MOSFET Power Dissipation Calculation with PWM gate drive - help needed

mishra87

Joined Jan 17, 2016
908
Hi Guys,

I want to calculation power dissipation of MOSFET. The parameters are as per below.
http://www.aosmd.com/pdfs/datasheet/AON7534.pdf

Vgs = 4V
Vds = 8.4V -6V
Id = 10A
Frequency = 16KHz assume 50% duty cycle
RQJA = 75 °C/W
Tj = 150

PD = ?

Edit : Vgs = 4.5V

Last edited:

Audioguru again

Joined Oct 21, 2019
3,536
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.

mishra87

Joined Jan 17, 2016
908
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.

Last edited:

Audioguru again

Joined Oct 21, 2019
3,536
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.

mishra87

Joined Jan 17, 2016
908
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
1,757
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

ronsimpson

Joined Oct 7, 2019
1,472
Lets Assume Vgs = 4.5V.
Don't drive the gate with 4.5V. This graph shows the resistance is lower at 10V.

Audioguru again

Joined Oct 21, 2019
3,536
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..