P-channel MOSFET as high-side switch gets hot
The problem
The /OE (output enable) pin of the 74CH595 toggles the output drivers between two states: driving the outputs (high or low) and high impedance (letting the output lines float). It is active low, so driving /OE low causes the 74CH595 to drive the outputs high or low depending on the data you have shifted in, while driving /OE high causes the outputs to float undriven.

You have tied the /OE pin to the 5 V supply trough the 10 kΩ resistor R29, so the output drivers are always in the high impedance state.
Ideally no current would flow in this state, and consequently the unnamed NPN bjt would never conduct any current, but in practice there is always some leakage current present:

There is no such thing as a &quot;BC447C&quot;, so I assume that the transistor is either a BC447, a BC547C or some other similar part. If say +1 μA was to leak from the 74CH595 output to the collector of the BJT, the transistors DC current gain (likely somewhere between 100 and 600) would cause it to conduct from 100 μA to 600 μA of current from the collector to the emitter. For example a collector current of 300 μA would lead to a voltage drop of 3 V over the 10 kΩ resistor R7, leading to a Vgs (gate to source voltage) of -3 V.
This hypothesis is consistent with what you saw (Ids of 2.58 A and Vds of 0.67V), the gate voltage must be very close to the threshold voltage (Vgsth):

The fix

Add a pull-down resistor between the base of the BJT and ground to stop stray currents from turning it on.

Remove the unnecessary 10 kΩ resistor R29, and connect the /OE pin directly to ground.

The LED would light up extremely dimly in its current configuration. Put it in parallel with the transistor base, with a separate current limiting resistor.

Add a bypass capacitor for the 74HC595, as close as possible to the 74HC595.

If this is going to be installed in a car, I would add transient protection for the MOSFET gate in order to protect it from any voltage spikes (the gate oxide is easily destroyed by overvoltage). A 15 V TVS diode would be ideal, but a zener will work as well.


The components you have chosen are just fine for what you are doing.



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