Synchronous rectifier control with adaptive minimum off-time

Patent metadata

Field	Value
Publication number	US-10079548-B2
Application number	US-201715413042-A
Country	US
Kind code	B2
Filing date	Jan 23, 2017
Priority date	Jan 23, 2017
Publication date	Sep 18, 2018
Grant date	Sep 18, 2018

Abstract

A switching power converter may include a power switch coupled to a primary winding of a transformer, and a primary controller configured to turn on and off the power switch, a synchronous rectifier switch coupled to a secondary winding of a transformer, and a synchronous rectifier controller configured to turn on and off the synchronous rectifier switch. The synchronous rectifier controller may monitor a voltage across the synchronous rectifier switch. The synchronous rectifier controller may determine a period of a resonant oscillation of the voltage across the synchronous rectifier switch following at least one cycling off of the synchronous rectifier switch. The synchronous rectifier controller may adjust the minimum off-time period for the synchronous rectifier switch based on the period of the resonant oscillation. The synchronous rectifier controller may adaptively adjust a minimum off-time period for the synchronous rectifier switch.

First claim

We claim:

1. A flyback converter, comprising:
a synchronous rectifier switch coupled to a secondary winding of a transformer; and
a synchronous rectifier controller configured to control a cycling of the synchronous rectifier switch and including a controller logic circuit configured to measure a resonant oscillation period for a terminal voltage for the synchronous rectifier switch in a first cycle of the synchronous rectifier switch, the controller logic circuit being further configured to time an adjusted minimum off-time period for the synchronous rectifier switch having a duration at least as long as the resonant oscillation period, and wherein the synchronous rectifier controller is further configured to maintain the synchronous rectifier switch off in a subsequent cycle of the synchronous rectifier switch for at least the adjusted minimum off-time period.

2. The flyback converter of claim 1 , wherein the synchronous rectifier controller is configured to both switch the synchronous rectifier switch off and to begin timing the adjusted minimum off-time period in the subsequent cycle of the synchronous rectifier switch responsive to the terminal voltage exceeding an off-time threshold voltage.

3. The flyback converter of claim 2 , wherein the synchronous rectifier controller further includes a minimum off-time timer configured to time the adjusted minimum off-time period.

4. The flyback converter of claim 2 , wherein the synchronous rectifier controller is further configured to switch on the synchronous rectifier switch on in another cycle of the synchronous rectifier switch responsive to the terminal voltage being less than an on-time threshold voltage following an expiration of the adjusted minimum off-time period.

5. The flyback converter of claim 4 , wherein the on-time threshold voltage is less than the off-time threshold voltage.

6. The flyback converter of claim 1 , wherein the synchronous rectifier controller further includes a comparator configured to compare the terminal voltage for the synchronous rectifier switch to an output voltage for the flyback converter to provide a comparator output signal, and wherein the controller logic circuit is further configured to measure the resonant oscillation period for the terminal voltage responsive to a delay between a first edge for the comparator output signal and a subsequent second edge for the comparator output signal.

7. The flyback converter of claim 1 , wherein the synchronous rectifier controller is further configured to set the adjusted minimum off-time period for the synchronous rectifier switch to equal a sum of the resonant oscillation period plus a fractional amount of the resonant oscillation period.

8. The flyback converter of claim 7 , wherein the fractional amount of the resonant oscillation period is no more than 10% of the resonant oscillation period.

9. The flyback converter of claim 1 , further comprising:
a power switch coupled to a primary winding of the transformer; and
a primary controller configured to cycle the power switch to regulate an output voltage across a load coupled to the secondary winding.

10. The flyback converter of claim 1 , wherein the synchronous rectifier switch is a field-effect transistor (FET).

11. A method comprising:
measuring a period of a resonant oscillation of a synchronous rectifier switch voltage for a synchronous rectifier switch in a flyback converter during a first power switch cycle to provide a measured period of the resonant oscillation;
adjusting a minimum off-time period of the synchronous rectifier switch to provide an adjusted minimum off-time period that has a duration at least as long as the measured period of the resonant oscillation;
switching the synchronous rectifier switch on during a second power switch cycle subsequent to the first power switch cycle; and
switching the synchronous rectifier switch off and maintaining the synchronous rectifier switch off for at least the adjusted minimum off-time period during the second power switch cycle.

12. The method of claim 11 , wherein the measuring of the period of the resonant oscillation comprises comparing the synchronous rectifier switch voltage in a comparator to form a comparator output signal.

13. The method of claim 12 , wherein the measuring of the period of the resonant oscillation comprises determining a time duration from a first binary transition for the comparator output signal to a second binary transition for the comparator output signal.

14. The method of claim 13 , wherein the first binary transition is a falling edge transition and the second binary transition is a rising edge transition.

15. The method of claim 11 , wherein the adjusted minimum off-time period equals the measured period of the resonant oscillation plus a fraction of the measured period of the resonant oscillation.

16. The method of claim 15 , wherein the fraction of the measured period of the resonant oscillation equals 10% of the measured period of the resonant oscillation.

17. A synchronous rectifier controller, comprising:
a controller logic circuit configured to measure a resonant oscillation period of a drain voltage for a synchronous rectifier switch during a first cycle of the synchronous rectifier switch and to set a minimum off-time for the synchronous rectifier switch that has a duration at least as long as the resonant oscillation period for a second cycle of the synchronous rectifier switch; and
a timer configured to time the minimum off-time, wherein the controller logic circuit is further configured to maintain the synchronous rectifier switch off in the second cycle of the synchronous rectifier switch for at least the duration of the minimum-off time as timed by the timer.

18. The synchronous rectifier controller of claim 17 , wherein the controller logic circuit further includes a comparator configured to compare the drain voltage to form a comparator output signal, and wherein the controller logic circuit is further configured to measure the resonant oscillation period through a measurement of a delay from a first edge in the comparator output signal to a second edge in the comparator output signal.

19. The synchronous rectifier controller of claim 18 , wherein the comparator is further configured to compare the drain voltage to an output voltage to form the comparator output signal.

20. The synchronous rectifier controller of claim 18 , wherein the comparator is further configured to compare the drain voltage to a threshold voltage to form the comparator output signal.

Assignees

• Dialog Semiconductor Inc

Inventors

• Kong Pengju
• Bui Hien
• Zhang Hanguang

Classifications

• H02M7/217 Primary

  using semiconductor devices only · CPC title

• H02M3/33592 Primary

  having a synchronous rectifier circuit or a synchronous freewheeling circuit at the secondary side of an isolation transformer · CPC title

• H02M1/08

  Circuits specially adapted for the generation of control voltages for semiconductor devices incorporated in static converters · CPC title

• H02M2001/0048

  Electricity · mapped topic

• H02M1/0054

  Transistor switching losses (periodically suspending operation of switching converter in low power mode H02M1/0035) · CPC title