In a tuned switch mode power supply, a supply inductance is coupled to a capacitance to form a tuned, resonant circuit. A transistor switch is coupled to the resonant circuit for generating current pulses in the inductance, coupled to a load circuit to provide an output of the power supply. Resonant voltage pulses are developed between a pair of main current conducting terminals of the transistor switch. A controller is coupled to the transistor switch for generating a switch control signal, during a run mode of operation. The transistor switch is turned on periodically, when the resonant voltage pulses reaches a zero voltage, to provide for zero voltage switching. The generation of the current pulses is disabled to provide for a burst mode operation, in a standby mode operation.
A switching power source apparatus has a switching element being connected with a DC power source through a primary winding of a transformer in which parasitic capacitance exists between terminals of the switching element. A voltage V4 of free oscillation appears at a gate of the switching element after a flyback period due to the parasitic capacitance. Based on the free oscillation voltage, the switching power source apparatus detects timing synchronized with a period of the free oscillation, outputs a trigger signal V10 based on the detected timing, uses the trigger signal to estimate bottom timing when the free oscillation reaches a bottom level the second time, outputs an ON start signal V9 synchronized with the bottom timing, uses the ON start signal to output an ON control signal to turn on the switching element, and drives the switching element in response to the ON control signal.
A power supply operates in a run mode of operation and in a stand-by, burst mode of operation. When, for example, a user initiates a power-on request command, an on/off control signal is applied to an input terminal of a microprocessor. The microprocessor monitors an end time of a dead time interval of a burst mode cycle and generates a synchronized control signal for turning on a switch. The switch turns on immediately after the end of the dead time interval. The turned on switch couples a run mode load to a filter capacitor of the power supply.
In a dual power supply, a main power supply is shut down by providing a signal from an auxiliary power supply. To generate the shutdown signal, a load is temporarily applied to the output of the auxiliary power supply for a predetermined time. The application of the load is detected using a hysteretic comparator in the auxiliary power supply that remains in a high state for a time period that exceeds a reference time period, which is established by a timer circuit. In response to exceeding the reference time period, the shutdown signal is sent to the main power supply.
