Control device for rectifiers of switching converters
A control device controls a rectifier of a switching converter that is supplied with an input voltage and provides an output current. The rectifier is configured to rectify the output current of the converter and has at least one transistor. The control device, when the at least one transistor is turned off, provides a slow discharge path to ground in a normal operation condition and provides a fast discharge path to ground for discharging the control terminal of the at least one transistor in response to detecting a zero cross event of the current flowing through said at least one transistor.
1. A control device for a rectifier of a switching converter, said control device comprising:
a zero crossing detector configured to detect a zero crossing of a current through a rectifier transistor of the rectifier and output a zero crossing signal indicating a detection of the zero crossing;
a selectable slow discharge path configured to discharge a control terminal of the rectifier transistor to ground;
a selectable fast discharge path configured to discharge the control terminal of the rectifier transistor to ground more quickly than the slow discharge path; and
a controller configured to enable the slow discharge path to discharge the control terminal of the rectifier transistor in a normal operation condition and enable the fast discharge path to discharge the control terminal of the rectifier transistor in response to the zero crossing signal indicating the zero crossing of the current through said rectifier transistor.
2. The control device according to claim 1 , wherein the controller is configured to enable the second discharge path in response to detecting that a voltage at the control terminal of the rectifier transistor is lower than a voltage threshold when in the normal operation condition.
3. The control device according to claim 1 , wherein:
said slow discharge path and said fast discharge path respectively comprise a first switch and a second switch configured to be arranged between the control terminal of said rectifier transistor and ground;
the second switch has a greater channel width than the first switch; and
said controller is configured to close said first switch and open said second switch in the normal operation condition and to close said second switch in response to the zero crossing signal indicating the zero crossing of the current through said rectifier transistor.
4. The control device according to claim 3 , wherein the controller includes:
a detector configured to detect that a voltage at the control terminal of the rectifier transistor is lower than a voltage threshold when in the normal operation condition;
a delay circuit configured to produce a ramp signal in response to receiving from the detector a detection signal indicating that the voltage at the control terminal of the rectifier transistor is lower than the voltage threshold; and
a third switch configured to close said second switch, while the first switch remains closed, in response to the ramp signal.
5. The control device according to claim 4 , wherein the delay circuit is configured to produce said ramp signal with a ramp that goes from ground to a maximum voltage value in a time period that is less than a time period from an instant in which the voltage at the control terminal of the rectifier transistor falls under said voltage threshold until a time instant of the zero crossing of the current through said rectifier transistor.
6. The control device according to claim 4 , wherein:
the control device is an integrated circuit and includes a supply voltage internal node
the third switch is an N-channel MOSFET transistor having a source terminal coupled to a control terminal of said second switch, a drain terminal coupled to the supply voltage internal node, and a gate terminal configured to be driven by the ramp signal.
7. The control device according to claim 3 , wherein controller is configured to drive said first switch of said slow discharge path with a ramp signal that goes from ground to a maximum voltage value.
8. The control device according to claim 1 , comprising a comparator configured to compare a voltage at the control terminal with a voltage threshold, wherein the controller is configured to enable the second discharge path in response to the comparator detecting that a voltage at the control terminal of the rectifier transistor is lower than a voltage threshold when in the normal operation condition.
9. A switching resonant converter, comprising:
a transformer configured to provide an output current of the converter, said transformer including a primary winding and a secondary winding;
a rectifier coupled to the secondary winding and configured to rectify said output current of the converter, said rectifier including a rectifier transistor; and
a control device configured to control the rectifier and including:
a zero crossing detector configured to detect a zero crossing of a current through the rectifier transistor and output a zero crossing signal indicating a detection of the zero crossing;
a selectable slow discharge path configured to discharge a control terminal of the rectifier transistor to ground;
a selectable fast discharge path configured to discharge the control terminal of the rectifier transistor to ground more quickly than the slow discharge path; and
a controller configured to enable the slow discharge path to discharge the control terminal of the rectifier transistor in a normal operation condition and enable the fast discharge path to discharge the control terminal of the rectifier transistor in response to the zero crossing signal indicating the zero crossing of the current through said rectifier transistor.
10. The switching resonant converter according to claim 9 , wherein the controller is configured to enable the second discharge path in response to detecting that a voltage at the control terminal of the rectifier transistor is lower than a voltage threshold when in the normal operation condition.
11. The switching resonant converter according to claim 9 , wherein:
said slow discharge path and said fast discharge path respectively comprise a first switch and a second switch configured to be arranged between the control terminal of said rectifier transistor and ground;
the second switch has a greater channel width than the first switch; and
said controller is configured to close said first switch and open said second switch in the normal operation condition and to close said second switch in response to the zero crossing signal indicating the zero crossing of the current through said rectifier transistor.
12. The switching resonant converter according to claim 11 , wherein the controller includes:
a detector configured to detect that a voltage at the control terminal of the rectifier transistor is lower than a voltage threshold when in the normal operation condition;
a delay circuit configured to produce a ramp signal in response to receiving from the detector a detection signal indicating that the voltage at the control terminal of the rectifier transistor is lower than the voltage threshold; and
a third switch configured to close said second switch, while the first switch remains closed, in response to the ramp signal.
13. The switching resonant converter according to claim 12 , wherein the delay circuit is configured to produce said ramp signal with a ramp that goes from ground to a maximum voltage value in a time period that is less than a time period from an instant in which the voltage at the control terminal of the rectifier transistor falls under said voltage threshold until a time instant of the zero crossing of the current through said rectifier transistor.
14. The switching resonant converter according to claim 12 , wherein:
the control device is an integrated circuit and includes a supply voltage internal node
the third switch is an N-channel MOSFET transistor having a source terminal coupled to a control terminal of said second switch, a drain terminal coupled to the supply voltage internal node, and a gate terminal configured to be driven by the ramp signal.
15. The switching resonant converter according to claim 11 , wherein controller is configured to drive said first switch of said slow discharge path with a ramp signal that goes from ground to a maximum voltage value.
16. The switching resonant converter according to claim 9 , wherein the control device includes a comparator configured to compare a voltage at the control terminal with a voltage threshold, wherein the controller is configured to enable the second discharge path in response to the comparator detecting that a voltage at the control terminal of the rectifier transistor is lower than a voltage threshold when in the normal operation condition.
17. A control method, comprising:
controlling a rectifier of a switching converter that is configured to provide an output current based on an input voltage, said rectifier being configured to rectify the output current of the converter and having a rectifier transistor, the controlling including:
providing a slow discharge path from a control terminal of the rectifier transistor to ground in a normal operation condition when the rectifier transistor is turned off;
detecting a zero crossing of a current through the rectifier transistor; and
providing a fast discharge path from the control terminal of the rectifier transistor to ground in response to detecting the zero crossing of the current through said rectifier transistor, the fast discharge path discharging the control terminal more quickly than the slow discharge path.
18. The method of claim 17 , comprising:
detecting that a voltage at the control terminal of the rectifier transistor is lower than a voltage threshold when in the normal operation condition; and
enabling the second discharge path in response to detecting that the voltage at the control terminal of the rectifier transistor is lower than the voltage threshold.
19. The method of claim 18 , wherein:
said slow discharge path and said fast discharge path respectively comprise a first switch and a second switch arranged between the control terminal of said rectifier transistor and ground;
the second switch has a greater channel width than the first switch; and
providing the slow discharge path includes closing said first switch and opening said second switch in the normal operation condition; and
providing the fast discharge path includes closing said second switch in response to the zero crossing signal indicating the zero crossing of the current through said rectifier transistor.
20. The method of claim 19 , comprising:
detecting that a voltage at the control terminal of the rectifier transistor is lower than a voltage threshold when in the normal operation condition;
producing a ramp signal in response to detecting that the voltage at the control terminal of the rectifier transistor is lower than the voltage threshold; and
closing said second switch, while the first switch remains closed, in response to the ramp signal.