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VN05N high side driver question...

Hi,

I am using an ST VN05N high side driver to switch power to a Mini-ITX motherboard (small form factor, low power consumption). The board draws a maximum of 6A with a hard-drive connected but approx. 4A the rest of the time; however, the HSD gets very hot within 30secs and then cuts out momentarily (presumably due to the over-temp protection). The device is rated for 13A constant current (according to

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am I missing something obvious here?! It is running in a car, so the supply is around 13V. I'd really appreciate some help with this; either I'm being stupid (not the first time), or I have 3 faulty parts (I have tried the same setup with 3 devices with the same result).

Thanks in advance!

Angus Thomson

Reply to
Angus Thomson
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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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Siol
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Reply to
SioL

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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.

Reply to
Watson A.Name - "Watt Sun, th

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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

40milliohms. This would lower your dissipation to 1.44W which is much more easily dealt with.
Reply to
Anthony Fremont

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

Reply to
Angus Thomson

something

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

100C) by adding a heatsink.

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...

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

Reply to
CWatters

a

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

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

6

Yes 720 C ! See my other reply.

Reply to
CWatters

6W at 60C/W is 360C differential between junction and ambiant. A little warm.

Line 2 page 3 of the specification.

Robert

Reply to
R Adsett

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

Reply to
Angus Thomson

Hi,

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

11-18V supply IIRC, and produces the relevant ATX voltages (-12V, +3.3V, +5V, +12V etc.). They were all bought from
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The circuit I have designed is simply to switch the supply for all the components of the PC; motherboard, LCD display and speakers.

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

Angus

Reply to
Angus Thomson

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

Reply to
CWatters

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