VN05N high side driver question...

motherboard (small form factor, low power consumption). The

rest of the time; however, the HSD gets very hot within

protection). The device is rated for 13A constant current

missing something obvious here?! It is running in a car,

I'm being stupid (not the first time), or I have 3

Are you using a heat sink and why not? You need to dissipate some 12W of heat at 6A. Resistance of this device is 0.36 ohms at 6A. Do the math.

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To get its on resistance down to a minimum, the gate should be much higher voltage than the source, probably 8 to 10V depending on the specs. If it's anything less than that, then you have a resistance in the FET that's dissipating excessive power. If this is a switching PS, then you may have problems with the FET staying too long in the linear region, between fully off or fully on. That's why the SMPSes often use a pair of complementary drivers to actively pull the gate on or off quickly.

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You need a bigger heatsink or a part that has a smaller RDSon. You are dissipating >6W in the device, that's why it gets hot.

The RDSon for your part is 180 milliohms. Since P=I*I*R, P = 6A * 6A * .18ohms. This means P=6.48W. The VN820 has an RDSon of only

Hi,

Thanks to all who replied; first off, you're right - I'm not using a heat sink! Mainly because according to the datasheet, the device is capable of a maximum power dissipation of 56W! Although this is a maximum rating I am not even approaching this, and assumed (maybe naively) that the device would cope with 6W. I was trying (unsucessfully I might add) to work out the temperature of the part with the current I'm pulling; according to the datasheet, it will shut down at 140 deg. Celcius - could I reach that with 6 Watts?

Thanks again!

Angus

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Needs a bigger heatsink. I think the maths looks like this....

RdsON is between 0.18 and 0.36 Ohms so the device is dissipating between 6 and 12 W when running at 6A.

The thermal resistance of this package without a heatsink is given as 60C/W in the spec so at 12W the junction temperature could soar to Tambient + (12x60) = 720C ! In fact it would fail if it didn't have built in thermal protection.

You need to keep the junction temperature below 125C (and preferably below

eg Tambient + (Pd * Rheatsink) + (Pd * Rjunction-case) < 100

or rearranging....

Rheatsink < (100 -Tambient - (Pd * Rjunction-case))/Pd

Known quantities: Tambient = 30C in summer Spec says Rjunction-case = 2.2 C/W . Pd = 12W

so worst case the thermal resistance of the heatsink needs to be..

Rheatsink < (100 - 30 - 26)/12

Rheatsink < 3.6 C/W

Example heatsink...

Out of interest what voltage rails does the Mini-ITX need and how are those being produced from the 12V car supply?

True - but only if the case temperature is 25C !

At 56W it would take water cooling to keep the case that cool!

Yes 720 C ! See my other reply.

Line 2 page 3 of the specification.

Robert

Fair enough - should really have noticed that bit of the datasheet! Thanks to all who replied - I think a heatsink is the answer!

Cheers! Angus

Hi,

I bought a seperate PSU for use with my Mini-ITX motherboard; it needs a

Anyway, thanks very much for the help - I'll put in a Farnell order tomorrow!

Angus

I would also order a device with a lower ON resistance while you are there.