For power supply devices such as a DC-DC (direct current to direct current) converter or AC-DC (alternating current to direct current) converter, the size of the device and its cost are two important considerations for its application. A converter usually includes at least one controller with certain external components. It is desired that minimum or ideally no external components are necessary to build the converter as well as to reduce the manufacturing cost of the controller portion of the converter.
FIG. 1 illustrates a schematic topology of a conventional off-line converter. As shown in FIG. 1, the converter comprises (1) a rectifier 11 to convert the high AC line voltage (HVAC) into DC line voltage (HVDC) and (2) a DC-DC converter to convert the HVDC into a low DC voltage (LVDC) with typically one controller. The off-line converter is designed to be a switch-mode voltage converter, such as the 110 VAC-3.6 DC battery charger for portable electronics. As seen, HVAC is rectified by the rectifier 11 and delivers the line voltage of HVDC. The DC-DC converter in FIG. 1 uses a flyback converter which converts the HVDC into a low AC voltage through the transformer T with the periodical turn-on and turn-off of the switch Q. The low AC voltage is rectified by D and filtered by Co into a low DC voltage LVDC which supplies the load such as a battery. The switching of Q is controlled by the controller.
In operation where the DC-DC converter functions normally, the controller is powered from the DC-DC converter, typically from an auxiliary winding L3 shown in FIG. 1 with a low DC voltage rectified by a rectifier D1. However, during startup of the converter, before the onset of the switching action of the switch Q, there is no power delivered from the auxiliary winding L3, and the switch Q enters into normal function only after the controller is powered and controls Q. One traditional approach to power the controller before normal operation is to get power from the high line voltage HVDC through a voltage divider comprising two resistors. HVAC can be 110 VAC, 220 VAC or other levels in applications, thus the rectified HVDC can reach up to about 700V in some applications. HVDC itself is too high to power the low voltage controller thus it needs a leaker device to reduce the voltage and to form a low current source first, such as adopting a resistor R with high value to divide the HVDC line voltage. But this approach needs extra component of high resistor R which increases the size and the cost of the system. For that reason, it is desirable to introduce a current leaker or, as formally called, a current source to supply the startup power. While it is possible to include a HV leaker device on the same substrate as a low-voltage controller, such inclusion requires expensive high-resistance substrates as well as several more masking stages in the process of manufacture, thus increasing the costs of the controller.