Application Note Overview 

Bias power supply circuit designs for offline AC/DC power supplies are critical because of their impact on the power supply startup sequence. This sequence takes a relatively long time because bias capacitors are charged with very low constant current from the rectified pulsating DC voltage source. After the capacitors have been precharged and the bias supply exceeds the internal turn on threshold, a controller can start switching operations. An auxiliary winding supplies bias voltage after a few milliseconds. This auxiliary winding improves converter efficiency in normal operations. However, since bias capacitors supply only limited energy after constant current charging is completed, an under voltage event may occur before the switching operation can supply sufficient power through the auxiliary winding. Measuring input voltage, DC bias voltage, PWM signals and output voltage is vital when validating overlapping converter functions. Any anomalies such as incorrectly activated status signals must be detected in the long startup sequence when the current source is active. This is challenging and requires a high sampling rate and sufficient vertical resolution over hundreds of milliseconds.