Eliminate The Guesswork When Selecting Primary Switch VDD Capacitors

by Ed Wenzel, STMicroelectronics, Schaumburg, Ill., How2Power Today, May 27 2011

Focus:
A primary switch, used for off-line applications, often contains a controller and a high-voltage MOSFET. The engineering documents of primary switch families state that the capacitor at VDD must be sized according to the time needed to start-up. Some provide a formula for the capacitor at the VDD pin to ensure start-up, and this formula which may contain an undefined start-up time, tss, which depends on many parameters. In this article, the author derives an equation to determine a capacitor value at VDD using known circuit component values and parameters from one primary switch specification. The derivation begins with a calculation of the time it takes to charge up the output capacitors, tss. We then use this parameter together with the required VDD switching current (IDD1) and VDDhyst to determine the value for the VDD capacitor. Two versions of this CVDD equation are derived-one for a power supply with a known number of transformer turns and another that can used for an estimate when the turns ratio is unknown.

What you’ll learn:

• How to calculate the primary switch VDD capacitor value in offline power supply designs

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