The quasi-square voltage at the output of full-bridge inverter may contain a DC component due to unequal voltage drops, rise and fall times of the switches. The series capacitor Cs also acts as a blocking capacitor and therefore prevents the saturation of the output transformer. This feature greatly simplifies the control circuit requirements. The only disadvantage of the series converter is that the output DC filter capacitor carries high ripple current (Fig. 18). This makes the converter less attractive for low voltage, high current applications. 4.4.2 Parallel-resonant Converter. The main advantage of the parallel resonant converter is that the output DC filter capacitor does not carry high ripple currents. This is because in the parallel resonant converter, output diodes rectify the voltage (not current as in case of the series converter). This makes it possible to use an L-C output filter in which the inductor limits the ripple current of the output capacitors. This feature is highly desirable for low voltage, high current applications. Another advantage of this converter, as discussed in Section 3.4, is that it has a self current limiting capability against an output short circuit. The main disadvantages of this converter are poor natural output voltage regulation and reduced-load efficiency. These limitations inhibit the use of this converter for relatively constant output loads. 4.4.3 Series-Parallel Resonant Converter. Both series and series-parallel converters have load independent output voltages. The main advantage of the series-parallel converter as compared to the series converter is that the DC output filter capacitor does not carry high ripple currents. This makes the series-parallel converter better suited for low voltage, high current applications than the series converter. For the same application, the series-parallel converter has lower no-load current and volt-ampere rating of the switches than the parallel-converter. The only drawback of this converter, as compared to the parallel-converter, is that it is not naturally output short-circuit-proof. However, as explained earlier in Section 3.4, it is relatively simple to make the series-parallel topology short-circuit-proof. Therefore, this converter has all the advantages over parallel topology. As compared to the series converter, the series-parallel converter has higher no- load current (no-load current is zero in series converter) and component count. This results in lower efficiency and higher total volt-ampere rating of the components than for the series converter. In fact the only advantage of series-parallel converter over series converter is the low ripple current rating of output filter capacitor. Therefore, the application of series-parallel converter is reserved only for low voltage, high current outputs. 5 AC/DC Converters In an advanced high frequency, high power space power distribution system, the AC/DC converters are employed to convert high frequency AC voltage to conditioned DC output voltage at a given power level. Some of the main design objectives for these converters are: (1) High efficiency. (2) Close to unity power factor. (3) Close to sinusoidal input current. (4) Low mass and volume. (5) Controlled output voltage. (6) Soft starting (input surge current).
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