Output Line Ripple on Power Factor Corrected AC-DC Power Supply Outputs (Part 8)

This is the eight 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.

What about the loss of two phases?

If the three phase power source is connected in a delta configuration, you cannot lose two phases. You go from one phase lost to all power lost. In a wye configuration, however, it is possible to operate with return current provided by the neutral conductor.

There is good reason to stay with a delta configuration. You need not worry about the long term operating conditions as the single phase converter tries to struggle with the three times the normal intended load conditions.

Yet, if you have a load that only requires higher power levels for a few modes of operation, it might make sense to run with only one phase present, have the controller detect that condition, and refuse to go into the few high power modes that would tax the power train.

In that case, you still need to consider the ripple current through the bulk capacitor, but the power level may be lower than full three phase power.

At first thought, because two phases are at 120 degrees to each other, there is a strong hunch that a sqrt(3)/2 factor should be involved as a multiplier going from single phase to two phase ripple currents.

A little deeper thought shows that not to be the case. As shown in the previous section, the current through the bulk capacitor is equal to the current of the missing phase current, which is also equal to each of the other two balanced phase currents.

If one of those two working PFM converters were to stop operating, the one remaining would need to increase its output level to 3x the original current, or 2x the 1.5x current when the first phase removed.

When this occurs, the bulk capacitor needs to handle the entire ripple that the single phase creates by PFC action, the same as single phase operation.

The net effect is the simple result: when you go from two phase operation to single phase operation, the ripple current of the bulk capacitor doubles.

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