Valley Detector for Fixed Frequency Resonant Converter

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SiCEngineer

Joined May 22, 2019
209
I have been playing around with ways to have a resonant converter, while operating it at a fixed frequency for my application. I have heard of valley detector circuits before, mostly in quasi-resonant flyback topologies.

Could someone explain what the purpose of these circuits are, and how they allow fixed frequency operation of the resonant converter down to zero load conditions? What are the main building blocks for a valley detector?

I understand the main idea is to detect when the voltage is at a minimum across the switch, and then drive the MOSFETs according to that. I have included some patents which show this used in half bridge resonant converters, but I cannot wrap my head around why the frequency stays constant considering load variations will affect the impedance of the tank.

If I was to attempt this method for my design, is there a simple way to implement it in LTSpice? Does anyone have an .asc file of a valley detector, or know where I could find one to see how the theory lines up with the simulation?

Here is an example patent of this method in a flyback converter: https://patents.google.com/patent/US8040119 and a half bridge resonant converter: https://patents.google.com/patent/US7948775B2/en/.
 
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