Offline synchronous rectifying circuit with current transformer for soft switching power converters
View Patent ↗A synchronous rectifying circuit of soft switching power converter is provided to improve the efficiency. The integrated synchronous rectifier includes a power transistor connected from a transformer to the output of the power converter for rectifying. A controller having a latch circuit generates a drive signal to control the power transistor in response to a switching-current signal. A current transformer generates the switching-current signal in response to the switching current of the transformer. The controller turns off the power transistor when the switching-current signal is lower than a second threshold. The power transistor is turned on once the switching-current signal is higher than a first threshold. Furthermore, a pulse-width detection circuit generates a pulse signal coupled to disable the drive signal and turn off the power transistor.
1. A synchronous rectifying circuit of soft switching power converter with a transformer, comprising:
a current transformer generating a switching-current signal in response to the switching of the transformer; and
an integrated synchronous rectifier comprising:
a power transistor coupled to the transformer and an output of the power converter for the rectifying; and
a controller generating a drive signal to control the power transistor directly in response to the switching-current signal; wherein the controller generates a start signal when the switching-current signal is higher than a first threshold; the controller generates a stop signal when the switching-current signal is lower than a second threshold; the start signal is coupled to enable the drive signal to turn on the power transistor; the stop signal is coupled to disable the drive signal to turn off the power transistor;
wherein the controller comprises a pulse-width detection circuit generating an integration signal, a ramp signal and a pulse signal; wherein the integration signal is generated in accordance with a period between the start signal and the stop signal, the ramp signal is generated in response to the start signal, the pulse signal is generated by comparing the integration signal and the ramp signal to disable the driver signal and turn off the power transistor.
2. The synchronous rectifying circuit of claim 1 , wherein the controller comprises an inner-lock circuit generating an inner-lock signal in response to the enablement of the drive signal;
wherein the drive signal is initiated when the inner-lock signal is disabled.
3. The synchronous rectifying circuit of claim 1 , wherein the controller comprises a maximum-on-time circuit generating a maximum-on-time signal, wherein the maximum-on-time signal is coupled to turn off the power transistor for limiting a maximum on-time of the power transistor.
4. The synchronous rectifying circuit of claim 1 , wherein the controller further comprises:
a latch circuit generating the drive signal to control of the power transistor; and
a plurality of comparators generating a start signal and a stop signal coupled to set or rest the latch;
wherein the drive signal is enabled when the switching-current signal is higher than the first threshold; the drive signal is disabled when the switching-current signal is lower than the second threshold.
5. The synchronous rectifying circuit of claim 1 , wherein the controller comprises a monitor circuit for generating a reset signal, wherein the reset signal is coupled to disable the power transistor when amplitude of the switching-current signal is lower than a third threshold voltage.
6. The synchronous rectifying circuit of claim 1 , further comprising a diode connected to the power transistor in parallel, and the power transistor is turned on after the diode is on.
7. A method of synchronous rectifying for a soft switching power converter, the soft switching power converter including a transformer, comprising:
generating a switching-current signal in response to a switching current of the transformer;
setting or resetting a latch in response to the switching-current signal; and
turning on/off a power transistor in accordance with a status of the latch;
wherein the power transistor is coupled to the transformer and an output of the power converter for rectifying, and the switching-current signal is generated in response to the switching current of the transformer;
wherein further comprises a step of generating an integration signal, a ramp signal and a pulse signal; the integration signal is generated in accordance with a period of the switching-current signal, the ramp signal is generated in response to enable of the switching-current signal, and the pulse signal is generated by comparing the integration signal and the ramp signal to turn off the power transistor.
8. The method according to claim 7 , further comprising:
generating an inner-lock signal in response to the enable of a drive signal;
wherein the driver signal is initiated when the inner-lock signal is disabled.
9. The method according to claim 7 , further comprising:
generating a maximum-on-time signal in response to the turn on of the power transistor, wherein the maximum-on-time signal turns off the power transistor for limiting a maximum on-time of the power transistor.
10. The method according to claim 7 , further comprising:
generating a reset signal to disable the power transistor when the switching-current signal is lower than a voltage threshold.
11. The method according to claim 7 , further comprising:
generating a reset signal to disable the power transistor when a supply voltage of the latch is lower than a VCC threshold.
12. The method according to claim 7 , further comprising:
generating a pass signal for allowing the enable of the power transistor;
wherein the pass signal is generated when a diode is conducted; the diode is connected in parallel to the power transistor.