POWER SUPPLY SWITCHING CIRCUIT FOR A COMPACT PORTABLE TELEPHONE SET

摘要

In order to reduce volume and weight of a power supply of a compact portable telephone set and thus reduce volume and weight of the compact portable telephone set itself, a P-channel power MOSFET is used as a power switching element, and a negative voltage, for example, -11 volts is applied across a source and a gate from a DC/DC converter to turn on the P-channel power MOSFET so that power is supplied from a battery 2 which serves as one power supply. When the gate and the source of the power MOSFET are maintained at the same potential, the power MOSFET is turned off and power is supplied from another power supply.