transistor q3 has become cut off or the transistor q2 has become conductive while the zener diode zd1 is used for the reverse operation the above transitional time length is normally shorter than 0 5 u03bcs fig1 is a graph showing the input current and efficiency versus the input power voltage v cc for alternating operation of a two stage darlington transistor set as the main transistor q5 the sum of the output fig8 h of windings n1 and n2 of reactor l2 is maintained relatively constant except for a ripple component and the variation in the efficiency is within 2 despite any power input voltage variation the reason for choosing the smoothing reactor l2 to allow some ripple component is to stabilize the current peak detection otherwise too large capacity of the reactor would make the current waveform of the switching transistor q1 resemble a rectangular waveform resulting in the unstable detection on this account a smaller smoothing reactor l1 is chosen provided that the ripple component of the base current does not result in the variation in the collector current of main transistor the output frequency of the oscillator 16 is chosen so as to produce a high ripple frequency beyond the response of the main transistor q5 although in the above embodiment the reverse bias current flows in the bias diode db if it is intended to drive a complementary transistor pair q5a and q5b as shown in fig1 a base drive circuit can be shared by these transistors advantageously as described above the inventive transistor drive circuit is configured in a single transistor forward switching mode base drive circuit and a constant base current is achieved.