# Mosfet Switching losses Calculation and confusion

Discussion in 'Power Electronics' started by QMESAR, Dec 14, 2017.

1. ### QMESAR Thread Starter New Member

Dec 9, 2017
15
4
I have a the situation a Mosfet switching a 12V / 15 Ohm solenoid valve. I run the switching circuits in SPICE ,I take the Ids and the Vds time the PWM duty cycle to get the Power ,interesting to see that people suggest to take the 1/2 to get the average ,
my question here is
what value of duty cycle should I use for my calculation to see the heat dissipation as it could range between 5%-95% at the moment I take a dc of 80%
in my case I have Vds = 12V,, Ids = 820mA
the Pq = ((12 V * 820mA )* 0.8) = 7.87 then Pavg = 7.87/2 =3.98W

(1)My confusion is this the Power that generates the heat over the Mosfet ?
(2) should I calculate for the loss at maximum Duty cycle ?
(3) what is the loss when there is no switching for example the Mosfet switch on and then the DC is 100% Mosfet is on all the time,

Any idea help and guidelines is highly appreciated
Kind Regards
QMESAR

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2. ### Picbuster Well-Known Member

Dec 2, 2013
916
119
Look at the fet spec's like input impedance, s-d resistance versus gate voltage, impedance solenoid and rise time duty cycle.
You are using pwm rise time will switch the mosfet. s-d impedance goes up when gate goes to slow in saturation.
That's your biggest heat source normal mosfet impedance is 10 milliohm @ 1 amp does 10mW heat power.
When switching takes a slow gate rice time then the mosfet output impedance could go from 10 Ohm to 10 milliohm during the switch on time.
But the mosfet data sheet is able to produce wanted answers.

Picbuster

Dec 9, 2017
15
4

4. ### crutschow Expert

Mar 14, 2008
22,238
6,487
One component of the MOSFET power dissipation is the ON-current times the MOSFET ON-resistance times the ON duty-cycle.
This will be highest at 100% duty-cycle, of course.

The other is the switching loss during the rise and fall times.
This can be roughly approximated by taking both the MOSFET output rise-time and output fall-time times the maximum current times the supply voltage times the PWM frequency divided by 2.
Psw = (Tr*I*V + Tf*I*V) *Fpwm * 1/2, where I = ON MOSFET load current and V = supply voltage.

QMESAR likes this.
5. ### QMESAR Thread Starter New Member

Dec 9, 2017
15
4
Thank you very much for your input it is highly appreciated 