APPLICATION NOTES

3

Power dissipation and maximum allowable temperature rise involve many variables working together. Collector current,

PWM duty cycle and switching frequency all factor into power dissipation. DC losses or "ON-TIME" losses are simply

cycle of 30%, DC losses equal 86 watts. Switching losses, in milli-joules, vary proportionally with switching frequency. The

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 MSK4853, at 15KHz, will exhibit switching power dissipation of 135 watts. The total

losses are the sum of DC losses plus switching losses, or in this case, 221 watts total.

221 watts x 0.26°C/W thermal resistance equals 57.5 degrees of temperature rise between the case and the junction.

Subtracting 57.5°C from the maximum junction temperature of 150°C equals 92.5°C maximum case temperature for this

example.

Turn-on switching loss + Turn-off switching loss = Total switching losses = 2.5mJ + 6.5mJ = 9mJ

Total switching loss x PWM frequency = Total switching power dissipation = 9mJ x 15KHz = 135 watts

Total power dissipation = DC losses + switching losses = 86 + 135 = 221 watts

Junction temperature rise above case = Total power dissipation x thermal resistance

221 watts x 0.26°C/W = 57.5°C temperature rise above case

Maximum junction temperature - junction temperature rise = maximum baseplate temperature

150°C - 57.5°C = 92.5°C

THERMAL CALCULATIONS

8548-20 Rev. F 12/14