator, because the output voltage is relatively unaffected by the input voltage. A single transistor such as this isn't necessarily a good voltage regulator on its own, but it could be the basis for something better. A good regulator starts with a reference such as this which might vary based on other parameters (output current, supply voltage, temperature), then isolating or compensating for those parameters from the reference.

There are three possible causes I can think of that would cause the FETs to be damaged:

1) Over dissipation. You say the LEDs are 15W - how much current is that? Do the FETs get hot?What voltage are they being driven from? How fast is the multiplexing from the TLC5940? If the gate drive is not enough to ensure saturation they could get excessively hot and be damaged.

2) Over voltage. You have no snubbers (an R and C) on the outputs to dissipate voltage spikes if there is inductance in the wiring, you could get these spikes at turn-off of the FETs.

3) SOA violation: Is there excess capacitance on the output lines that could cause excessive currents at turn-on of the FETs.

To start with I would look in detail at the turn on and turn off of the output stages. Ensure there is enough gate drive, that the drain voltage goes down to a very low voltage when the FET is on. Look for spikes at the drain when the FET turns off.

Looking at the FET current is not so easy - a low value resistor in the source of one of the FETs would be one way or a current probe.