Conventionally, a DC/DC converter having a DC input terminal has been used to boost or drop a DC voltage and output it (see, e.g., Japanese Patent Application Publication No. 2010-022077). In such a type of the DC/DC converter having the DC input terminal, when reverse polarity is connected to the DC input terminal, an internal circuit of the converter is broken. Various configurations as shown in FIGS. 28A to 28C have been known as countermeasures against the reverse connection to the DC input terminal.
FIG. 28A illustrates the configuration in which a diode is connected in series to one input terminal, thereby preventing a voltage from being applied to the internal circuit of the DC/DC converter in reverse connection. In this configuration, when polarity is normally connected to the DC input terminal, voltage loss is always generated by a voltage drop of the connected diode. Since the voltage is not applied to the internal circuit of the DC/DC converter in the reverse connection, the DC/DC converter does not operate.
FIG. 28B illustrates the configuration in which a diode is connected from one input terminal to the other input terminal and the internal circuit is short-circuited by the diode in reverse connection, thereby preventing a voltage from being applied to the internal circuit of the DC/DC converter in the reverse connection. In this configuration, a protection circuit is additionally required to protect a circuit connected to the DC input terminal from the short-circuit current. Alternatively, this configuration is limitedly applied to a power supply, such as a solar photovoltaic power generating panel, to which current limiting acts. Also, since the voltage is not applied to the internal circuit of the DC/DC converter in the reverse connection, the DC/DC converter does not operate.
FIG. 28C illustrates the configuration in which a bridge circuit is provided in an input unit, so that a normal voltage is applied to the internal circuit of the DC/DC converter even in any one of forward and reverse connections, such as when an AC power supply is connected to the DC/DC converter. In this configuration, the DC/DC converter operates even in any one of the forward and reverse connections, but voltage loss corresponding to the voltage of two diodes included in the bridge circuit always occurs.