What is the maximum possible temperature a MOSFET can reach?

Question

I'm choosing a switch for a power supply design. I would like to know the highest temperature the MOSFET can reach. The switch that I'm using is DMT3020LDV.

An input having 24 V and 8 A is supplied to the drain of the switch. The following are my calculations for the junction temperature:

\$T_j = T_a + (P_t \times R_{ja})\$

Where:

\$T_j\$ = Junction Temperature;
\$T_a\$ = Ambient Temperature;
\$P_t\$ = Total Power Dissipation = 0.9 W (from datasheet);
\$R_{ja}\$ = Thermal Resistance, junction to ambient = 138 °C/W (from datasheet)

So, \$T_j\$ = 25(room temperature) + (0.9 x 138) = 150°C.

But if the outside temperature is 85°C, will \$T_j\$ = 85 + (0.9 x 138) = 210°C?

Are my calculations correct? Because 210°C is too high in my opinion.

Answer 1

Work backwards.
For desired power level and known other parameters you can calculate maximuj allowed ambient.
Rearrange as desired.

1 ... Tambient_max = Tjmax - Power(Rth_ca + Rth_jc)
or
2 ... Power_max = (Tamb-Tjmax) / (Rth_ca + Rth_jc)
or
3 ... Rthca = (Tjmax - Tambmax)/ Power - Rthjc

So (1) - Tambient_max = Tjmax - Power(Rth_ca + Rth_jc) tells you how hot the ambient can be for a given desired Power level, given Rthjc (fixed by manufacturer) and Rthca - set by heatsinking.

eg at P = 10 Watts, Tjmax = 150 c, Rjc = 3 c/W, Rthca = 10 c/W
(1) gives ... Tambient max = 150 - 10W x (3 + 10) = 150 - 130
= 20c :-(.

Change to a 2c / W heatsink and you get
Tambmax = 150 - 10 x (3+2) = 150-50 = 100 c.

ie IF you can get the case within 2c of the water then you can cool it with boiling water :-)

Adjust assumptions to suit.

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You need to understand power dissipation.
If you have a low Rdson MOSFET and it is usually hard on or off then dissipation at 8A is "lowish". eg I^2R at say 30 milliOhm = 64 x 0.030 ~= 2 Watts.
Add to that some switching loses and it is still liable to work well with sensible minimal heatsinking.

If the FET operates partially NOT in on/off mode then it will be different or much different.

Show us your circuit

Answer 2

It depends on the MOSFET. The key parameter is the junction temperature. Current parts with a 175 C rating are available. There are silicon carbide "MOSFETs" that have a much higher rating. I used them as custom parts, I do not know if they are available on the open market. They were not inexpensive.

Using the calculations given earlier you can work backwards to determine how much heat you can dissipate. From that you can calculate your max current.