Electronic switch circuit, converter and method of operation
View Patent ↗An electronic switch circuit includes an electronic switch having a first terminal, a second terminal and a third terminal, control means for applying to the first terminal a cyclic drive waveform which causes the electronic switch to conduct between the second and third terminals during a selected portion of each cycle of the waveform, operably coupled to the control means to control synchronisation of the cyclic drive waveform a detector operable to detect a change of direction of current flow at the second terminal of the electronic switch and means for monitoring and, where required, compensating for offset error of the detector. The electronic switch may beneficially be a synchronous rectifier. The synchronous rectifier may be used in DC-DC power converters to improve efficiency.
1. An electronic switch circuit comprising:
an electronic switch having a first terminal, a second terminal and a third terminal;
control means for applying to the first terminal a cyclic drive waveform which causes the electronic switch to conduct between the second and third terminals during a selected portion of each cycle of the waveform, operably coupled to the control means to control synchronisation of the cyclic drive waveform a detector operable to detect a change of direction of current flow at the second terminal of the electronic switch and means for repetitively monitoring for offset error of the detector and, where required, compensating for the offset error, wherein the detector comprises:
a voltage comparator operable to compare a measured voltage between the second and third terminals of the electronic switch with a reference voltage and to determine when the measured voltage changes sign with respect to the reference voltage and wherein the means for monitoring and compensating for offset error of the detector includes:
(i) a state sensor operable to sense a polarity of the second terminal at a selected point in the cycle of the cyclic drive waveform when conduction of the electronic switch is turned off by the waveform and to produce a corresponding output signal; and
(ii) means for adding to or withdrawing from the detector in response to the output signal produced by the state sensor a compensating current which adjusts a reference voltage of the detector.
2. A circuit according to claim 1 wherein the detector includes a differential gain stage and a gain stage and the means for adding or withdrawing current is operable to add current to or withdraw current from the differential gain stage.
3. A circuit according to claim 1 , wherein the means for adding or subtracting current is operable to adjust the reference voltage of the detector in incremental steps in a given direction until the state sensor detects a change in polarity of the second terminal at a selected point in a cycle of the cyclic drive waveform.
4. A circuit according to claim 1 , which includes, operably coupled to the state sensor, a current producing DAC (digital to analog converter) which produces a current flow for use in producing the adjustment current and, operably coupled to and controlled by the state sensor, a switching device to control a direction of the adjustment current flow.
5. A circuit according to claim 1 , s wherein the electronic switch comprises a synchronous rectifier.
6. A circuit according to claim 1 , wherein the electronic switch comprises a voltage controlled semiconductor device.
7. A circuit according to claim 6 wherein the voltage controlled semiconductor device comprises a metal oxide semiconductor power transistor.
8. A DC to DC power converter including at least one synchronous rectifier circuit which comprises an electronic switch circuit according to claim 1 .
9. A DC to DC converter according to claim 8 which is a converter selected from the group of converters consisting of: Buck converters, Boost converters, Buck-Boost converters, Sepic converters, charge pump converters, isolating converters, flyback converters and forward converters.
10. A converter according to claim 8 wherein the electronic switch comprises:
a synchronous rectifier and including a voltage source, a further electronic switch having a first terminal connected to the control means, a second terminal connected to the voltage source and a third terminal connected to a terminal of the synchronous rectifier;
the control means being operable to apply to the first terminal of the further electronic switch a further cyclic drive waveform to cause the further electronic switch to conduct between its second and third terminals during selected portions of each cycle of the further drive waveform, wherein the cyclic drive waveforms applied respectively to the synchronous rectifier and the further electronic switch are mutually synchronised so that only one of the synchronous rectifier device and the further electronic switch conducts at a time.
11. A converter according to claim 10 which is a Buck converter wherein the second terminal of the synchronous rectifier and the third terminal of the further electronic switch are connected through an inductor to a load impedance, and a capacitor connected to a node between the inductor and the load impedance, the converter having an output terminal connected to the load impedance.
12. A converter according to claims 10 including a further rectifier connected in parallel with the synchronous rectifier.
13. A converter according to claim 10 , wherein the further electronic switch comprises a voltage controlled semiconductor device.
14. A method of operation of an electronic switch circuit including an electronic switch having a first terminal, a second terminal and a third terminal, the method comprising:
applying from control means to the first terminal a cyclic drive waveform which causes the electronic switch to conduct between the second and third terminals during a selected portion of each cycle of the waveform;
detecting by a detector a change of direction of current flow at the second terminal of the electronic switch;
providing an output signal from the detector to the control means to control synchronisation of the cyclic drive waveform and repetitively monitoring for offset error of the detector and, where required, compensating for the offset error;
comparing by a voltage comparator of the detector a measured voltage between the second and third terminals of the electronic switch with a reference voltage and determining when the measured voltage changes sign with respect to the reference voltage;
sensing a polarity of the second terminal at a selected point in the cycle of the cyclic drive waveform when conduction of the electronic switch is turned off by the waveform and to produce a corresponding output signal; and
adding or withdrawing a compensating current from the detector in response to the output signal produced by the state sensor, the compensating current adjusting a reference voltage of the detector.
15. A circuit according to claim 1 , wherein the detector includes a differential gain stage and a gain stage and the means for adding or withdrawing current is operable to add current to or withdraw current from the differential gain stage.
16. A converter according to claim 9 , wherein the electronic switch comprises:
a synchronous rectifier and including a voltage source, a further electronic switch having a first terminal connected to the control means, a second terminal connected to the voltage source and a third terminal connected to a terminal of the synchronous rectifier;
the control means being operable to apply to the first terminal of the further electronic switch a further cyclic drive waveform to cause the further electronic switch to conduct between its second and third terminals during selected portions of each cycle of the further drive waveform, wherein the cyclic drive waveforms applied respectively to the synchronous rectifier and the further electronic switch are mutually synchronised so that only one of the synchronous rectifier device and the further electronic switch conducts at a time.
17. A converter according to claims 11 , including a further rectifier connected in parallel with the synchronous rectifier.