A BJT switch operating at frequency, f_s = 15 kHz and duty cycle, D = 0.8 modula

Question

A BJT switch operating at frequency, f_s = 15 kHz and duty cycle, D = 0.8 modulates power in a power converter from a dc source, V_in = 180 V to an RL load with resistance 5 ohm. BJT switch operates in the hard saturation region with an on-stage voltage drop. V_CE (sat)=1.6 V. The BJT manufacturer provides following specifications of the switching times; delay time. t_d = 1 mu s current rise time, t_ri = 2.5 mu s, and voltage fall time, t_fv = 3 mu s. the turn off storage time, t_s = 4 mu s. voltage rise-time, t_rv = 1.3 mu s, and current fall-time, t_fi = 1.7 mu s. The base drive consumes 9 W of power continuously. For this BJT under given operating conditions, determine Conduction power loss, Switching power loss (on and off) and If it is desirable to limit the total power dissipated in the switch to = 100 Watts, what change in the converter design or operation should be made?

Explanation / Answer

f = 1 1 1 2 L C or C1 = 2 3 12 2 1 1 1 L f (2 (1 10 ) (2 1 10 ) = ) × × × = 253 × 10– 23 F = 253 × 10– 11 pF

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