Switching mode power supply with synchronous rectifying control circuit

Patent metadata

Field	Value
Publication number	US-8976547-B2
Application number	US-201213478575-A
Country	US
Kind code	B2
Filing date	May 23, 2012
Priority date	May 24, 2011
Publication date	Mar 10, 2015
Grant date	Mar 10, 2015

Abstract

A switching mode power supply comprising a synchronous rectifying control circuit. The synchronous rectifying control circuit comprising an integrating circuit, a first comparison circuit and a logic circuit. The integrating circuit is configured to provide an integrating signal. The first comparison circuit comprises a first input coupled to the output of the integrating signal, a second input configured to receive a first threshold signal, and an output. The logic circuit comprises a first input coupled to the output of the first comparison circuit and an output coupled to a control terminal of the secondary switch, and the secondary switch is configured to be turned OFF when the integrating signal is less then the first threshold signal.

First claim

We claim:

1. A synchronous rectifying control circuit for a switching mode power supply, the switching mode power supply having an output configured to provide an output signal, wherein the switching mode power supply comprises a transformer having a primary winding and a secondary winding, and a secondary switch coupled to the secondary winding, and wherein the synchronous rectifying control circuit comprises:
an integrating circuit having a first input terminal, a second input terminal and an output terminal, wherein the first input terminal is coupled to a first terminal of the secondary winding, wherein the second input terminal is coupled to a second terminal of the secondary winding, and wherein the integrating circuit integrates the voltage across the secondary winding and generates an integrating signal at the output terminal;
a first comparison circuit having a first input terminal, a second input terminal and an output terminal, wherein the first input terminal is coupled to the output terminal of the integrating circuit to receive the integrating signal, wherein the second input terminal is configured to receive a first threshold signal, and wherein the first comparison circuit compares the integrating signal with the first threshold signal and generates a first comparing signal at the output terminal; and
a logic circuit having a first input terminal and an output terminal, wherein the first input terminal is coupled to the output terminal of the first comparison circuit to receive the first comparing signal, the output terminal is coupled to a control terminal of the secondary switch, and wherein based on the first comparing signal, the logic circuit generates a control signal at the output terminal; and wherein
the secondary switch is turned OFF when the integrating signal is less than the first threshold signal.

2. The synchronous rectifying control circuit of claim 1 , further comprising:
a second comparison circuit having a first input terminal, a second input terminal and an output terminal, wherein the first input terminal is configured to receive a drain-source voltage of the secondary switch, wherein the second input terminal is configured to receive a second threshold signal, and wherein the second comparison circuit compares the drain-source voltage of the secondary switch with the second threshold signal and generates a second comparing signal at the output terminal; wherein
the logic circuit further comprises a second input terminal, wherein the second input terminal is coupled to the output terminal of the second comparison circuit to receive the second comparing signal, and wherein the control signal is responsive to the second comparing signal; and wherein
the secondary switch is turned ON when the drain-source voltage of the secondary switch is less than the second threshold signal and the integrating signal is larger than the first threshold signal.

3. The synchronous rectifying control circuit of claim 1 , wherein the integrating circuit comprises:
a voltage sampling circuit configured to receive the voltage across the secondary winding, and configured to provide a voltage sampling signal;
a voltage to current conversion circuit having an input terminal and an output terminal, wherein the input terminal is coupled to the voltage sampling circuit to receive the voltage sampling signal, and wherein the output terminal is configured to provide a first current signal responsive to the voltage sampling signal; and
a first capacitor having a first terminal and a second terminal, wherein the first terminal is coupled to the output terminal of the voltage to current conversion circuit to receive the first current signal and is configured to provide the integrating signal, the second terminal is coupled to a secondary ground.

4. The synchronous rectifying control circuit of claim 3 , wherein the synchronous rectifying control circuit further comprises:
a first switch, comprising a first terminal, a second terminal and a control terminal, wherein the first terminal is coupled to the first terminal of the first capacitor, and wherein the second terminal is coupled to the second terminal of the first capacitor, and wherein the second terminal is coupled to the second terminal of the first capacitor;
a third comparison circuit, comprising a first input terminal, a second input terminal and an output terminal, wherein the first input terminal is coupled to the voltage sampling circuit to receive the voltage sampling signal, wherein the second input terminal is configured to receive a third threshold signal; and
a RS trigger, comprising a set terminal, a reset terminal and an output terminal, wherein the set terminal is coupled to the output terminal of the first comparison circuit, wherein the reset terminal is coupled to the output terminal of the third comparison circuit, and wherein the output terminal is coupled to the control terminal of the first switch; and wherein
the first switch is turned ON when the integrating signal is less than the first threshold signal, and the first switch is turned OFF when the voltage across the secondary winding is larger than the third threshold signal.

5. The synchronous rectifying control circuit of claim 4 , wherein the third threshold signal is a feedback signal indicative of the output signal.

6. The synchronous rectifying control circuit of claim 1 , wherein when the integrating signal is less than the first threshold signal, the integrating signal is decreased to about zero volts until the voltage across the secondary winding is larger than a third threshold signal.

7. The synchronous rectifying control circuit of claim 1 , wherein the synchronous rectifying control circuit further comprising:
a threshold start up circuit, having an input terminal and an output terminal, wherein the input terminal is coupled to the output terminal of the logic circuit, wherein the output terminal is configured to provide a first threshold varying signal;
a first threshold adjusting circuit, having an input terminal and an output terminal, wherein the input terminal is coupled to the output of the switching mode power supply, and wherein the output terminal is configured to provide a second threshold varying signal;
a second threshold adjusting circuit, having an input terminal and an output terminal, wherein the input terminal is coupled to the output terminal of the integrating circuit, and wherein the output terminal is configured to provide a third threshold varying signal; and
an adder, having a first input terminal, a second input terminal, a third input terminal, a fourth input terminal and an output terminal, wherein the first input terminal is coupled to the output terminal of the threshold start up circuit, the second input terminal is coupled to the output terminal of the first threshold adjusting circuit, the third input terminal is coupled to the output terminal of the second threshold adjusting circuit, the fourth input terminal is coupled to a constant threshold signal, and the output terminal is configured to provide the first threshold signal, and wherein the first threshold signal is based on adding together the constant threshold signal, the first threshold varying signal, the second threshold varying signal, and the third threshold varying signal.

8. The synchronous rectifying control circuit of claim 7 , wherein the first threshold adjusting circuit further comprising:
a first variation detecting circuit, having an input terminal and an output terminal, wherein the input terminal is coupled to the output of the switching mode power supply; and
a first proportional circuit, having an input terminal and an output terminal, wherein the input terminal is coupled to the output terminal of

Assignees

• Ren Yuancheng
• Miao Lei
• Chengdu Monolithic Power Sys

Inventors

• Ren Yuancheng
• Miao Lei

Classifications

• Y02B70/1475

  Cross-Sectional Technologies · mapped topic

• H02M3/33592 Primary

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

• Y02B70/10

  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 · CPC title

• H02M1/32

  Means for protecting converters other than automatic disconnection · CPC title