# MOSFETs and heat dissipation

Discussion in 'General Electronics Chat' started by HiProfile, Aug 12, 2009.

1. ### HiProfile Thread Starter Member

Mar 15, 2009
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I'm really curious about the how they rate the power capabilities of MOSFETs, in particular the IRFZ46N. I'm not really sure what rating to go by.

I plan on running about 4-5 amps through it at 12-13 volts, so 65 watts at most. The datasheet says the TO-220 package can dissipate 50 watts, but then they list the max power dissipation at 25C at 107 watts and max current at 37 amps. When my circuit is near/at 100% duty cycle, will 65 watts be too much? It is unlikely I'll be running over 50% anyways, and I can slap a large heat sink on it.

In less words, can you run 60-70 watts through an IRFZ46N w/heatsink?

btw, http://www.dprg.org/tutorials/2005-11a/index.html is the circuit in question. It will be opperating four fuel injectors.

2. ### ELECTRONERD Senior Member

May 26, 2009
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I have an old transistor book, and they depict a graph that shows the relationship between current and voltage. They say you can simply multiply the two (depending on your requirements) to find the power. This, of course, was for BJT so I'm not sure if you can do it with MOSFET's or not.

3. ### jpanhalt AAC Fanatic!

Jan 18, 2008
5,558
839
The heat dissipation of a mosfet is calculated from the RDS(on) resistance and current in the conventional way. It has nothing to do with the total voltage, but rather is calculated from the drop in voltage across the mosfet and current.

RDS on at the conditons you will be operating can be found in the datasheet.

edit: Using an RDSon of 0.024 ohm and current of 5 A, I get 0.6 W, so your mosfet will be fine under those conditions.

John

4. ### bearing01 Member

Jul 29, 2009
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But don't confuse RDS with the output conductance of the hybrid-pie model for a MOS transistor. Early voltage effect (created by upward slope in Id -vs- Vds curve in pinchoff / saturation region) gives you an Rds in your small signal model for the transistor. This is not RDS.

DC Power dissipated by the MOS biased up in Class A mode (always continually passing DC bias current) is Power = Id * Vds where Id is the average DC current and Vds is the average voltage across the drain to source.

If you're operating the device in class AB or class B or C then the power it has to dissipate will be less and there's a bit more to the calculation. But if you're design this type of amplifier then you should know how to calculate the power dissipation.

5. ### HiProfile Thread Starter Member

Mar 15, 2009
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Thanks, I'm glad I asked. I never would have guessed heat dissipation for a MOSFET would be calculated that way. I tried my "driver" out on a single injector, and the MOSFET didn't even get warm, just the injector.

I'm not technically designing it, just using a simple PWM circuit to pulse injectors (solenoids). My goal is to pulse them to get cleaning solution through them w/o burning them up. They would probably be classified as intermittent duty solenoids, so I can't just hold them open.

6. ### jpanhalt AAC Fanatic!

Jan 18, 2008
5,558
839
I'm a little late out of the blocks in responding, but here is an application note from Fairchild that addresses using simply the RDSon value to calculate power dissipation when the mosfet is acting as a switch, as I assumed was the case in your design. I mentioned referring to the datasheet, because of the variation of RDSon with temperature.

The image is a little blurry, but AN-558 is quite clear. Bottom line, your mosfet should have no problem with that current, if it is properly turned on.

John

7. ### Audioguru New Member

Dec 20, 2007
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If the drain and source of the mosfet are across your 13V power supply (the Mosfet is the load) and it conducts 5A then it dissipates 65W. It would need a huge heatsink.

But if the Mosfet is used as a switch then its on-resistance of 24milli-ohms will dissipate only 0.6W max. It will be slightly warm without a heatsink.

8. ### jpanhalt AAC Fanatic!

Jan 18, 2008
5,558
839
That is clearly not the case here.

The OP gives this schematic:

The mosfet is not simply shorted across a supply and being operated in its linear region. It is being used as a switch. Although the schematic shows an allowable supply range of 3 to 18V, the 12 to 13V supply being used by the OP should be adequate to turn it on completely.
John

Last edited: Aug 13, 2009

Jul 29, 2009
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