# MOSFET over heating at lower than expected load currents

Hello there,

I'm testing a new switch method for my battery cut off circuit and when testing the FQP27P06 P channel mosfet at 12V at the source and 0V at the input it generates quite alot of heat without a heat sink at 5A and when measuring the Vds voltage and multi pling it by the current I get around 2.15Ohm which is way above what is being said in the datasheet.

Why is that?

Thanks for helping

Maybe something in the circuit that we can't see?

Not Much that's how it looks:

That's an N channel you've drawn, what is the voltage of your input?

outsider:
That's an N channel you've drawn, what is the voltage of your input?

Well just used a P channel with 12V

Please redraw with G,D,S instead of 1,2,3, are you sure it's not backward and current is going through the body diode?

You didn't define the input to the mosfet. Could it be the Mosfet is not turning off completely?

If the Vbatt is 12V and the input is derived from a 5V device, there is still 7 volts on the Mosfet S->G

The circuit shown in reply #2 won't work with a P-channel MOSFET, especially a non-logic-level MOSFET like the FQP27P06.

Use an NPN transistor and pullup from the gate to +12V as shown for Q6/Q7 below.

If you wanted to keep the P Channel MosFet you could do this:

Noone ever spoke about running 5V with it all I was asking for was an explanation for why would the mosfet get so hot at 5A load when the datasheet states such a low Rds ON and when
i measure it is more around 2.1Ohm instead of 0.7Ohm even at dead short to ground at the gate...

HI,

Given no other information folks try to help with the info provided and some reasonable assumptions.

So, what is your Vgs under the conditions you suspect are causing the overheat?

Yes, this is the wrong symbol for your P channel MOS FET.
Do make sure the P MOS FET is correctly installed, i.e. Source to Supply + Drain to load to ground.

A good image of the wiring would help.

.

i measure it is more around 2.1Ohm instead of 0.7Ohm even at dead short to ground at the gate

Exactly how did you arrive at that value?

when measuring the Vds voltage and multi pling it by the current I get around 2.15Ohm

By multiplying voltage and current you get power not resistance. To get the resistance take the voltage and divide it by the current.

You only need one pling not multiple ones.

larryd:

What is wrong with this picture, shouldn't it have a pull down resistor from the gate to - BAT. Any input signal will go directly to - BAT. Not good.

At the moment, OP really needs to re-do the circuit diagram so that everybody gets the right idea about what is the 'real' deal.

MikeLemon:
Hello there,

I'm testing a new switch method for my battery cut off circuit and when testing the FQP27P06 P channel mosfet at 12V at the source and 0V at the input it generates quite alot of heat without a heat sink at 5A and when measuring the Vds voltage and multi pling it by the current I get around 2.15Ohm which is way above what is being said in the datasheet.

Why is that?

Thanks for helping

That device has 55 milliohms on-resistance, so at 5A it will run very hot without a heatsink as
its trying to dissipate 1.4W into thin air. Anything over about 0.5W needs some attention to heatsinking.

Multiplying current by voltage gives power, not resistance. Vds/I gives the resistance.

Hmm that was the answer I was looking for thanks for the rich info,

It'd be awesome if you could refer the equations you used to the datasheet.

About the symbol it is a P channel mos taken from KiCad and the gate is shorted straight to ground to simulate a "full flow"

Any way's so you say in order to operate at around this current it is required to lower the Rds ON at least 3 times?

Is there such a solution available for this voltages(12V) at a reasonable price?

Thanks for your time.

MarkT:
That device has 55 milliohms on-resistance, so at 5A it will run very hot without a heatsink as
its trying to dissipate 1.4W into thin air. Anything over about 0.5W needs some attention to heatsinking.

Multiplying current by voltage gives power, not resistance. Vds/I gives the resistance.

BTW if you are a mosfet expert maybe you could explain me that:

in the below datasheet:

How can I know what would be the Rds ON at a particular Vgs voltage if they say the Vgs th is -4V max and the normal Vgs is +-20V?

Could this work with the current(Not amps ^_^) requirements for example?

Refer to this line of text in the data sheet:

RDS(on) Static Drain-to-Source On-Resistance 0.02Î© VGS = -10V, ID = -38A

VGS(th) is the cut off point.