Accurate verification of power supply startup sequences for auxiliary bias supplies

Your task

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.

Rohde & Schwarz solution

The R&S®MXO 5 series oscilloscope is ideal for this task because it can measure details at a high sampling rate with slow timebase settings thanks to the large standard memory of 400 Mpoints.

The 12 bit ADC vertical resolution provides greater detail of measured voltage levels when evaluating the turn on/ turn off thresholds for bias voltage. In combination with the highly sensitive digital trigger, distant trigger events can be captured to precisely catch a critical overlapping event when the regulator starts a switching operation after several hundred illiseconds.

The zoom function can be used to view details from the high sampling rate of the PWM pulse. Fig. 1 shows a typical startup sequence.

Measuring the startup sequence

After arranging the setup, the AC power source must be switched on to start the measurement. As soon as the trigger detects the minimum turn on threshold for the bias voltage, the waveforms will be displayed in the screenshot (see Fig. 2). The top window reveals the entire sequence and shows the delay between input and output voltage (channels 1 and 4). The duration here is 585 ms and the time the power supply needed before entering steady state operation can also be seen. More signal details can be seen with the zoom function. Moving the cursor over the image reveals the maximum turn on threshold for the bias voltage (channel 2) which is 17.4 V and the turn off voltage of 10.6 V. This level exceeds the critical data sheet value of 10.4 V, ensuring that the bias voltage supply from the auxiliary is fast enough.

The PWM pulses (channel 3) illustrate how the controller works in detail. Another zoom window of the same measurement data highlights the PWM pulses in the converter steady state region (see Fig. 3). Moving the cursor over the area reveals the frequency for the steady state. The zoom can be adjusted for other important points in the sequence.