２．The operation and feature of the secondary phase-shifted PWM control scheme

　The high-frequency AC link single-phase DC-AC converter is composed of the high-frequency inverter, the high-frequecny transformer and the cycloconverter connected with secondary side. Its feature is to output the commercial power by performing frequency conversion directly using cycloconverter circuit at the secondary side. Since this system does not include conversion into DC power by the diode rectifier circuit after the high-frequency power converted by high-frequency inverter insulates using high-frequency transformer, the power flow can be enabled the both direction processing. In this circuit system, commonly, the output voltage is controlled by symmetrical control scheme. The circuit configuration of the high-frequency AC link single-phase DC-AC converter employing the symmetrical control and that switching pattern are shown in Fig.1 and Fig.2 respectively.

　The conventional scheme has some inherent problems. For instance, the reactive power will be continued to regenerate in order to continue the load current io to flow through on D2,D3,S5,D5,S8,D8 and the high-frequency transformer after S1,S4 are turned off at time t1 with ZVS operation. As a result, the conduction losses of the power devices and high-frequency transformer are increased and conversion efficiency is deteriorated. On the other hand, in the case of the light-load or no-load, or reactive power secures insufficiently by condition of power factor, the loss-less snubber capacitor can not be charged (or discharged) by lagging current because the load current own becomes small. As a consequence, the ZVS operation can not be realized. Hence, the high-frequency AC link single-phase DC-AC converter employing symmetrical control scheme is effective with respect to specific load such as the fixed load. But if the load has a wide range from full-load to no-load or the power factor becomes nearly 1, it is difficult for this schem to apply to DC-AC converter. Therefore, the novel proposal is necessary to be able to keep up with these requirement. Accordingly, we have proposed the novel high-frequency AC link DC-AC converter employing the secondary phase-shifted PWM control scheme.
The circuit configuration of the high-frequency AC link single-phase DC-AC converter employing the secondary phase-shifted PWM control scheme is similarly to the conventional scheme shown in Fig.1. The switching pattern at the secondary phase-shifted PWM control is shown in Fig.3 .

　In the secondary phase-shifted PWM control scheme, S6 is already turned on before S1,S4 is turned off. As a result, the load current io is commutated from S5,D5 to S6 D6 because V1 is a forward bias in D6 and a reverse bias in D5. In this reason, since the load current io will start the circulation only secondary side, there is almost no flow of i1,i2. This state is the single circulation interval. At this point, since the reactive power will not be also regenerated, the charge and discharge of loss-less snubber capacitor will be performed as the component of the reactive power with electromagnetic energy stored in leakage inductance of high-frequency transformer. However, the coupling coefficient of high-frequency transformer used such as this system is large and it can not be secured the electromagnetic energy which is enough to charge and discharge the loss-less snubber capacitor only a leakage inductance. Therefore, that deficiency is supplemented by connecting the auxiliary inductance La,Lb. Moreover, in the single circulation interval without supply or regeneration , since the active power and the reactive power of the load are processed only secondary side, there is no current on power devices of high-frequency inverter and transformer. Hence, the conduction losses generating on them can be reduced. For these reasons control scheme becomes resonant pole type circuit configuration, the fixed losses become small and the conduction losses can be reduced comparing with the symmetrical control scheme. Moreover, this scheme have many merits which it can be performed the ZVS stated above, though the secondary phase-shifted PWM operation with reliability to every load of the power factor and may be effective to apply for high-frequency AC link single-phase DC-AC converter. The operating mode of the high-frequency AC link single-phase DC-AC converter can be classified into two mode. One is inverter mode which will supply the power from primary side to secondary side by the power factor of the load, the other is rectifier mode which will regenerate the power from secondary side to primary side. The operation of inverter mode becomes similarly to the high-frequency AC link DC-DC converter in order to correspond with output voltage and current.[8] So, we will explain the operation of high-frequency inverter period in rectifier mode which will be become inverse polarity between output voltage and current and will be regenerated the power from secondary side to primary side.

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