If the isolation or a high ratio of the voltage conversion is required, a transformer is usually integrated into the system. It is preferred to place the transformer in the high-frequency section of the circuit rather than at the ac output-frequency, since a low- frequency transformer is bulky and expensive. In a dc/dc dc/ac converter system, the dc/dc converter is used for isolation and voltage step-up and the inverter is needed for ac output. In general both dc/dc converters and dc/ac in- verters have many topologies for selection, including the hard- switching and soft-switching circuits. 1) DC/DC Converters in Fuel Cell Conditioning Systems: A dc/dc converter is usually put between the fuel cell and the in- verter to perform two functions. One is the dc isolation for the inverter because a low-frequency transformer is placed at the output of the inverter is very bulky, and the second is to pro- duce sufficient voltage for the inverter input, so that the required magnitude of the ac voltage can be produced. For example, only 200-V fuel cell stack cannot produce 380-V line voltage, then a step up dc converter is needed. The classical dc/dc converter such as H-bridge type forward converter shown in Fig. 9(a) is well-developed and proven technology. However, in order to reduce the switching loss, a soft-switching type of the converter, the H-bridge series resonant converter shown in Fig. 9(b) may be used. The main advantage of the converter is its inherited short-circuit protection and there is no saturation problem of the transformer whereas the hard-switching forward converter requires very accurate bi-polar waveform or current mode control otherwise Fig. 10. Nonisolated dc/dc converters: (a) a boost dc/dc converter and (b) a buck dc/dc converter. Fig. 11. Single-phase dc/ac inverters: (a) full bridge inverter and (b) half bridge inverter. the transformer may experience saturation and cause extra losses. Fig. 9(c) shows a push pull type of a dc/dc converter, which requires the high dc voltage and current. The diode rectifier bridges shown in Fig. 9 can be replaced with half bridge diode rectifiers as well. If the isolation is not required and the voltage conversion ratio is not high, then nonisolated dc/dc converters as shown in Fig. 10 may be used. 2) DC/AC Converters in Fuel Cell Conditioning Sys- tems: The dc/ac conversion circuit may be chosen from a selection of various circuit topologies, Fig. 11 shows two single-phase inverters. Fig. 11(a) is an H-bridge inverter while Fig. 11(b) shows that the dc-link capacitor is composed by two capacitors in series, where the midpoint is used in the inverter stage. This makes it possible to save two switches at the cost of a twice as high dc-link voltage compared with the full-bridge in order to produce the same level of the ac voltage.
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- Winter '15