Base drive circuit for a switching power transistor

Claim 1

A base drive circuit for use in a switching-type power supply comprising: first and second transistors each having an emitter, a collector and a base electrode, the base electrode of said first transistor being coupled to the emitter electrode of said second transistor via a first current path, the base electrode of said second transistor being coupled to a bias voltage source, the collector electrode of said first transistor being coupled to said base electrode of said second transistor via a second current path, means coupled to said second transistor for causing said last mentioned transistor to selectively assume a conducting and nonconducting state, said first transistor being responsive to the state of conduction of said second transistor whereby said first transistor is caused to assume a like state of conduction, the conduction of said second transistor causing current flow via said first current path to the base of said first transistor thereby determining the degree of conduction of said first transistor, said second current path including a unidirectional current conducting device poled to permit current to flow in said last mentioned path when the potential on the collector electrode of said first transistor falls below a predetermined value consistent with the desired level of saturation current in said first transistor, the current flow in said second current path effecting a diminution of said current flow in said first current path thereby reducing the current flow in said first transistor to said desired level.