| 摘要 |
PURPOSE:To prevent malfunction due to parasitic coupling capacitance by providing a bypass line detouring electric noise driving in error a photodetector between a light emitting element and the photodetector of a photocoupler giving a control signal to a drive circuit. CONSTITUTION:A potential Vn at a node N1 is changed depending on the transient state when a load current IL is switched, and a noise current Ib induced by the said change passes almost through an added capacitor Cb and is bypassed to the emitter of a photo transistor (TR) Qb. On the other hand, the noise current Ib going to flow to the open base of a photo transistor Qp through a parasitic coupling capacitor Cs is decreased in such a manner to be almost neglected. As a result, the possibility of the open base of the photo TRQp of the secondary side driven in error by a light other than that from the primary side light emitting diode Ld is almost less and then a fault that MOS field effect TRs Q2, Q4 or Q1, Q3 of the said current changeover circuit 10, for example, have a short- circuit current Is is prevented surely. |
