A switching power supply, comprising: a storage capacitor (Caux) coupled to a power magnetic element to maintain a steady state value of an auxiliary voltage (Vaux) 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 (Vin) of the switching power supply rises from zero toward a steady state operating value; switching and control circuitry configured to couple energy from the input-side DC voltage to the power magnetic element during the steady state operation, the switching and control circuitry including 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 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 (Ic) thereto based on the absence of an inhibitory control signal (Vg), the startup current source characterised by an emitter-switched current source (Q2, Q3) having a normally on switching transistor that with the inhibitory control signal applied to it, such that it conducts an enabling current (Ic) 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 in the absence of the inhibitory control signal when the normally on transistor is also conducting, wherein the bipolar transistor stops conducting in the presence of the inhibitory control signal when the normally on transistor is not conducting. a storage capacitor (Caux) coupled to a power magnetic element to maintain a steady state value of an auxiliary voltage (Vaux) 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 (Vin) of the switching power supply rises from zero toward a steady state operating value; switching and control circuitry configured to couple energy from the input-side DC voltage to the power magnetic element during the steady state operation, the switching and control circuitry including 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 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 (Ic) thereto based on the absence of an inhibitory control signal (Vg), the startup current source characterised by an emitter-switched current source (Q2, Q3) having a normally on switching transistor that with the inhibitory control signal applied to it, such that it conducts an enabling current (Ic) 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 in the absence of the inhibitory control signal when the normally on transistor is also conducting, wherein the bipolar transistor stops conducting in the presence of the inhibitory control signal when the normally on transistor is not conducting.