Synchronous rectification controller applied to power converter and start-up method for the same in start-up stage

A synchronous rectification controller applied to a power converter, the synchronous rectification controller coupled to a rectification switch at a secondary side of the power converter, the rectification switch comprising a rectification switch control terminal, a rectification switch first terminal, and a rectification switch second terminal, the synchronous rectification controller comprising:

• a driving signal terminal, coupled to the rectification switch control terminal,
• a power receiving terminal, configured to receive an operating voltage, wherein when the operating voltage is higher than an operating voltage threshold, the synchronous rectification controller outputs a driving signal through the driving signal terminal,
• a transistor, comprising a transistor first terminal, a transistor second terminal, and a transistor control terminal, the transistor first terminal coupled to the rectification switch control terminal, and the transistor second terminal coupled to the rectification switch second terminal, and
• a start-up control module, coupled to the transistor control terminal and the power receiving terminal, wherein the start-up control module comprises:
• a voltage control circuit, comprising an input terminal and an output terminal, the input terminal is coupled to the rectification switch first terminal, and the output terminal is coupled to the transistor control terminal,
• wherein when the operating voltage is lower than the operating voltage threshold, the transistor is turned on and a cross voltage between the rectification switch control terminal and the rectification switch second terminal is lower than a first threshold voltage of the rectification switch to prevent conduction of the rectification switch,
• wherein the transistor is a n-channel transistor and the synchronous rectification controller further comprises a driving circuit;
• wherein the driving circuit is coupled to the rectification switch control terminal through the driving signal terminal, and provides the driving signal to drive the rectification switch to be turned on when the operating voltage is higher than the operating voltage threshold,
• wherein the input terminal receives a drain voltage, and the output terminal outputs a control voltage positive correlated to the drain voltage,
• when the operating voltage is lower than the operating voltage threshold and the control voltage makes the voltage difference across the transistor control terminal and the transistor second terminal higher than a second threshold voltage, the transistor is turned on, and
• when the operating voltage is lower than the operating voltage threshold and the control voltage makes the voltage difference lower than the second threshold voltage, the transistor is turned off.

A start-up method for a rectification switch of a power converter in a start-up stage, the rectification switch coupled to a secondary side of the power converter, and the rectification switch comprising a rectification switch control terminal, a rectification switch first terminal, and a rectification switch second terminal, the start-up method comprising steps of:

• receiving an operating voltage, and limiting a cross voltage across the rectification switch control terminal and the rectification switch second terminal to be lower than a first threshold voltage of the rectification switch by turning on a transistor when the operating voltage is lower than an operating voltage threshold, wherein the transistor is a p-channel transistor,
• normally turning off the transistor when the operating voltage is higher than the operating voltage threshold,
• driving the rectification switch to be turned on by a driving signal provided by a driving circuit,
• turning on the transistor when the operating voltage is lower than the operating voltage threshold and a voltage difference across the transistor first terminal and the transistor control terminal is lower than a second threshold voltage of the transistor, and
• driving, by the operating voltage, the voltage difference to be always higher than the second threshold voltage to normally turn off the transistor when the operating voltage is higher than the operating voltage threshold.

A start-up method for a rectification switch of a power converter in a start-up stage, the rectification switch coupled to a secondary side of the power converter, and the rectification switch comprising a rectification switch control terminal, a rectification switch first terminal, and a rectification switch second terminal, the start-up method comprising steps of:

• receiving an operating voltage, and limiting a cross voltage across the rectification switch control terminal and the rectification switch second terminal to be lower than a first threshold voltage of the rectification switch by turning on a transistor when the operating voltage is lower than an operating voltage threshold, wherein the transistor is a n-channel transistor,
• normally turning off the transistor when the operating voltage is higher than the operating voltage threshold,
• driving the rectification switch to be turned on by a driving signal provided by a driving circuit,
• receiving a drain voltage of the rectification switch first terminal, and providing a control voltage corresponding to the drain voltage,
• turning on the transistor when the operating voltage is lower than the operating voltage threshold and a voltage difference across the transistor control terminal and the transistor second terminal is higher than a second threshold voltage due to the increase of the control voltage, and
• turning off the transistor when the operating voltage is lower than the operating voltage threshold and the voltage difference is lower than the second threshold voltage due to the decrease of the control voltage.