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Soft-Switching Buck Converter with Quasi-Resonant Operation

Fig. 1. Single-phase self-driven soft switching Buck with very small Cb.
The single phase self-driven soft switching Buck with main switches ZVS turn on, duty cycle extension and self-driven of SR can reduce switching related loss dramatically. But the main switches of this converter still have large turn off loss. In order to reduce turn off loss, a quasi-resonant operation is proposed for this soft switching buck converter. If a very small capacitor Cb is used in this converter, the Lr and Cb will become a resonant tank, and a resonant period will be introduced. By carefully design resonant frequency of Lr and Cb, the turn off current of Q1 can be controlled to be very small to reduce turn off loss. A four-phase 700kHz 1.2V/130A output VRM prototypes are built to verify this concept. The experiment results show that the proposed converter can achieve almost same full load efficiency with the 12V self driven. In addition, it has single phase option to further increase light load efficiency. Therefore, the new topology is more flexible and can achieve higher efficiency under not only full load but also light load, which is important for POL and VR applications.

Fig. 2. Hardware of a 4-phase three-switch quasi-resonant buck converter.
Fig. 3. Key waveform of three-switch quasi-resonant Buck converter.
Fig. 4. Efficiency of three-switch quasi-resonant Buck.
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