High-side MOSFET switch

Question

I would like to switch 3 separate 60 V, 20 A fused loads on/off high side from an ATMEGA328P. It needs to be switched high side for 2 reasons,

1. I don't want live to have the potential to short to ground while off as ground is chassis.

2. The output plug only has 3 pins (48-57 V, GND, DATA) and data shares the same ground plane.

My circuit is likely to draw about 12 A per channel for a few hours at a time.

Attached is my testing in the circuit simulator before printing PCBs and wasting money can anyone point out where this may fail as I am confident there is lots i don't understand about MOSFETs.

I have selected P-chanel MOSFET NCE60P25K Datasheet though I have only put this to the test via simulation, I have just tried to fill in as many of the specks from the datasheet as possible in the simulator.

I'm not sure what value resistors are acceptable, I have tried to use high enough values to not waste power but still work.

Feedback would be much appreciated.

Edit: I have just noticed that the positive gate resistor is connected to VIN not VIN1, reality it would be much more practical to wire to VIN1 the same fuse as the respective power. would this make any difference or not matter.

Based on comments, would I be correct in saying that to drive a P-channel MOSFET at 60 V I would need to range BASE between 40 V and 60 V, NOT 0 V and 60 V? In this picture I have done it with a switch rather than a transistor but you should get what I mean.

Please can you explain as this is likely the missing knowledge I don't understand about MOSFETs.

Answer 1

• You need to limit the peak gate-source voltage to significantly less than 20 volts (read the data sheet for the MOSFET used). This can be done with an added resistor and a 12 volt zener diode.
• The MOSFET has only got a maximum drain-source voltage of 60 volts and your supply is 60 volts <-- that's a no-no.
• You will probably need to use lower values than 500 kΩ and, the resistor that connects between MOSFET drain and GND is unlikely to be required.
• With, for example, a 20 amp load, the MOSFET will dissipate a power of 12 watts and, will need significant heat-sinking. This is based on figure 1 from the data sheet: -

• VIN doesn't need a fuse

Answer 2

If you want to use a SMD MOSFET and keep it on for hours, and you have three, you need to keep the dissipation down to avoid cooking the board. Another option would be to use a heat sink but that's bulky and more expensive, SMD is preferable if it is possible.

So the SMD MOSFET should not dissipate much more than 1-2W at 12A current, which means its RdsON should be 10-15 mOhms or less.

Unfortunately there is only one PMOS available on digikey with this RdsON and adequate voltage, and it is very expensive.

With your power supply voltage, you need at least 80V MOSFETs.

So you would either need to use several PMOS in parallel, for example 3 in the much easier to find 30-40 mOhm range, or use a NMOS with a charge pump to drive it.

I would recommend overcurrent or short circuit protection too.

I'm not sure what value resistors are acceptable, I have tried to use high enough values to not waste power but still work.

Power wasted heating the MOSFET due to high RdsON will be much worse.