Choosing mosfet heatsink ?

[tron87]

How do i choose a suitable heat sink for a STP36NF06L MOSFET LOGIC N 60V 30A to dissipate up to 70watt at 25c ambient temperature ?

 

[dknguyen]

You have the following information from the datasheet and your operating environment:
-your maximum allowable temperature of the MOSFET junction (or the maximum temperature you are willing to tolerate)
-the maximum ambient temperature (usually 25C at least, but maybe 30C or even 40C especially if you're outside or it's a hot day)
-the thermal resistance of the heatsink to ambient
-thermal resistance of thermal grease
-thermal resistance from junction to heatsink (it might be called junction to case, or junction to board, or other names depending on the package and how the heatsink is supposed to mount to it).

The Thermal Resistance is the C/W, it will rise X degrees Celcius for every Y Watts of power dissipated. The MOSFET's thermal resistance for junction to ambient is what you use if there is no heatsink. Even if there is a heatsink, it plays a role. If you choose to ignore it, it just means your MOSFET will run cooler than predicted which is always good. It just means there are two paths to cool the MOSFET: through the heatsink to air, and directly from MOSFET package to air.

So

[Maximum Junction Temp] >= [Max Temp Rise]+[Maximum Ambient Temp] = [Power Dissipated] * ( [Junction to case/heatsink C/W] + [Thermal grease C/W] + [ Heatsink C/W] )

You just pick a heatsink with a C/W that gets you a low enough temperature rise so that it does not exceed your chosen junction temeprature at the ambient temperature. The thermal grease is usually very low impedance if applied properly so it does play a role but mainly you choose the heatsink only.

Heatsinks have different thermal resistance with different airflows and it is sometimes listed for different airflows. So if you know your airflow (like a fan) or if you know that you don't have forced airflow you use the thermal resistance that is your operating condition.

The hardest part of all this is actually calculating your MOSFET power dissipation from conduction losses (which is pretty easy) and switchign losses (which can get very complicated to get an accurate number).

EDIT: I looked at teh MOSFET datasheet and what you are asking is not possible. Because maximum allowable temperature is 175C and with an ambient of 25C, your maximum rise is 150C. So EVEN if your MOSFET and thermal grease had 0C/W you would still need a heatsink with a thermal resistance to ambient air of less than 2.14C/W. But the junction to case thermal resistance of the MOSFET is already 2.14C/W. It's already too high. You would need a heatsink with 0C/W to ambient. Without a heatsink, the package is the heatsink which adds on an additional 62.5C/W (junction to ambient thermal resistance). You need to parallel MOSFETs or find another MOSFET.

 

[tron87]

Ive recalculated the expected power dissipation to be 15 watt for what im using it for the ambient temperature is to be max of 40c knowing that what c/w heatsink is required ?

If i parallel mosfets do i need balancing resistors to evenly distribute the load ?

If this particular mosfet has a thermal resistance junction-ambient (free air) max of 62.5 °C/W does that mean its good for about 2 watts without heat sink keeping it within core limit 175c ?

How do i find the rds(on) for a gate source voltage of 2.5v for this particular mosfet if it only has in the datasheet the rds(on) for 5v gs and 10v gs ?

 

[MikeMl]

Your chosen FET will barely turn on (or not at all) with a Vgs of 2.5V. Post a link to its datasheet.

 

[tron87]

http://www.datasheetcatalog.org/datasheet2/8/0iyt26iogeofcyaowtrx4z7iwiky.pdf .

 

[MikeMl]

Why are you driving the gate with only 2.5V when all of the specs are taken at a minimum gate voltage of 5v?

 

[tron87]

Why are you driving the gate with only 2.5V when all of the specs are taken at a minimum gate voltage of 5v?

I want pwm it at 10khz 50 %duty which means 2.5v on the gate if 5v is 100 % duty, need to know what the rds(on) is at 2.5v gs ideas ?

 

[MikeMl]

You should be driving the gate with a 5V square wave which has a rapid rise/fall time. This means the FET is either fully off or fully on. The "averaging" of the PWM is done in the load that the FET is switching on/off; not at the gate of the FET.

To minimize the time as the FET switches, you will need a gate driver capable of sourcing/sinking almost 1A. They make ICs for that purpose.

 

[ronv]

Duty cycle means time not amplitude. So 50% is on half the cycle and off half cycle - it's digital. The FET itself needs to be either on or off so it needs full gate drive.

 

[dknguyen]

Ive recalculated the expected power dissipation to be 15 watt for what im using it for the ambient temperature is to be max of 40c knowing that what c/w heatsink is required ?

Surely you're not asking me to do the math for you a second time? It's all right there in my first post on how to calculate it yourself.

 

[tron87]

Couple of questions:

Does the resistance between drain and source on a mosfet change much depending on junction temperature ? for example if i had a mosfet with a heat sink fitted will it have a different vds at the same Ids than one without a hs fitted ?


How do i find the max safe ID vs VDS in the safe operating area figure for 100% duty on all the time where on the data sheet they only short pulses in the ms ?

 

[ronv]

Resistance changes a bit: see fig. 10
The DC values can be found in Table 1.

https://www.st.com/stonline/books/pdf/docs/9616.pdf

 

[tron87]

Regarding figure 10 is that how much the ds resistance changes +- with temperature or the expected ds resistance at a tj temperature ?

Without using a temperature probe on the case of a mosfet how do you know if the the mosfet is overheating ?

How do you if a mosfet heatsink is actually working doing the job its supposed to do ?

 

[MikeMl]

Touch the FET with a moistened finger. If you leave skin behind, it is TOO HOT

btw: ever heard of an IR Thermometer?

 

[ronv]

Figure 10 is the change in RDS(on) with junction temperature. In this case the resistance would be 1.5X as high at 100C as it would be at 25C.
You can get an idea of the temperature by doing the calculations based on the power dissipation and the temperature coefficient of the heat-sink. (table 2)
I usually try to keep the case temperature below 50C. That way if I can hold my finger on it it's OK. But I suppose measureing it is better. Then using table 2 you can work backwards to get the junction temperature.

 

[tron87]

If this mosfet is good for 175c max tempeture in reality is it safe to run it at around 130c just seems very hot ?

 

[ronv]

It can be run there but... My friend has a really nice Samsung TV. The power supply mosfets run really hot. The mosfets are all fine but the filter caps keep going bad because they are close to the hot heat-sink. Same problem with my DVD player. The outside case must be 45C. Same problem the caps are to close to the heat sink. Change them every 2 years.