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How many MOSFET's do I need? Answered

I am building a 2000W inverter powered by a car battery at 200A and I am unsure what to look at to find how many MOSFET's I need. Do I look at continuous current (in Amps) or power dissipation(in watts)?

If it's power dissipation might you be able to suggest a MOSFET with a high power dissipation that is under a continuous current rating of 75A?


We've tried to explain to you before. You are SWITCHING these devices, not using them as amplifiers, so you are PURELY interested in Rds on.

Look at Rds on.
Look at Icontinuous.

If I cont < I load, you need more devices, or, if you are VERY careful, you can rely on the slight uprating from pulse operation.

If I continuous^2 x Rdson > dissipation limit, you need more devices.

As I have ALSO explained you need GOOD gate drivers to make sure the gate is saturated to ensure the device reaches Rds On.

Be aware too that the devices derate as the heatsink temperature rises.

Power specs for semiconductors are somewhat meaningless, unless you have some idea of the heat sinking that is going to be in place. The datasheet will list one or more "thermal impedance" parameters. There will often be notes at the bottom of the page of the datasheet telling how much area of copper on the PCB will be needed to achieve that thermal impedance.

The most important thing is to not exceed the rated juntion temperature of the part. Note that the juntion will be always be hotter than the ambient air and the surface of the heat sink.

There are several good application notes out there from different semicoductor manufacturers on heatsinking.

If you are doing any kind of fast switching in your inverter, such as using PWM to generate a true sinusoidal shape output, then you will need to account for the switching losses in the MOSFET. At high switching frequencies, the power dissipation due to switching can easily exceed the power dissipated in the Rds_on of the part itself.

Continuous current is how much power can go through the mosfet.
Dissipation is when you are using the mosfet to partially conduct, acting as a big resistor, and how many watts of heat it can output without 'asploding'.

Say with MOSFET's with a power dissipation of 300W and cont drain current of 20A would I need 2000W / 300W MOSFET's or 200A-20A MOSFET's?

That, I cannot answer, simply because I don't know the specifics, and its dependant on the efficiency of the circuit.