This PFET is really HOT ... normal?

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Rexlan

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I am trying to use a Fairchild FQU11P06TU and at times I need to run 6.5 amps through it steady which is within the specification. The load is a headlight that is ~ 80 watts @ 12.V
Data sheet is here:
Discrete MOSFETs - Fairchild Semiconductor Site Search

I have a circuit hooked up to test and the part is really hot to the touch. Will I need to have a heat sink on this to keep it cool or will it be OK?
Thanks
 
Hi Rexlan,

according to the data sheet the case temperature may be about 90deg/C at 6.67A.

I suggest to use a heat sink anyway for that current since the transistor dissipates around 8.2W at that current.

(calculation is based on an RDS(on) of 0.185 at 6.67A.)

Boncuk
 
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RCinFLA said:
You have around 1 watt of dissipation on device.
Click to expand...
No.
The on-resistance is 0.185 ohms max when its temperature is 25 degrees C and it has 10V Vgs. But it is hot so its on-resistance is higher, probably 0.28 ohms. Simple arithmatic shows a dissipation of (6.5A squared x 0.28 ohms)= 11.8W!

Maybe the Mosfet is not being fed enough Vgs for it to fully turn on.
 
I'm using the circuit Mike helped me with and I picked the PFET because of it's small case size.

I am using it as a modulator for a motorcycle headlight and it works well. However, at dark when I turn it on steady and run ~ 6.6 amps through the PFET then I see big trouble ahead.

I think it is turned on fully
 

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Have you measured the voltage drop across the transistor? Measure from drain to source with the lights on.
 
gerty said:
Have you measured the voltage drop across the transistor? Measure from drain to source with the lights on.
Click to expand...

Not sure what you mean but here are the numbers.

Input source is 12.48 Vdc

PFET voltage across the S-D is 1.68 Vdc
Transistor across E-C is 48 mills
 
The Mosfet has a poor on-resistance that is causing a lot of heat. Use a Mosfet with a much lower on-resistance that will barely get warm.
If the on-resistance is 0.02 ohms then it will dissipate (6.5A squared x 0.02 ohms)= 0.845W.
 
Hi Rexlan,

you might want to take a look at the IRF7410 (International Rectifier).

It has an RDS(on) of 7mΩ at VGS-4.5V and ID-16A. It's also pretty small (SO-8 package).

Properly mounted you might use the PCB for a heatsink.

Here's the datasheet.

Boncuk
 

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Looks like Audio hit this one again, you need a different transistor with a lower rds on, as per Audios' post
 
My bad on power dissipation. Yes it's Rds_on x Id^2 and Rds_on does go up when device is hot. You need lower Rds_on MOSFET.

What freq are you PWM'g the circuit. Using the resisitive pull up is going to limit the PWM speed. It can also cause device to get hot because rise time limitation results in insufficient Vgs decay on turn off cycle.

As you go for a low Rds_on you're get more gate capacitance making rise time worse.

Consider using a CMOS hex buffer IC will all buffers stacked in parallel to drive MOSFET or a regular MOSFET lowside driver.
 
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