Synchronous rectifier and alternator using the same
A rectifier ( 107 ) includes a rectifying MOSFET ( 101 ) that performs synchronous rectification, a control circuit ( 106 ) that inputs a voltage across a pair of a positive-side main terminal TK and a negative-side main terminal TA of the rectifying MOSFET ( 101 ) to determine an ON or OFF state of the rectifying MOSFET ( 101 ) based on the inputted voltage, and a capacitor ( 104 ) that supplies power to the control circuit ( 106 ). The control circuit ( 106 ) includes a blocking circuit ( 105 ) that inputs the voltage across the pair of main terminals of the rectifying MOSFET ( 101 ), to block power supply to the control circuit ( 106 ) when the inputted voltage across the pair of main terminals is higher than or equal to a first voltage, and to unblock power supply to the control circuit ( 106 ) when the inputted voltage across the pair of main terminals is lower than the first voltage.
1. A synchronous rectifier for synchronously rectifying an AC input voltage to generate and externally output a DC voltage for use in an alternator, the rectifier comprising:
a switching transistor;
a pair of external terminals that is connected to a pair of main terminals of the switching transistor;
a control circuit that provides a control signal to and turns on the gate of the switching transistor; and
a capacitor that supplies power to the control circuit,
wherein the control circuit includes:
a determining circuit that inputs a voltage across the pair of external terminals; and
a blocking circuit that blocks power supply to the determining circuit when the voltage across the pair of main terminals of the switching transistor is equal to or higher than a predetermined voltage, and unblocks power supply to the determining circuit when the voltage across the pair of main terminals is lower than a predetermined voltage.
2. The synchronous rectifier according to claim 1 , wherein
when the blocking circuit is blocking power supply to the determining circuit, a current flowing via the control circuit through the pair of external terminals is lower than a current flowing via the capacitor through the pair of external terminals.
3. The synchronous rectifier according to claim 1 , wherein
when the blocking circuit is blocking power supply to the determining circuit, a current flowing via the capacitor through the pair of external terminals is larger than a current flowing via the switching transistor through the pair of external terminals.
4. The synchronous rectifier according to claim 1 , wherein
a voltage across the pair of main terminals when the blocking circuit starts blocking power supply to the determining circuit is larger than a voltage across the pair of main terminals when the blocking circuit stops blocking power supply to the determining circuit.
5. The synchronous rectifier according to claim 1 , wherein the blocking circuit inputs a voltage across the pair of main terminals of the switching transistor and a voltage across a pair of terminals of the capacitor.
6. The synchronous rectifier according to claim 5 , wherein
the blocking circuit includes a transistor that blocks power supply to the determining circuit, and turns on or off the transistor based on a result of comparing a voltage across the pair of main terminals of the switching transistor and a voltage across the pair of terminals of the capacitor.
7. The synchronous rectifier according to claim 1 , wherein
the switching transistor is a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor).
8. The synchronous rectifier according to claim 7 , wherein
a voltage across the pair of main terminals when blocking power supply to the determining circuit is stopped is higher than a negative voltage across the pair of main terminals when a rectifier current flows through a parasitic diode of the MOSFET.
9. The synchronous rectifier according to claim 1 , further comprising
a circuit that fixes a logical state of the control circuit so that the switching transistor is turned off when power supply to the determining circuit is blocked.
10. The synchronous rectifier according to claim 1 , wherein
the control circuit includes a gate drive circuit that drives the gate of the switching transistor, wherein
the blocking circuit unblocks power supply to the gate drive circuit.
11. The synchronous rectifier according to claim 1 , wherein
the control circuit includes a gate drive circuit that drives the gate of the switching transistor, wherein
the blocking circuit blocks power supply to the gate drive circuit when a voltage across the pair of main terminals of the switching transistor is equal to or higher than a predetermined voltage, and unblocks power supply to the gate drive circuit when the voltage across the pair of main terminals is lower than a predetermined voltage.
12. The synchronous rectifier according to claim 1 , wherein
when the blocking circuit is blocking power supply to the control circuit, at least one transistor is in an OFF state in all paths for a current to flow in the control circuit through the pair of main terminals.
13. The synchronous rectifier according to claim 1 , wherein
the determining circuit has an input terminal that is connected to a positive-side main terminal of the pair of external terminals, wherein
the input terminal is connected to the gate of a MOSFET that constitutes the determining circuit or the cathode of a diode that constitutes the determining circuit.
14. The synchronous rectifier according to claim 13 , wherein
the determining circuit is a comparator.
15. An alternator comprising the synchronous rectifier according to claim 1 .