# Setting correct operating frequency for LLC resonant SMPS

I need to figure out if an LLC half-bridge resonant SMPS is designed (more or less) correctly. I am not an SMPS designer, and neither is the manufacturer, so I'm just reading a bunch of app notes and learning what I can. It's producing ±35 V 400 W from a constant 400 VDC PFC output.

As I understand from my reading:

1. The resonant frequency is a property of the LLC transformer network, increasing with load from fp (unloaded resonant frequency) towards f0 (shorted resonant frequency).
2. The switching frequency is controlled by the driver IC, and decreases with load to regulate the output by riding up the resonant gain peak, getting closer to resonance and increasing the output as the load increases.
3. The switching frequency must never drop below the resonant frequency at any given load, or it will enter capacitive ZCS mode, which is Bad.

Is this all correct?

They've set the switching frequency to always be below f0. They claim this is ok, because Fairchild AN-4151 says you can do above or below resonant operation:

"Below resonance operation is preferred for high output voltage applications, such as Plasma Display Panel (PDP) TV ... above resonant operation ... can show better efficiency for low output voltage applications, such as Liquid Crystal Display (LCD) TV or laptop adapter"

Would 70 V out be considered low or high voltage in this case? I'm guessing high voltage means hundreds of volts, so this is low voltage. So does that mean it would ideally be operating always above f0? Maybe as long as point 3 is met (ZVS mode), it's not too important? This article says:

Most of these converters are operated above resonance so that at light-load conditions the switching frequency is higher than the resonant design points (region 1) [fs > f0]. As load increases, the switching frequency droops. At maximum load, the converter operates slightly below the LlkCr resonant point [fs < f0] in region 2 [but should never enter region 3].

The Q and M curves show that the resonant frequency is a property of the transformer, the load and the series capacitor. $f_o$ isn't necessarily a 'shorted' resonant frequency but a mostly-load-independent second resonant point predominantly controlled by the leakage inductance and series capacitor.