Reducing Wasted Startup Current in Offline Supplies

by Christophe Basso, Application Manager, ON Semiconductor, Toulouse, France, Power Electronics Technology, Aug 01 2004

Focus:
A resistor connected to the bulk capacitor (the high-voltage rectified rail) typically charges up a VCC capacitor to power a PWM controller chip at startup. This approach wastes power during normal operation, but a small startup supply built with discretes dramatically reduces this power consumption.

What you’ll learn:

• How to calculate the value of the startup resistor used to initially power a PWM controller
• How to build a power-saving start-up power supply for a PWM controller using a bipolar transistor, zener diode and a few other discretes.

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