Page 69 - Power Electronics Handbook

62 Thermal design
350r---l

0 50 100 150 200 250
Average forward current (A) 2.1 Power loss curves for a thyristor
They can occur due to the gradual turn-on and turn-off processes,
which enable large amounts of currents to flow whilst the voltage
across the device is still high. Figures 1.8(c) and 1.8(d) show one
example of the turn-on phenomenon, whilst Figures 1.8(e) and l.S(f)
show an example of a turn-off characteristic. These turn-on and
turn-off losses give rise to power spikes in the overall power-
dissipation curve, as in Figure 2.2, the mean power loss due to
switching being seen to be relatively small at low frequencies.

01 I 1 I
(b) t0 tl t2
2.2 Power losses in a semiconductor during a typical cycle: (a) current; (b) power loss