MOSFET Power Dissipation Calculation with PWM gate drive - help needed

Audioguru again

Joined Oct 21, 2019
1,733
The Mosfet is spec'd for a Vgs of 4.5V and 10V, not 4V.
At only 4V and a whopping 10A the Mosfet might smoke badly.
If you buy many of them then you might find one with spec's better than the worst ones.

Heat caused by the switching frequency depends on how much current the gate drive circuit can provide to quickly charge and discharge the gate capacitance.
 

Thread Starter

mishra87

Joined Jan 17, 2016
851
The Mosfet is spec'd for a Vgs of 4.5V and 10V, not 4V.
At only 4V and a whopping 10A the Mosfet might smoke badly.
If you buy many of them then you might find one with spec's better than the worst ones.

Heat caused by the switching frequency depends on how much current the gate drive circuit can provide to quickly charge and discharge the gate capacitance.
Thanks !

Lets Assume Vgs = 4.5V.
Could you please help me to calculate power dissipation ?
 
Last edited:

Audioguru again

Joined Oct 21, 2019
1,733
The on-resistance of a Mosfet increases as it heats up so yours might be 0.01 ohms at the 10A current.
Simple math shows 10A into 0.01 ohms= 1W but 50% switching causes only 0.5W. But the Mosfet heats much more if the on and off switching is slow since it will be linear and getting hotter.
 

Thread Starter

mishra87

Joined Jan 17, 2016
851
The on-resistance of a Mosfet increases as it heats up so yours might be 0.01 ohms at the 10A current.
Simple math shows 10A into 0.01 ohms= 1W but 50% switching causes only 0.5W. But the Mosfet heats much more if the on and off switching is slow since it will be linear and getting hotter.
Thanks !

What is impact of 16KHz frequency in power dissipation ?

Thanks again !
 

andrewmm

Joined Feb 25, 2011
526
When your switching, you have three power dissipation numbers to work out.
when the Fet is off or on , you can find the resistance form the data sheet, and you know the voltage , so you can find the power watts, and you know the time its on and off, so you know the average power. That gives you two of the numbers.

The third number, is when the fet is switching on and off. during that period, the power is "dynamic" . It can be calculated by area under the triangle type calculations similar to above assuming you know the time over which the fet is switching .

But , most designs, the switching time is a very small percentage of the time, say 1 % , so has little effect on the overall power estimation.

And please remember, it is an approximation, and you tend to err on the side of caution ,and put a bigger heat sink on than is needed
 

Audioguru again

Joined Oct 21, 2019
1,733
Graphs on a datasheet are for "typical" devices. You cannot buy a typical one, you get whatever is available.
The text says how bad some of them are, yours might be a bad one but it still passes the spec's in the text..
 
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