MSK4804_15 Datasheet, PDF (3/7 Pages) M.S. Kennedy Corporation – Silicon Carbide Mosfet Provides Ultra Fast Switching

APPLICATION NOTES
THERMAL CALCULATIONS
Power dissipation and maximum allowable temperature rise involve many variables working together. Drain current,
PWM duty cycle and switching frequency all factor into power dissipation. DC losses or "ON-TIME" losses are simply
VDS x Drain Current x PWM duty cycle. For the MSK 4804, VDS=2.4V max., and at 100 amps and a PWM duty cycle of
30%, DC losses equal 240 watts. Switching losses vary proportionally with switching frequency. The MSK 4804 typical
switching losses at VDS=600V and IDS=100A are about 3.99mJ, which is simply the sum of the turn-on switching loss
and the turn-off switching loss. Multiplying the switching frequency times the switching losses will result in a power
dissipation number for switching. The MSK 4804, at 5KHz, will exhibit switching power dissipation of 19.95 watts. The
total losses are the sum of DC losses plus switching losses, or in this case, 91.95 watts total.
91.95 watts x 0.190°C/W thermal resistance equals 17.47 degrees of temperature rise between the case and the junc-
tion. Subtracting 17.47°C from the maximum junction temperature of 150°C equals 132.53°C maximum case tempera-
ture for this example.
VDS x ID x PWM duty cycle = 2.4V x 100 amps x 30% =72 watts DC losses
Turn-on switching loss + Turn-off switching loss = Total switching losses = 2.08mJ + 1.91mJ = 3.99mJ
Total switching loss x PWM frequency = Total switching power dissipation = 3.99mJ x 5KHz = 19.95 watts
Total power dissipation = DC losses + switching losses =72W + 19.95W = 91.95 watts
Junction temperature rise above case = Total power dissipation x thermal resistance
91.95 watts x 0.190°C/W = 17.47°C temperature rise above case
Maximum junction temperature - junction temperature rise = maximum baseplate temperature
150°C - 17.47°C = 132.53°C
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8548-101 Rev. D 4/13