Power Dissipation for FET?

[sunny_nites]

Anyone know the answer to this one?

When you spec out the power dissipation requirement for an FET, do you do the calculation using true or apparent power?

For example I have a circuit running from a 3.7V Lipo using an FET as a switch. I want to hit the load with 400ma.

Using the apparent power calculation;
IxE or 3.7 x .4 = 1.4w

Using the true power calculation;
IxIxr or .4 x (.3 x .3)(according to the datasheet) = .036w

Big difference in power dissipation requirements!

 

[moderator007]

Try here

 

[Mr Happy]

There is only true power--which is the power dissipated inside the FET, causing it to heat up. The transistor neither knows nor cares what you are doing with the current passing through it.

Current through FET = 0.4 A.
Resistance of FET = 0.3 ohms (you must check this with the data sheet)
Voltage across FET = I x R = 0.4 x 0.3 = 0.12 V
Power dissipated in FET = I x V = 0.4 x 0.12 = 0.036 W

Alternatively:

Power dissipated in FET = I²R = 0.4 x 0.4 x 0.3 = 0.036 W

Unsurprisingly, both answers are the same. What method will you use to limit the current to 400 mA? This will not happen automatically.

 

[sunny_nites]

moderator007 - thanks for the link to that website, looks like a good one.

Mr Happy - thanks for the explanation. I knew I had to be missing something. Didn't think about calculation the voltage drop across the FET.
I will be just using a resistor to limit the current. I'll have to make sure I consider the higher current at full charge as well when considering the maximum power dissipation.

Thanks again all!

 

[gSPIN]

Power dissipated in FET = I x V = 0.4 x 0.12 = 0.036 W

:duh2:

:tsk::fail:



:whoopin:

 

[HooNz]

The FET parameters are important , as some FETs have a turned on resistance of .001 ohm and most/quite alot seem to be .01 ohm , so for 400ma practically a nothing .:wave:

 

[Mr Happy]

The FET parameters are important , as some FETs have a turned on resistance of .001 ohm and most/quite alot seem to be .01 ohm , so for 400ma practically a nothing .:wave:

Also, the FET resistance depends on how "hard" it is turned on, i.e. what the gate voltage is. If the gate voltage is too low, the resistance will be higher.

 

[HooNz]

Also, the FET resistance depends on how "hard" it is turned on, i.e. what the gate voltage is. If the gate voltage is too low, the resistance will be higher.

Ok , but i really meant "ON" as in on ,not half on or a 1/4 on , as getting to that level of importance might be a bit too informational overloadish for some , and sticking to the basic's i deemed posting keeping it simple and then the user or reader of the data sheet of said device if at the correct level of understanding would pick that up , maybe! :naughty:

 

[MikeAusC]

. . . Resistance of FET = 0.3 ohms (you must check this with the data sheet) . . . .

The resistance of a FET varies over a great range, depending on many things - Gate Voltage, Drain Voltage, Temperature etc.

The graphs on the datasheet make that very clear.

 

[Mr Happy]

The resistance of a FET varies over a great range, depending on many things - Gate Voltage, Drain Voltage, Temperature etc.

The graphs on the datasheet make that very clear.

Yup. The 0.3 ohms was just the example value used in the OP that I carried forward for illustrative purposes. Nobody should assume that this is a fixed value or even an estimated or approximate value without referring to the datasheet for the particular type of FET and the expected operating conditions.