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).
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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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.
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?
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
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