US Patent 9735663 BJT drive scheme

The invention generally relates to switch mode power converters (SMPCs). and methods for providing supplementary base drive to a bipolar transistor of an SMPC, and more particularly to a switching control circuit for a SMPC, a said SMPC having an inductive component coupled to receive power from an input to the SMPC, a bipolar transistor coupled to control current flow in the inductive component, an auxiliary power rail, a charge store for supplying power via the auxiliary rail to a switching control circuit for driving the bipolar transistor, and a bootstrap circuit for bleeding current from the input to the bipolar transistor to thereby provide an amplified current to the charge store, the switching control circuit comprising: a base current line for coupling to a base terminal of a said bipolar transistor, an emitter current line for coupling to an emitter terminal of a said bipolar transistor, an auxiliary line for coupling to a said auxiliary power rail, and at least one reference line for coupling to a reference voltage; a base current switch configured to controllably couple a base current source to the base current line; an emitter current switch configured to controllably couple the emitter current line to a said reference line; a bootstrap element for bleeding current from the emitter current line to the auxiliary line, the bootstrap element further operable to block a said current according to a current or voltage bias; a base discharge switch configured to controllably bleed current from the base current line to a said reference line; and a supplementary base drive line for coupling to a series circuit comprising a supplementary base drive resistor and current control element coupled in series, a said series circuit for bleeding current from a said input to a said base terminal, a said current control element for blocking current flow from a said bootstrap circuit to a said supplementary base drive resistor.