Flyback converter with secondary side regulation

• US 10,008,947 B2
• Filed: 07/06/2016
• Issued: 06/26/2018
• Est. Priority Date: 07/31/2015
• Status: Active Grant

• Current Assignee: Texas Instruments, Inc.
• Original Assignee: Texas Instruments, Inc.
• Inventors: Vemuri, Satish Kumar, Walden, James M., Cohen, Isaac
• Primary Examiner(s): Gblende, Jeffrey
• Application Number:
  US15/202,629
• Publication Number:
  US 20170033698A1
• Time in Patent Office: 720 Days
• Field of Search: None
• CPC Class Codes: H02M 1/0009 Devices or circuits for detecting current in a converter
• CPC Class Codes: H02M 3/33523 with galvanic isolation between input and output of both the power stage and the feedback loop
• CPC Class Codes: H02M 3/33592 having a synchronous rectifier circuit or a synchronous freewheeling circuit at the secondary side of an isolation transformer
• CPC Class Codes: Y02B 70/10 Technologies improving the efficiency by using switched-mode power supplies [SMPS], i.e. efficient power electronics conversion e.g. power factor correction or reduction of losses in power supplies or efficient standby modes

Abstract

Disclosed examples include flyback converters, control circuits and methods to facilitate secondary side regulation of the output voltage. A primary side control circuit operates a primary side switch to independently initiate power transfer cycles to deliver power to a transformer secondary winding in a first mode. A secondary side control circuit operates a synchronous rectifier or secondary side switch to generate a predetermined cycle start request signal via a transformer auxiliary winding to assume secondary side regulation and to cause the primary side controller to initiate new power transfer cycles.

Claims

1. A flyback converter, comprising:

a transformer, including;

a primary winding, including a first end to receive an input voltage signal, and a second end,

a secondary winding, including a first end to provide an output voltage signal, and a second end, and

an auxiliary winding including a first end, and a second end coupled with a first constant voltage node;

a first switch, including a first terminal coupled with the second end of the primary winding, a second terminal coupled with the first constant voltage node, and a first control terminal to receive a first switching control signal;

a second switch, including a first terminal coupled with the second end of the secondary winding, a second terminal coupled with a second constant voltage node, and a second control terminal to receive a second switching control signal;

a first control circuit, including an input coupled with the first end of the auxiliary winding to receive a first signal representing a voltage of the auxiliary winding, and an output to provide the first switching control signal to the first switch, the first control circuit operative in a first mode to independently turn the first switch on and off in a plurality of power transfer cycles, the first control circuit operative in a second mode to initiate a power transfer cycle by turning the first switch on and off, in response to detecting a predetermined cycle start request signal at the input; and

a second control circuit, including an output to provide the second switching control signal to the second switch, the second control circuit operative to generate the predetermined cycle start request signal to regulate the output voltage signal by turning the second switch on and off after a current of the secondary winding returns to zero in a given power transfer cycle to cause the first control circuit to initiate a new power transfer cycle.

2. The flyback converter of claim 1, the second control circuit is operative to generate the predetermined cycle start request signal to place the first control circuit in the second mode in response to the output voltage signal rising above a predetermined threshold value after the flyback converter is powered up.

3. The flyback converter of claim 1, wherein the second control circuit includes a first input coupled with the second end of the secondary winding to receive a switch voltage signal representing a voltage of the second switch, wherein the second control circuit turns the second switch on and off twice in the given power transfer cycle when the first control circuit is in the second mode, wherein the second control circuit turns the second switch on a first time in the given power transfer cycle in response to detection of switch voltage signal indicating that the first switch has been turned off, and wherein the second control circuit turns the second switch off a first time in the given power transfer cycle in response detecting that the current of the secondary winding has returned to zero.

4. The flyback converter of claim 3, wherein the second control circuit is operative to generate the predetermined cycle start request signal by turning the second switch on and off a second time in the given power transfer cycle, including turning the second switch on when a new power transfer cycle is desired to regulate the output voltage signal.

5. The flyback converter of claim 3, wherein the second control circuit is operative to generate the predetermined cycle start request signal by turning the second switch on and off a second time in the given power transfer cycle, including turning the second switch on near a next valley in a ringing waveform of the switch voltage signal after a time in the given power transfer cycle at which a new power transfer cycle is desired to regulate the output voltage signal.

6. The flyback converter of claim 1, wherein the second control circuit includes a first input coupled with the second end of the secondary winding to receive a switch voltage signal representing a voltage of the second switch, and wherein if there is no ringing in a waveform of the switch voltage signal when a new power transfer cycle is desired to regulate the output voltage signal, the second control circuit is operative to generate the predetermined cycle start request signal by turning the second switch on and off in the given power transfer cycle, including turning the second switch on when the new power transfer cycle is desired to regulate the output voltage signal.

7. The flyback converter of claim 6, wherein if there is a ringing waveform of the switch voltage signal when the new power transfer cycle is desired to regulate the output voltage signal, the second control circuit is configure to generate the predetermined cycle start request signal by turning the second switch on near a next valley in the ringing waveform of the switch voltage signal after a time in the given power transfer cycle at which a new power transfer cycle is desired to regulate the output voltage signal.

8. The flyback converter of claim 1, wherein the second control circuit includes a first input coupled with the second end of the secondary winding to receive a switch voltage signal representing a voltage of the second switch, wherein the second control circuit is operative to generate the predetermined cycle start request signal by turning the second switch on and off in the given power transfer cycle, including turning the second switch on near a next valley in a ringing waveform of the switch voltage signal after a time in the given power transfer cycle at which a new power transfer cycle is desired to regulate the output voltage signal.

9. The flyback converter of claim 1, wherein the first control circuit is operative to change from operation in the first mode to operation in the second mode in response to a first detection or a subsequent detection of the predetermined cycle start request signal at the input.

10. The flyback converter of claim 1, wherein the first control circuit is operative to detect the predetermined cycle start request signal as an interruption of a ringing waveform of the first signal at the input.

11. The flyback converter of claim 10, wherein the first control circuit is operative to detect the predetermined cycle start request signal as a spike in the first signal at the input.

12. The flyback converter of claim 1, wherein the first control circuit is operative to detect the predetermined cycle start request signal as a spike in the first signal at the input.

13. The flyback converter of claim 1, wherein the first control circuit and the second control circuit are included in a single integrated circuit.

14. A control circuit to operate a switch connected to a primary winding of a transformer in a flyback converter, the control circuit comprising:

a driver circuit including an output to provide a switching control signal to the switch; and

a logic circuit coupled with the driver circuit, the logic circuit including an input coupleable to an auxiliary winding of the transformer to receive a signal representing a voltage of the auxiliary winding;

wherein the logic circuit is operative in a first mode to independently cause the driver circuit to turn the switch on and off in a plurality of power transfer cycles to deliver power to a secondary winding of the transformer, the logic circuit operative in a second mode to initiate a power transfer cycle by turning the first switch on and off, in response to detecting a predetermined cycle start request signal at the input,

wherein the logic circuit is operative to detect the predetermined cycle start request signal as an interruption of a ringing waveform of the signal at the input.

15. The control circuit of claim 14, wherein the logic circuit is operative to change from operation in the first mode to operation in the second mode in response to a first detection or a subsequent detection of the predetermined cycle start request signal at the input.

16. The control circuit of claim 14, wherein the logic circuit is operative to detect the predetermined cycle start request signal as a spike in the signal at the input.