UPDATE AT THE BOTTOM
in a battery powered project with high current I'm using a P MOSFET as load switch in place of a mechanical switch to prevent inrush current and consequent spark on the switch contacts. The battery is rated 36V (as being Li-ion let's consider 42V at full charge) and 15A at max discharge.
The SPDT switch that originally was on the positive rail of the battery (this way sparking when turned on) now is located on the gate of the P MOSFET.
What accidentally happened is that, being the first time using and experimenting with discrete MOSFETS, I measured Vgs in strict reference to GND.
This is clearly a mistake: to turn the MOSFET off, I placed a resistor of 1.2M between source and gate to obtain something under 20V (measured towards GND) to stay under the absolute max rating for this MOSFET; to turn it on, the GATE is directly routed to GND by the SPDT switch.
Still before realizing that, in reference to source, Vgs is something near 22V when off and 42V when on and then always beyond the absolute max ratings, I obtained a fully working circuit and further I get my resulty with no damage to the MOS.
where the resistor to source is just a current limiter but Vgs must reach virtually 0V; the resistor to ground must be measured in order to obtain something near -20V for Vgs and it obviously needs a pull up resistor to make a divider.
Assuming that's common that with newest generation Power MOSFETS Vgs can exceed until 80V without damage, I can't still figure out why the circuit is working both on and off also if Vgs is always far beyond any expected rating.
The MOSFET never gets warm, because being an on/off main switch is not used often, but some days are passed in off state and I assume Vgs is always near 40V (and it's still working fine!)
I would notice that in early experimentation I was using an old, discontinued Power MOSFET with something near 0,1 Ohm as Rds and the device was dissipating near 26W, with a minor reduction of battery lasting with the same load. For this reason I changed to a newest Power MOSFET with the minimum Rds available to reduce the loss.
What do you think about this strange behaviour?
Being thankful to You in advance, I give my best regards
UPDATE: Hello again.
I tried to build the circuit suggested by Simon, having a Goford Power P-MOSFET with the source connected to the positive supply of the battery (+42V), the drain to the load, a 100k resistor between source and gate to provide 15Vgs, the switch that shorts this gate configuration to another 150k resistor ending to the negative supply of the battery (0V) AKA the whole ground load.
Only one board worked, some months ago, so I sold the product. It's very far from me, but it's working good. So I produced 30 boards with this exact circuit for the MOSFET implementation. But also if nothing has been changed in the board design, any other attempt on new boards is producing a fail on the MOSFET. I can say this because something like 9 MOSFETS has gone into failure in a board that's exactly the same revision of the working one.
Actually 11 MOSFETs of 12 are gone. The last Goford was placed on a brand new board with brand new components. It's important to say that everything on the load side is working perfectly. I tried with a IRFP9140N too to exclude the consideration of a defective batch. No luck. The majority of the MOSFETs went short at the first power up with no external signs, some lasted just a bit. Basically we have short on source-drain junction, or also the gate with source and drain. The failure seems due to Avalanche effect.
The load board, that works perfectly as expected, provide the parallel supply of two audio power amplifiers (automotive class D and class AB ICs) with power exceeding 350WRMS plus three voltage regulators (one LM317 and two LM338) for auxiliary supply; the board has been designed with a full ground plane and has been cared as requested from the class-D data sheet: as you know, class-D amplifiers must be routed with great care to work properly. Of course, there are capacitors for more than 13500uF as reservoir.
One of the three regulators supplies a secondary board with preamp section and MCU, but for sure this is not the issue, as the MOSFETs fail also with this secondary board disconnected.
I'm suspecting the MOSFET must be protected with a 42V zener diode or a snubber network between drain and source, but I wait for your precious opinions.
It's also important to say that a product unit I have with me adopts the original configuration with just a 1.2M resistor from source and the switch hanging around between this resistor and ground, IRFP9140N P MOS. It still works good with no stress signs.
Thank you very much
UPDATE AGAIN: Here came the brand new Infineon automotive high side switches in TO-220 sipmos package, with a rating so much higher than I need. These fails exactly the same way, at the first connection of the battery the MOSFET inside becomes shorted.
Apparently a 100nF capacitor between source and gate, as Tobalt suggested, solved the problem on my last IRFP9140N. Still have not tried the same solution on the smart high side switches, expecting that a specifically designed device for inrush currents would take this into account. Probably it gets the same: next boards will say.
Having made a lot of trials when I packed the only working system I sold, I suspect I adopted the capacitor and I forgot it between the processes as I forgot to add it to the last PCB revision, because seems there's no other possibility to get the thing to work properly without this one. As I can't check neither open this only working piece, honestly I was lacking for considerations.
Thank you all, especially to Tobalt, Simon Fitch and Greybeard.