Output Line Ripple on Power Factor Corrected AC-DC Power Supply Outputs (Part 1)

This is the first in a series of blog posts by Harry Vig, Principal Applications Engineer at Vicor.

Line Ripple on the Output Voltage Bus

Vicor FE175 and PF175 AC power supplies are universal AC-DC converters with power factor correction.

As a result of that power factor correction, because we want to draw current proportional to the rectifier input voltage, the output DC bus voltage has ripple at twice the line frequency. Doubling of the line frequency occurs because of the input rectifier.

The oscilloscope graph below illustrates that when the input voltage is highest in the half cycle, more power is drawn to charge the bulk capacitor. When the input voltage is lower, the load current on the 48V output bus exceeds the rate at which charge is replenished into the bulk capacitor, and the bus voltage drops. During the very lowest parts of the input power cycle, the converter is even turned off to improve efficiency by avoiding switching losses when little power is processed.

The waveforms look like this with 230V 50Hz input, 330W constant current load and 6800uF bulk capacitor:


Figure 1: Output ripple with 230V 50Hz input, 330W constant current load and 6800uF bulk capacitor

Channel 2 has the rectified input sine wave, and it shows that the converter switched off as the input crossed the 125V threshold. With the converter turned off, the rectified sine wave does not discharge further.

Even after the input rises from the 2.54ms long flat region, there is another 0.4ms delay until it turns on and starts processing power in the next half cycle.

The dark blue waveform shows not only the line ripple, but the switching ripple as well. It confirms that the converter is off when the input voltage is low not only by the fact that the bulk cap is discharging, but also because the switching ripple is visible during the period when it is switching. It can be seen that the power processed from AC line input to the DC output is not sufficient to keep the bus voltage up, and it confirms that little output power is gained during the period of time that the converter is not switching.

Read part two of this series: A Simple Average Model for Output Current Ripple.

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