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Resonant Switched-capacitor Converter with Multiresonant Frequencies

Converter diagram
Fig. 1. 2 to 1 Multi-Resonant Switched-Capacitor Converter
Due to the increase in data processing, data center servers require high performance multi-core CPU and GPU installations that increase power consumption. This higher power consumption induces higher conduction loss along the bus and degrades whole system efficiency. Many academia and industry members have proposed intermediate bus architecture using a 48 V distribution system in order to reduce conduction loss along the bus. Google and North Dakoda University expanded the concept of a resonant switched-capacitor converter (RSCC) introduced by Shoyama in 2004 into a higher voltage conversion ra-tio (VCR) DC transformer termed a switched-tank converter (STC). By operating switching frequency (fsw) at an exact resonant frequency (fo), a STC utilizes a zero-current switching (ZCS) soft-charging mechanism to achieve high efficiency. Regardless of ZCS, drain to source capaci-tance (Coss) loss persists. ZCS operations of a STC requires stable resonant frequency. Conse-quently, class I ceramic capacitors, such as U2J dielectric capacitors, are appropriate. However, any component comes with tolerance which will affect the performance of STC unless the com-ponent it utilizes zero current detection control. Derived from a resonant switched-capacitor converter (RSCC), a multi-resonant switched-capacitor converter (MRSCC) adds a small capacitor (Cr) in parallel to the resonant inductor (Lr) to form a high frequency resonant tank. By operating at fsw > fo, the MRSCC provides immunity towards resonant frequency variation of the tank. Lr resonates with Cr for half-period during dead-time (td) to reverse the current direction of Lr before the next conduction state starts, as shown in Eq. 2. By doing so, the MRSCC reshapes the inductor current to more like a square-wave like and further reduces conduction loss. Although losing ZCS, the MRSCC experiences has similar switching loss as a RSCC as and determined by Coss charging and discharging loss. By achieving high immunity tolerance towards component variation, the MRSCC allows utilization of a class II capacitor, such as an X7R capacitor, in order to reduce cost. Similar to a RSCC, the MRSCC is expandable to higher VCR topology, such as Dickson star topology. As a result of having two resonant tanks, a STC must always have two exact resonant tanks. But due to operating a non-ZCS mechanism, the MRSCC need not have the exact same resonant tanks. In conclusion, due to immunity towards variation, the MRSCC reduces produc-tion cost by using class II capacitors and a simple control mechanism (no zero current detection).
Operational waveform
Fig. 2. MRSCC Operational Waveform
Fig. 3. Equations
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