A switching power converter (100, 500) comprising: an energy storage section (160, 566) with a primary side and a secondary side, said energy storage section being configured to be responsive to a selective application of an input voltage to a primary side of the switching power converter to produce an energy that provides an output voltage (120, 520) on a secondary side of the switching power converter; a switching control circuit configured to produce a waveform (252, 256) that has a period and a duty cycle; a primary switch (190, 590) on the primary side of the switching power converter and a synchronous rectifier switch (195, 595b) that, during normal operation, switch in opposition to each other responsive to the waveform such that, when the primary switch is CLOSED and the synchronous rectifier switch is OPEN, the energy storage section is charging and, when the primary switch is OPEN and the synchronous rectifier switch is CLOSED, the energy storage section is discharging; wherein during a PRESTART operation, the primary switch and the synchronous rectifier switch are configured to be responsive to the waveform such that the primary switch is held OPEN and the synchronous rectifier switch is pulsed between OPEN and CLOSED; characterised in that: said duty cycle is set based on a value of the output voltage, the input voltage or the energy produced by the energy storage section; the switching power converter is an isolated switching power converter and that said isolated switching power converter further comprises a prestart control circuit that, during PRESTART operation when the synchronous rectifier switch is CLOSED, is configured to measure a reflected voltage (135, 535) on the primary side of the isolated switching power converter that is proportionate to the output voltage on the secondary side of the isolated switching power converter, and is further configured to establish an initial duty cycle value of the switching control circuit for normal operation using the reflected voltage. an energy storage section (160, 566) with a primary side and a secondary side, said energy storage section being configured to be responsive to a selective application of an input voltage to a primary side of the switching power converter to produce an energy that provides an output voltage (120, 520) on a secondary side of the switching power converter; a switching control circuit configured to produce a waveform (252, 256) that has a period and a duty cycle; a primary switch (190, 590) on the primary side of the switching power converter and a synchronous rectifier switch (195, 595b) that, during normal operation, switch in opposition to each other responsive to the waveform such that, when the primary switch is CLOSED and the synchronous rectifier switch is OPEN, the energy storage section is charging and, when the primary switch is OPEN and the synchronous rectifier switch is CLOSED, the energy storage section is discharging; wherein during a PRESTART operation, the primary switch and the synchronous rectifier switch are configured to be responsive to the waveform such that the primary switch is held OPEN and the synchronous rectifier switch is pulsed between OPEN and CLOSED; characterised in that: said duty cycle is set based on a value of the output voltage, the input voltage or the energy produced by the energy storage section; the switching power converter is an isolated switching power converter and that said isolated switching power converter further comprises a prestart control circuit that, during PRESTART operation when the synchronous rectifier switch is CLOSED, is configured to measure a reflected voltage (135, 535) on the primary side of the isolated switching power converter that is proportionate to the output voltage on the secondary side of the isolated switching power converter, and is further configured to establish an initial duty cycle value of the switching control circuit for normal operation using the reflected voltage. said duty cycle is set based on a value of the output voltage, the input voltage or the energy produced by the energy storage section; the switching power converter is an isolated switching power converter and that said isolated switching power converter further comprises a prestart control circuit that, during PRESTART operation when the synchronous rectifier switch is CLOSED, is configured to measure a reflected voltage (135, 535) on the primary side of the isolated switching power converter that is proportionate to the output voltage on the secondary side of the isolated switching power converter, and is further configured to establish an initial duty cycle value of the switching control circuit for normal operation using the reflected voltage.