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A Novel Soft Switching ZVS, Sinusoidal Input Boundary Current Mode Control of 6-Switch Three-Phase 2-Level Boost Rectifier for Active and Active + Reactive Power Generation

Image of phase currents in soft switching TCM.
Fig. 1. Phase currents in soft switching TCM (a) UPF; (b) 30° phase lag; and (c) 90° phase lag.
The switching frequency of three phase converter systems in continuous conduction mode (CCM) is limited by the switching loss of devices. The major contribution to these switching losses is from turn-on losses, since turn-off losses of wide bandgap devices (silicon-carbide and gallium-nitride) are significantly lower. However, varying the switching frequency so that the converter always operates on the boundary of continous conduction mode (CCM) and discontinuous conduction mode, also referred to as boundary conduction mode (BCM) or critical conduction mode (CRM) helps achieve a zero voltage switching (ZVS) turn-on. ZVS turn-on is achieved in BCM by making the body diode conduct before the device is turned on, thus necessitating a bidirectional current in one switching cycle.

Conventional CRM for a three phase single switch boost power factor correction (PFC) does not generate sinusoidal average input current. This issue has been addressed for a Vienna rectifier (VR) operating as a PFC, and it is shown that sinusoidal average input current can be achieved by adding one more switching state in between three switching states of conventional CRM. The three phase 6-switch boost converter has lesser semiconductors and thus lower losses as compared to VR. However, the above modulation cannot be directly applied to a three phase 6-switch 2-level boost converter.

A new BCM/transition conduction mode (TCM) control with different switching states, a complete ZVS turn-on, and sinusoidal average input current for three phase 6-switch 2-level boost PFC is presented in this paper. This modulation is further extended for varying power factors. Two cases, active and reactive power, are elaborated. The basic idea of TCM in unity power factor (UPF) mode comes from expressing average phase currents as directly proportional to phase voltages as shown in Fig. 2 (a). The reactive power generation case can be expressed as the sum of two voltage phasors as shown in Fig. 2 (b). The loss breakdown comparison between CCM and TCM for UPF mode is shown in Fig. 2 (c).

Image of phasor average currents and loss comparison of three phase 2-level converter between CCM and TCM in u.p.f. mode.
Fig. 2. (a) Phasor average currents for u.p.f. mode (b) Phasor average currents with a phase lag (c) Loss comparison of three phase 2-level converter between CCM at 50 kHz and TCM at 1 MHz in u.p.f. mode.