Switching power supply startup circuit having normally on emitter-switched current source

摘要

A power converter startup circuit establishes an operating voltage for control circuitry during startup and is then disabled to reduce no-load power dissipation. The startup circuit has a normally on characteristic to automatically provide startup charging current for a startup capacitor. The control circuitry begins operating as the startup capacitor voltage reaches an operating value, and it generates an inhibitory signal that disables the startup circuit to stop the startup charging current and reduce power dissipation. The normally on characteristic is achieved by an emitter switched current source employing a normally on device such as a depletion-mode J-FET. A resistor divider network provides both biasing for the startup current source and a point of monitoring the power supply input voltage during steady state operation.

主权项

1. A switching power supply, comprising:
a storage capacitor coupled to a power magnetic element to maintain a steady state value of an auxiliary voltage during steady state operation of the switching power supply, the steady state operation being preceded by a startup period in which an input-side DC voltage of the switching power supply rises from zero toward a steady state operating value; switching/control circuitry configured to couple energy from the input-side DC voltage to the power magnetic element during the steady state operation, the switching/control circuitry including powered control circuitry powered by the auxiliary voltage; and a startup circuit configured and operative during the startup period to generate a startup value of the auxiliary voltage to enable the powered control circuitry to establish the steady state operation, the startup circuit including (i) a startup current source coupled between the input-side DC voltage and the storage capacitor to provide charging current thereto based on the absence of an inhibitory control signal, the startup current source including an emitter-switched current source having a normally on switching transistor that conducts an enabling current in the absence of the inhibitory control signal and that does not conduct the enabling current in the presence of the inhibitory control signal, the enabling current controlling the delivery of the charging current by the startup current source, and (ii) a generator of the inhibitory control signal, the generator being configured and operative during the startup period to generate the inhibitory control signal in response to the auxiliary voltage reaching the startup value by charging action of the startup current source and to maintain the inhibitory control signal during subsequent steady state operation, wherein the startup current source includes an output current source that generates the charging current in response to the enabling current, and wherein the emitter-switched current source includes a bipolar transistor having an emitter in series with the normally on transistor, the bipolar transistor conducting the enabling current as collector current when the normally on transistor is also conducting.