LLC topology driving methods and output inductor

[AstRii]

Hello,

I've recently discovered LLC switch mode power supply topology which I have studied ever since, however I have a few questions for which I have not found an answer or I have failed to understand:

LLC converters seem to be always using a frequency modulation to set the output power, however the efficiency drops when the converter runs away from the resonance.
Wouldn't it be better to always drive it in resonance but skip switching pulses to module output power? I believe this method is called "bang-bang" control (please correct me if I'm wrong).
Or perhaps always drive it in resonance but limit the duty cycle of the transistors?

What would be the advantages/disadvantages or these control methods?

I have also noticed the LLC power supplies do not use any output inductors as most power supplies do.

They just rectify the output from the transformer and feed that directly to an output capacitance.
Doesn't this yield a poor power factor? Wouldn't current flow just near the peaks of the secondary voltage (also leading to big output ripple)?

One other thing I have failed to understand is how phase shifted full bridge achieves ZVS.

I will be thankful for any answers

[Konkedout]

I think your questions require answers that would be at least a long chapter in a textbook; more than most forum participants would want to write.

I have only had brief exposure to LLC converters but I think that your missing piece may be some controlled amount of transformer leakage inductance.

[johansen]

I think what textbooks fail to teach is that to transform a range of voltages into another voltage requires energy storage.

LLC converters just store that energy in a resonant state with an inductor, in order to achieve better switching transitions, so when you change the frequency in order to change the power transfer, or the output voltage, the switching transitions become more difficult because the turn on and turn off current has increased (perhaps significantly) from the ideal situation in which ZCS or ZVS or both is the intended full load condition.


Pulse skipping at full resonance condition would still involve weird start up conditions which might destroy any efficiency improvements. Also the output would have low frequency ripple..