Three-pin integrated synchronous rectifier and a flyback synchronous rectifying circuit

A flyback synchronous rectifying circuit, comprising:

• a flyback converter having a first power transistor and a transformer having a primary side and a secondary side, wherein the first power transistor is coupled with the primary side;
• an output capacitor coupled with the secondary side of the transformer;
• a three-pin integrated synchronous rectifier having
• a first electric connection point,
• a second electric connection point;
• a third electric connection point,
• a second power transistor having a first output/input terminal, a second output/input terminal and a control terminal, wherein the first output/input terminal is coupled with the first electric connection point, and the second output/input terminal is coupled with the second electric connection point;
• and
• a synchronous rectifying control circuit for controlling the electric status of the second power transistor and having an input control terminal, two detection terminals, and an output terminal, wherein the input control terminal is coupled with the third electric connection point, the two detection terminals respectively are coupled with the first output/input terminal and the second output/input terminal of the second power transistor, and the output terminal is coupled with the control terminal of the second power transistor;
• wherein the synchronous rectifying control circuit comprises:
• (i) a detection circuit coupled with the first output/input terminal and the second output/input terminal to form the two detection terminals and used for detecting a current flowing through the second power transistor, wherein the detection circuit outputs a turn-off signal when the current is smaller than the pre-determined current;
• (ii) a delay circuit coupled with the detection circuit and outputting a confirm signal when the turn-off signal lasts a first pre-determined period;
• and
• (iii) a driver coupled with the delay circuit and the input control terminal, wherein the driver turns the second power transistor on when the driver receives the synchronous signal at the input control terminal, and the driver turns the power transistor off when the driver receives the confirm signal;
• wherein, the synchronous rectifying control circuit turns the power transistor on when the synchronous rectifying control circuit receives a synchronous signal at the input control pin, and the synchronous rectifying control circuit turns the power transistor off when the synchronous rectifying control circuit detects that a current flowing through the power transistor is smaller than a pre-determined current by the two detection pins;
• wherein the first electric connection point and the second electric connection point respectively are coupled with the secondary side of the transformer and the output capacitor, and the third electric connection point is used for receiving a synchronous signal;
• an output detection unit coupled with the secondary side of the transformer for detecting the electric status of the secondary side to generate an output detection signal;
• an electric isolation unit coupled with the output detection unit for outputting the output detection signal with electric isolation, and
• a primary PWM controller coupled with the flyback converter and the electric isolation unit for turning on or turning off the first power transistor according to the output detection signal;
• wherein, the three-pin integrated synchronous rectifier is turned on for conducting an output current when receiving the synchronous signal, and the three-pin integrated synchronous rectifier is turned off when the output current is smaller than a pre-determined value.