VN05N high side driver question...

[Angus Thomson]

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
http://www.st.com/stonline/books/pdf/docs/1082.pdf); 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

 

[SioL]

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 http://www.st.com/stonline/books/pdf/docs/1082.pdf); 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

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.

 

[Watson A.Name - \Watt Sun, the Dark Remover\]

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
http://www.st.com/stonline/books/pdf/docs/1082.pdf); 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).

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.

 

[Anthony Fremont]

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
http://www.st.com/stonline/books/pdf/docs/1082.pdf); 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).

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.

 

[Angus Thomson]

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

 

[CWatters]

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
http://www.st.com/stonline/books/pdf/docs/1082.pdf); am I missing something
obvious here?! It is running in a car, so the supply is around 13V.

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...
http://uk.farnell.com/jsp/endecaSearch/partDetail.jsp?SKU=170088&N=401

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

 

[CWatters]

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!

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

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

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?

Yes 720 C ! See my other reply.

 

[R Adsett]

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?

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

Line 2 page 3 of the specification.

Robert

 

[Angus Thomson]

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

 

[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 http://www.linitx.com. 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

 

[CWatters]

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

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