Heat Sink for Mosfets question:

[siliconmike]

I have 3 mosfets:

a) 1 for line powered flyback convertor switching, Peak Drain Voltage
is 500V
b) 1 for high side of 12V DC motor control (for free wheeling)
c) 1 for low side of 12V DC motor control

The circuit is such that when the power supply is on, battery charges
and the motor is disabled and when the power supply is off, the motor
may be running on battery.

So at any instant, either (a) is on or (b) and (c) are on.

- I would like to mount all 3 mosfets on a single heat sink, using mica
insulators.. Is this ok?

- Somehow if I could mount all 3 on the metal case, it would be great,
but there is an issue of safety .. Any advise on how to do this safely?

- This box is going to be used in a vehicle, so I plan to air-seal it
for protection against moisture, water and dust [that's why avoiding
separate heat sinks while using case as heat sink is better]. I assume
that the heat dissipation out of the box will be a problem.
Any ideas for this?

Mike

 

[exxos]

I have 3 mosfets:

a) 1 for line powered flyback convertor switching, Peak Drain Voltage
is 500V
b) 1 for high side of 12V DC motor control (for free wheeling)
c) 1 for low side of 12V DC motor control

The circuit is such that when the power supply is on, battery charges
and the motor is disabled and when the power supply is off, the motor
may be running on battery.

So at any instant, either (a) is on or (b) and (c) are on.

- I would like to mount all 3 mosfets on a single heat sink, using mica
insulators.. Is this ok?

- Somehow if I could mount all 3 on the metal case, it would be great,
but there is an issue of safety .. Any advise on how to do this safely?

- This box is going to be used in a vehicle, so I plan to air-seal it
for protection against moisture, water and dust [that's why avoiding
separate heat sinks while using case as heat sink is better]. I assume
that the heat dissipation out of the box will be a problem.
Any ideas for this?

Mike

Most powe mosfets are plastic package with metal back, such as you onlu need
a mica waster to isolate from the heatsink, you do not normally need the
inserts and nylon bolts. It depends how how your mosfets get, if only "warm"
you can leave them on the case, though remember that case metal is very thin
and will create a hotspot where the mosfets are bolted on to the case. For
highpower applications the heatsink needs to be 5-10mm thick where the
mosfets bolt on. Cheaper heatsinks are normally 0.5mm-3mm thick, they are
"ok" for most cases though if your switching some KW of power I would look
for something "chuncky".

I normally run my mosfets connected to nothing or a very small clip on
heatsink, if they run hot to the touch (as in burn your finger) then you
need better cooling. Note that it could take several hours for a larger
heatsink to rise in temp. You may want to invest in a digital thermal
tester, then you can see how hot you are running.

Get mosfets with low RDS on, Mosfets ive tested here, then can vary from a
couple of watts heat loss to a over 100watts loss. Invest in a good mosfets
for the job and you probably won't even need a heatsink at all.

HTH
Chris

 

[phatty mo]

I have 3 mosfets:

a) 1 for line powered flyback convertor switching, Peak Drain Voltage
is 500V
b) 1 for high side of 12V DC motor control (for free wheeling)
c) 1 for low side of 12V DC motor control

The circuit is such that when the power supply is on, battery charges
and the motor is disabled and when the power supply is off, the motor
may be running on battery.

So at any instant, either (a) is on or (b) and (c) are on.

- I would like to mount all 3 mosfets on a single heat sink, using mica
insulators.. Is this ok?

- Somehow if I could mount all 3 on the metal case, it would be great,
but there is an issue of safety .. Any advise on how to do this safely?

- This box is going to be used in a vehicle, so I plan to air-seal it
for protection against moisture, water and dust [that's why avoiding
separate heat sinks while using case as heat sink is better]. I assume
that the heat dissipation out of the box will be a problem.
Any ideas for this?

Mike

Most powe mosfets are plastic package with metal back, such as you onlu need
a mica waster to isolate from the heatsink, you do not normally need the
inserts and nylon bolts. It depends how how your mosfets get, if only "warm"
you can leave them on the case, though remember that case metal is very thin
and will create a hotspot where the mosfets are bolted on to the case. For
highpower applications the heatsink needs to be 5-10mm thick where the
mosfets bolt on. Cheaper heatsinks are normally 0.5mm-3mm thick, they are
"ok" for most cases though if your switching some KW of power I would look
for something "chuncky".

I normally run my mosfets connected to nothing or a very small clip on
heatsink, if they run hot to the touch (as in burn your finger) then you
need better cooling. Note that it could take several hours for a larger
heatsink to rise in temp. You may want to invest in a digital thermal
tester, then you can see how hot you are running.

Get mosfets with low RDS on, Mosfets ive tested here, then can vary from a
couple of watts heat loss to a over 100watts loss. Invest in a good mosfets
for the job and you probably won't even need a heatsink at all.

HTH
Chris

Perhaps a "heat spreader" between the mosfets and case would be
good..maybe a fairly thick piece of aluminum plate.