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[FAQ] PMP20946: Why is the LCC topology used as the isolated DC-DC stage of the PMP20946 design?

Part Number: PMP20946

Why LCC topology is applied as the isolated DC-DC stage of the PMP20946 design? Where should I pay extra attention to on using the UCC25630x LLC controller for wide output voltage power supply design like PMP20946?

  • PMP20946 is an AC-DC power supply reference design targeting LED lighting applications. Power supplies for lighting applications generally need to have pretty wide output voltage range. In PMP20946 case, the output voltage range is in 100V to 200V range. Some LED lighting applications requires even wider output voltage range. Resonant topologies become popular in power supply industry because of its benefits of soft-switching and high efficiency. However, most resonant converters (like the popular LLC resonant converter) can’t handle wide output voltage range without sacrifice converter efficiency. Therefore, we use LCC instead of LLC resonant converter to handle wide output voltage range while still maintain decent efficiency.
    UCC25630x implements hybrid hysteretic control (HHC) by taking resonant tank current at primary side to improve response performance. The sensed resonant tank current signal feed into VCR pin of the controller along with internal compensation currents to compare with two voltage thresholds generated by feedback network for fast transient response. This is analogous to current mode control in PWM converters. Using UCC25630x HHC in wide output range converter design like PMP20946 generally requires to have lower percentage of resonant tank current signal (compare to the internal compensation currents). That is why the VCR top capacitor (C210) is relatively small in PMP20946.