SLUSBL5A February 2015 – June 2019 UCC28730
The extra-low operating frequency capability and minimal bias power of the UCC28730, in conjunction with its companion micro-power wake-up device UCC24650, allow for the achievement of less than 5-mW input stand-by power consumption under no-load conditions. This is often referred to as zero-power stand-by.
Assuming that no-load stand-by power is a critical design parameter, determine the estimated no-load input power based on the target maximum switching frequency and the maximum output power. The following equation estimates the stand-by power of the converter.
For a typical flyback converter, ηSB may range between 0.5 and 0.7, but the lower factor should be used for an initial estimate. Also, fMIN should be estimated at 3x to 4x fSW(min) to allow for possible parameter adjustment.
If the PSTBY calculation result is well below 5 mW, there is an excellent chance of achieving zero-power stand-by in the actual converter. If the result is near 5 mW, some design adjustment to fMAX, fMIN, and ηSB may be needed to achieve zero-power. If the result is well above 5 mW, there is little chance to achieve zero-power at the target power level unless additional special circuitry and design effort is applied.