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Series-Capacitor Buck Converter with Soft Turn-On for Datacenters

Converter schematic
Fig. 1. Resonant series-capacitor buck (RSCB) converter
  Interleaved multiphase buck converters have been adopted for point-of-load regulation below 3.3 V and above 10 A from input voltage exceeding 12 V. The series-capacitor buck (SCB) converter was synthesized by adding a series capacitor in a two-phase buck converter to reduce voltage stresses under steady state. Side benefits include doubling of the duty cycle, reduction of the switching loss, and equalization of the phase currents. Hard switching has hindered efforts to reduce volume via increasing the switching frequency, although a monolithically integrated SCB converter has boosted current density exceeding 60 A/cm3 in. The efficiency dropped by 5%, owing to the switching loss, as the frequency increased from 1 MHz
to 3 MHz.
  The resonant series capacitor buck (RSCB) converter with soft turn-on is synthesized by adding a parallel resonant tank next to the series-capacitor Cs, as demonstrated in Fig. 1. All switches turn on into zero-voltage (ZVOn), and low-side switches turn off from zero- current (ZCOff). No snubber is needed to keep the switchesÂ’ stresses within the input voltage. A 2 MHz prototype with 48 V at the input, and 7V - 20 A at the output, was built to verify the design.
  The peak efficiency of the RSCB prototype is 98.5%, and the full load efficiency is 97.3%, as shown in Fig. 2. The measured drain- source voltages of all switches at nominal conditions are shown. All switches turn on into zero-voltage.
Efficiency curve and waveform
Fig. 2. Efficiency and experimental waveforms of the prototype
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