| 摘要 |
<p>PURPOSE:To inhibit heat loss due to base currents by reducing the maximum allowable value of the collector currents of a power transistor at a low temperature. CONSTITUTION:A base current control circuit 12 determines base currents IB required for saturating voltage between a collector and an emitter in a power transistor 7 on the basis of current IM detected by a current detector 5, and controls a transistor 13. On the other hand, a conduction ratio decision circuit 11 reduces the conduction ratio of a pulse signal transmitted over a transistor 14 when currents IM detected by the current detector 5 reach a maximum allowable-current set value V1 or more acquired by dividing constant voltage V0 by resistors R4 and R5. When a thermoswitch fitted near the power transistor 7 is turned OFF at a low temperature, the voltage dividing resistor R4 is short- circuited, and the maximum allowable-current set value V1 is minimized.</p> |
