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Method for Increasing ac-dc Voltage Gain of Three Phase Critical Conduction Mode Converter

Converter schematics
Fig. 1. Converter schematics and triplen harmonics injection concept of a three-phase critical conduction mode DC/AC converter with decoupling capacitors.
Increasing power density as well as efficiency, has always been the major concern in power electronics technology development. With the use of wide bandgap power semiconductors, operating a capacitive decoupled three-phase converter (shown in Fig. 1) in critical conduction mode, could largely boost converter power density and efficiency. However, due to the decoupled three phases, the ac-dc voltage gain is reduced by at least 15 %. An example of this is shown in Fig. 1, where a dc to ac converter is shown; the gain is defined as VANÂ’, rms / VDC. Compared to conventional pulse width modulation (PWM) modulated three phase converters, this largely limits the adoption of this type of converter. This work provides a way for the capacitive coupled three-phase converter to maintain similar ac-dc voltage gain comparable to conventional PWM modulated three-phase converters. A controlled triplen order harmonic current is injected into the decoupling capacitors (Cf in Fig. 1). This induces triplen order harmonic voltage to be superimposed on the fundamental frequency voltage, which allows for increasing ac-dc voltage gain in a similar manner as a space vector modulated three-phase converter. The triplen order harmonics currents in the filter inductor Lf flows entirely through the decoupling capacitors Cf, as shown in Fig. 1. By controlling the triplen order harmonics in Lf, the triplen order harmonic current injected into Cf is directly controlled, and the triplen voltage superimposed on the capacitor fundamental voltage is controlled according to Eq. 1. CfdV3rd/dt=I3rd (1) With properly superimposed triplen order harmonics, peak capacitor voltage can be reduced to 85 % of the non-superimposed voltage, as shown in Fig. 2. This allows higher ac-dc gain to be achieved (which may also refer to dc voltage utilization). A detailed theoretical derivation for this concept can be found in the slides submitted along with this document. Using this technique, the capacitive decoupled three phase converters, which achieve higher power density and efficiency, become good replacements for conventional PWM modulated three-phase converters. The target applications of this type of converter include telecom power supplies, server power supplies, aircraft power supplies, uninterrupted power supplies, motor drives, and other applications where three-phase ac to dc or dc to ac power conversion in kW range is required.
Simulation waveforms
Fig. 2. Capacitor voltage waveforms with and without superimposed triplen order harmonics. With properly superimposed triplen order harmonics, an increase of 15 % AC/DC voltage gain could be achieved.
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