I am currently designing a SMPS based on the half-bridge LCC converter. Due to my application, it is desired to operate the converter at a fixed frequency. I also wish to optimize the passive components, which is not possible at variable frequency. Since I can't regulate through frequency modulation, here are some of my alternatives:
Use a pre-regulator. A buck or a boost converter before the LCC converter can regulate the input voltage to the half bridge according to Vout, hence achieving regulation. This is simple, but we need extra active switches, inductors etc.
"Bang Bang" control - turning the switches ON or OFF depending on the output voltage. Seems like a simple technique, but from what I understand, it still modulates through frequency while keeping duty cycle and phase constant.
Hybrid hysteretic control - With LLC converters, new IC's are available that use the resonant capacitor current to modulate the switches ON and OFF. Again, this uses frequency modulation to achieve output voltage regulation. I like this method as it reduces the second order, non-linear system to one that is first order and easier to control as a result.
Are there any other ways that can be used to achieve hysteretic control while maintaining a fixed frequency? I have seen some research use a phase-locked loop to "lock" the hysteretic window to a fixed frequency deviation. Could this be added to standard HHC control IC's to manipulate the hysteretic window?