２．Secondary Phase-Shifted PWM Control Scheme

The circuit configuration of the high-frequency AC link DC-DC converter using secondary phase-shifted PWM control scheme (below called "proposed control scheme") is shown in Fig.1. This circuit is composed of the high-frequency inverter, the high-frequency transformer, the diode rectifier circuit and the LC filter. A difference between conventional DC-DC converter and proposed DC-DC converter is two power devices are newly connected inside the diode rectifier circuit. Fig.2 is shows its switching pattern, inverter output voltage waveform and output current waveform. In conventional control scheme, the converter output voltage was controlled by giving phase-difference between the right and left arms of the full-bridge inverter on the primary side. In proposed control scheme, the converter output voltage is controlled by giving phase-difference between the primary side and the secondary side which is synchronized with the primary side. From this reason, between the right and left arms of the full-bridge inverter don't have phase-difference. Namely, it operates as a square-wave generator like a symmetrical drive with 50% duty including dead time. By using proposed control scheme, secondary switches S5 and S6 are both off during the circulation interval (t1<t<t2) in which the power is not supplied form the primary side. At this point, the circulating current will flow only the secondary circuits via the high-frequency transformer. Therefore, the circulating current can not flow on the primary side (this state is called "the self circulation interval"). As a result, almost conduction losses are eliminated because circulating current can be removed. This is the distinctive feature of proposed control scheme. In addition to this, all power devices can operate under soft-switching condition, independent of changing load resistance. From these reasons, the conversion efficiency become high compared with conventional DC-DC converter because proposed control scheme can effectively solve the problems mentioned above.

Fig.1. Circuit configuration of high-frequency DC-DC converter

Fig.2. Switching pattern of secondary phase-shifted PWM control scheme

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