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May 15, 2001

Synchronizing Two Converters

I have few doubts regarding synchronizing a PFC circuit with the down stream PWM converter. J. J. 05/15/2001

Question: Why is synchronizing necessary?

Answer: To avoid two drifting switchers from causing beat frequencies that cause EMI and other problems. These beats can be low frequency noise that is impractical to filter, so it is best to avoid them when possible. Also the noise spikes can drift and periodically become additive, giving EMI spikes that are hard to see and hard to fix. This is especially troublesome in digital systems where the noise may cause a random bit change for no apparent reason.

Question: When we do so it is a common practice to select PFC RT & CT values for a lower frequency (typically 20% less than PWM designed frequency). Please explain why?

Answer: Because you can terminate a cycle before it is complete but not after. The synch signal always has to be higher than the free frequency. Note that if magnetics are involved that can ratchet up the hysteresis curve and saturate, synchronization is often at half (or twice) the frequency in order to keep imbalances from saturating the magnetics.

Question: What frequency should we consider when we design the circuits & magnetics of PFC (the frequency defined by RT & CT or the PWM frequency to which PFC is going to be synchronized)?

Answer: Both. You do not want the magnetics to saturate at the lower frequency, but you probably want to optimize efficiency at the normal operating frequency.

Question: In many of the PFC application notes a voltage doubler kind of circuit (with two diodes & two capacitors) is used to derive the VCC from the auxiliary winding on the boost inductor. Why is this preferred instead of a simple rectifier/capacitor combination?

Answer: Don't know off the top of my head, but you have to consider abnormal conditions such as start-up, max-min duty cycles, etc. and one may be better than the other for any given application and design criteria (minimum cost, maximum reliability, etc.) Also the vendor may be selling voltage doubler circuits.

Posted by Jerrold Foutz at May 15, 2001 07:31 PM