Patent ID: 7843173

Claim:
A charger comprising: input terminals whereto positive and negative voltages from a power source unit are applied; output terminals generating an output for charging a secondary battery, upon being applied with the voltage from the input terminals via positive and negative power source lines; a first field effect type transistor having a region between a drain and a source inserted and connected in at least one of the positive and negative power source lines, and having a first parasitic diode with the reverse polarity with respect to a charge current; a second field effect type transistor having a drain and a source inserted and connected in at least one of the positive and negative power source lines, and having a second parasitic diode with a forward polarity with respect to the charge current; a first detection circuit having series connections of a plurality of resistors inserted between the positive and negative power source lines, between the input terminals and the first field effect type transistor, and allowing extraction of a first detection voltage from a divisional point of the resistors; a second detection circuit having series connections of a plurality of resistors inserted between the positive and negative power source lines, between the first field effect type transistor and the second field effect type transistor, and allowing extraction of a second detection voltage from a divisional point of the resistors; and a charge control unit which is supplied with the first and second detection voltages, and generates ON/OFF control signals for gates of the first and second field effect transistors; wherein the charge control unit calculates a first difference voltage between the first and second detection voltages, while turning the first field effect type transistor OFF and turning the second field effect type transistor ON, and judges whether the first difference voltage has a value not turning the second parasitic diode ON; if the first difference voltage is judged as having a value turning the second parasitic diode ON, the first and second field effect type transistors are turned ON to perform a charging operation; whereas, if the first difference voltage is judged as having a value not turning the second parasitic diode ON, the first field effect type transistor is turned ON and the second field effect type transistor is switched OFF to calculate a second difference voltage while applying a same voltage to the first and second detection circuits; and the end-of-charge state is detected by comparing a third difference voltage with a reference voltage, the third voltage being corrected by the second difference voltage.