Apparatus to provide synchronous rectifying circuit for flyback power converters

• US 7,787,264 B2
• Filed: 07/25/2007
• Issued: 08/31/2010
• Est. Priority Date: 07/25/2007
• Status: Active Grant

• Current Assignee: Semiconductor Components Industries LLC (ON Semiconductor Corporation)
• Original Assignee: System General Corporation (ON Semiconductor Corporation)
• Inventors: Yang, Ta-yung, Chen, Tso-Min
• Primary Examiner(s): BERHANE, ADOLF D
• Application Number:
  US11/782,901
• Publication Number:
  US 20090027926A1
• Time in Patent Office: 1,133 Days
• Field of Search: 363/21.06, 363/21.14, 363/84, 363/89, 363/125, 363/127
• US Class Current: 363/21.06
• 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

A synchronous rectifying circuit is provided for flyback power converter. A pulse generator is utilized to generate a pulse signal in response to a leading edge and a trailing edge of a switching signal. The switching signal is used for switching the transformer of the power converter. An isolation device such as pulse transformer or small capacitors is coupled to the pulse generator for transferring the pulse signal through an isolation barrier of a transformer. A synchronous rectifier includes a power switch and a control circuit. The power switch is connected in between the secondary side of the transformer and the output of the power converter for the rectifying operation. The control circuit having a latch is operated to receive the pulse signal for controlling the power switch.

Claims

1. A synchronous rectifying circuit for power converter, comprising:

a pulse generator, for generating a pulse signal in response to a rising edge and a falling edge of a switching signal;

an isolation device, coupled to the pulse generator for transferring the pulse signal from a primary side of a transformer to a secondary side of the transformer; and

a synchronous rectifier, having a power switch, a diode and a control circuit, wherein the power switch is coupled to the secondary side of the transformer for the rectifying operation, and wherein the control circuit is operated to receive the pulse signal for turning on/off the power switch,

wherein the switching signal is used for switching the transformer of the power converter;

the diode is coupled to the power switch in parallel, the pulse signal is generated to turn off the power switch in response to a magnetization of transformer; and

the pulse signal is generated to turn on the power switch in response to a demagnetization of the transformer.

2. The synchronous rectifying circuit as claimed in claim 1, wherein the pulse generator further generates another pulse signal to turn off the power switch in accordance with a pulse width of the switching signal and an input voltage of the power converter.

3. The synchronous rectifying circuit as claimed in claim 1, wherein the control circuit of the synchronous rectifier turns off the power switch in accordance with the switching signal of the transformer and the output voltage of the power converter.

4. The synchronous rectifying circuit as claimed in claim 1, wherein the power switch is turned on by the pulse signal once the diode is conducted.

5. The synchronous rectifying circuit as claimed in claim 1, wherein the isolation device is a pulse transformer or is composed of a plurality of capacitors.

6. The synchronous rectifying circuit as claimed in claim 1, wherein the pulse signal is a trig signal, the pulse width of the pulse signal is shorter than a pulse width of the switching signal.

7. The synchronous rectifying circuit as claimed in claim 1, wherein the pulse generator further generates a drive signal in response to the switching signal;

the drive signal is coupled to switch the transformer; and

a time delay is developed between the enabling of the switching signal and the enabling of the drive signal.

8. The synchronous rectifying circuit as claimed in claim 1, wherein the pulse generator comprises:

an input voltage terminal, coupled to receive an input voltage signal representative of an input voltage of the transformer; and

a program terminal, coupled to generate a program signal,

wherein the pulse generator produces the pulse signal to turn off the power switch in accordance with an input voltage signal, the program signal and a pulse width of the switching signal.

9. The synchronous rectifying circuit as claimed in claim 1, wherein the pulse generator comprises:

an input signal terminal, coupled to receive the switching signal;

a first output terminal; and

a second output terminal;

wherein the pulse signal is generated at the first output terminal and the second output terminal;

the pulse signal is a differential signal;

a polarity of the pulse signal determines the pulse signal and is utilized to turn on or off the power switch.

10. The synchronous rectifying circuit as claimed in claim 1, wherein the synchronous rectifier comprises:

a rectifying terminal, coupled to the secondary side of the transformer;

a ground terminal, coupled to the output of the power converter;

a first input terminal; and

a second input terminal;

wherein the power switch is connected in between a rectifying terminal and a ground terminal; and

the first input terminal and the second input terminal are coupled to receive the pulse signal for turning on/off the power switch.

11. The synchronous rectifying circuit as claimed in claim 1, wherein the control circuit comprises a latch circuit coupled to the first input terminal and the second input terminal to receive the pulse signal for setting or resetting the latch circuit;

and the latch circuit is coupled to control the power switch.