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# Mosfet power dissipation

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#### kkdelabaca

##### Full Member level 2
Hello,

I'm looking for one MOSFET transistor with Low resistance to drive one DC motor.

I think that one of the best options is the mosfet 3034 from IRF (international rectifier), but now I have some doubts about the heat dissipation:

The TO-247AC is bigger than the TO-220AB and I suppose that is better in high power switching, but what parameter is important in the datasheet to compare the transistors? Maybe the Junction-to-Ambient Thermal Resistance?

Somebody can explain to me if in my application I need one external heatsink? What is the formula to calculate the power?

Thanks!

#### zainka

##### Full Member level 2
Look at the thermal resistance, it gives you a clue of what rice in temperature you could expect based on dissipated effect. For the TO247 case you have
RθJA (Junction to ambient) set to max 40 °C/W. It means that for each W dissipated you could expect a 40°C rise in temperature in comparison to ambient temperature without heatsink. But, the other package TO220 gives you a 62°C rise in temperature. Do the math.

Now, we know nothing about your motor, thus we know nothing about your application, if its a small DC motor like those found in childrens RC toys you would do fine without a heatsink,.

Then again... What is 1W? Remember that this is dissipated effect in the mosfet and not in your DC motor so at 2m ohm internal ON resistance it means a current of >20A to reach 1W dissipated effect. Thats not the kind of motor you have, right?

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kkdelabaca

### kkdelabaca

Points: 2

#### kkdelabaca

##### Full Member level 2
Thanks a lot for your help.

Just one other question:

- According the datasheet, the maximum Operating Junction temperature is 175ºC. Considering that the ambient temperature is 25ºC, it means that the transistor only can dissipate 3.75W without heatsink?
(175-25) / 40 = 3.75W

I understand that the power dissipation in the transistor is the addition in DC conditions and switching mode. Somebody can tell me the formula of the power in fuction of the frequency, transistor capacitance, etc ?

Gracias!

#### Audioguru

I never operate a semiconductor anywhere near its maximum allowed temperature, voltage or current. Then I do not need to replace any.
I also do not drive my car as fast as it can go. Guess why not.

#### goldsmith

I'm looking for one MOSFET transistor with Low resistance to drive one DC motor.

Hi kkdelabaca

You've forgot to mention specifications of your DC motor ( it's voltage and current and if there is any kind of load existing ? )
Then how do you expect to be advised by a good mosfet type ?

What is the formula to calculate the power?
You mean the dissipated power ?
If so , if we neglect the gate dissipation then we can use this equation for that :
P loss :1/T integral over Rdson * Id^2 dt from zero up to ton .

Merry christmas + Best Wishes
Goldsmith

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