A number of DC/DC converter topologies have been developed (Fig. 16) to satisfy the input and output characteristics for a given application. At moderately high power levels (approximately greater than 1 kW), the resonant mode bridge type converter topologies satisfy the input and output requirements and meet the above design objectives. Since high power DC/DC converters will be required in near future space applications, the resonant mode bridge type topologies are only considered within the scope of this paper. 4.2 Control Techniques The following two basic techniques are normally employed in DC/DC resonant power processors: (1) Variable frequency control. (2) Pulse width modulation control. 4.2.1 Variable Frequency Control. In this type of control the output voltage of a DC/DC resonant converter is controlled by varying the operating frequency of the converter with respect to the resonant frequency of the resonant circuit. A comprehensive investigation of various resonant mode topologies for variable frequency control has been presented in [10], The variable frequency technique is easy to implement; however, it is unsuitable for applications with a constant frequency requirement for AC power. 4.2.2 Pulse Width Modulation Control. In many advanced space applications, constant switching frequency is required for power conversion to ease the EMC/EMI requirements. DC/DC resonant converters employing the pulse width modulation technique satisfy the constant frequency requirements. In this technique the output voltage of the converter is controlled by varying the pulse width angle (d) of the converter. This technique has been discussed in Section 3.2.2. 4.3 DC/DC Resonant Mode Converter Topologies [11] In this section the performance of pulse width modulated DC/DC resonant mode converter topologies are investigated. The following three are the commonly used resonant mode converter topologies in high power applications:
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