Output Line Ripple on Power Factor Corrected AC-DC Power Supply Outputs (Part 3)

This is the third in a series of blog posts by Harry Vig, Principal Applications Engineer at Vicor. To go to Output Line Ripple on Power Factor Corrected AC-DC Power Supply Outputs (Part 1) click here.

Effect of Load Type on Bulk Capacitor Ripple

The three load types are constant current, constant resistance, and constant power.

A constant current load will always draw the same current as the voltage on the 48V bus rises and falls. A typical example is LED lighting. The output brightness varies with average current through the LED, yet because of the diode terminal characteristics, the output voltage will vary much less as the current is changed. This makes it easier to control brightness using the current as the control variable, and reject changes in the output voltage (like reflected input ripple).

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Figure 5 – Constant Current Load

Constant resistance loads increase the amount of current they draw when the applied voltage is increased. Typical constant resistance loads are heaters, incandescent lighting, pullup resistors, and CMOS integrated circuits.

A constant resistance load will reduce the ripple voltage as compared to a constant current load. As the output voltage sags, the current demanded by the load will decrease, reducing the amount of discharge at the bottom of the cycle. Likewise, the load current will increase at the top of the cycle, reducing the amount of excess current near the peak that the bulk capacitor needs to store.

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Figure 6 – Constant Resistance Load

A constant power load, on the other hand, will cause the opposite effect. When the output sags near the zero crossing of line input voltage, a constant power load draws more current. This pulls more charge out of the bulk capacitor when it is at its lowest voltage, and pulls less current out of the bulk capacitor when is at its highest voltage, exasperating the disturbance that causes the line ripple.

A constant power load is how a downstream voltage regulator looks to its power supplier as it tries to keep its own output bus constant. It supplies the same output voltage and output current to its load, so the only change seen in power supplied to the downstream regulator is due to a change in efficiency as the input voltage varies.

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Figure 7 – Constant Power Load

Unfortunately, the most common type of load is a regulating power supply that is being fed by an upstream PFM, which results in the highest output ripple.

In the system below, not only are PRM regulators constant power loads as seen by the PFM converter, but the ZVS Buck regulators are a constant power load to the upstream PRM regulators.

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Figure 8 – Typical Four-Output, 300 W Application

 

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

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