Isolated switching converter with continuous conduction mode and discontinuous conduction mode

1 . A controller for an isolated switching converter with a transformer, a primary switch and a secondary switch, the controller comprising:

an error amplifying circuit configured to provide a compensation signal based on an output feedback signal indicative of an output signal of the isolated switching converter and a reference voltage;

a mode judgement circuit configured to provide a mode signal to determine a first drive mode or a second drive mode based on a first voltage across the secondary switch and the compensation signal, and the mode signal has a first level indicating the first drive mode and a second level indicating the second drive mode; and

a gate driver configured for being switchable in the first drive mode and the second drive mode to provide a gate voltage for controlling the secondary switch, wherein in the first drive mode, the secondary switch is turned on for a second ON-time after a current flowing through the secondary switch crosses zero, and in the second drive mode, the ON-state of the secondary switch is extended for an extended ON-time.

2 . The controller of claim 1 , wherein:

if a valley number of the first voltage across the secondary switch is zero and the compensation signal is higher than a first threshold voltage, the mode signal becomes the second level;

if the valley number of the first voltage across the secondary switch is higher than zero, the mode signal becomes the first level; and

if the compensation signal is less than a second threshold voltage, the mode signal becomes the first level, wherein the first threshold voltage is higher than the second threshold voltage.

3 . The controller of claim 1 , wherein in the first drive mode:

in a first time interval, the gate voltage of the secondary switch is controlled and is kept at a third voltage;

in a second time interval, the gate voltage of the secondary switch is pulled down;

in a third time interval, the gate voltage of the secondary switch is substantially zero; and

in a fourth time interval, the gate voltage of the secondary switch is pulled high to the third voltage.

4 . The controller of claim 1 , wherein in the second drive mode:

in a first time interval, the gate voltage of the secondary switch is controlled and is kept at a third voltage;

in a second time interval, the gate voltage of the secondary switch is pulled down; and

in a third time interval, the gate voltage of the secondary switch is pulled high to the third voltage.

5 . The controller of claim 1 , wherein the gate driver comprising:

a timer circuit configured to start timing in response to a valley number of the first voltage across the secondary switch being zero, if the timing duration of the timer circuit reaches a time threshold and the current flowing through the secondary switch is still higher than zero, the ON-state of the secondary switch is extended.

6 . The controller of claim 1 , wherein the gate driver is configured to provide a primary on enable signal at an end point of the second ON-time or at an end point of the extended ON-time, and the controller further comprising:

an isolation circuit having a first input terminal to receive the primary on enable signal and a first output terminal for outputting a first synchronous signal electrically isolated from the primary on enable signal;

a zero cross detection circuit configured to provide a voltage zero-crossing detection signal by detecting whether a second voltage across the primary switch crosses zero; and

a primary logic circuit configured to provide a primary control signal for controlling the primary switch based on the first synchronous signal and the voltage zero-crossing detection signal.

7 . The controller of claim 6 , further comprising:

an observation window setting circuit configured to receive the first synchronous signal and to provide an observation window based on the first synchronous signal;

a second comparison circuit configured to compare a primary current sensing signal representative of a current flowing through the primary switch with a current threshold voltage, and to provide a second comparison signal; and wherein

the primary logic circuit provides the primary control signal based on the observation window, the voltage zero-crossing detection signal and the second comparison signal.

8 . The controller of claim 6 , further comprising:

a pulse signal generator configured to provide a pulse signal when a ramp signal increases to a difference between a second reference voltage and the compensation signal, wherein the ramp signal increases from zero with a slope when the primary on enable signal is activated;

the isolation circuit further has a second input terminal to receive the pulse signal and a second output terminal for outputting a second synchronous signal electrically isolated from the pulse signal; and

a threshold voltage generator coupled to the second output terminal of the isolation circuit to receive the second synchronous signal and configured to provide a current threshold voltage based on the second synchronous signal.

9 . An isolated switching converter, comprising:

a transformer having a primary winding and a secondary winding;

a primary switch coupled to the primary winding;

a secondary switch coupled to the secondary winding; and

a controller, comprising:

an error amplifying circuit configured to provide a compensation signal based on an output feedback signal indicative of an output signal of the isolated switching converter and a reference voltage;

a mode judgement circuit configured to provide a mode signal to determine a first drive mode or a second drive mode based on a first voltage across the secondary switch and the compensation signal, and the mode signal has a first level indicating the first drive mode and a second level indicating the second drive mode; and

a gate driver for being switchable in the first drive mode and the second drive mode to provide a gate voltage for controlling the secondary switch, wherein the secondary switch is turned on for a second ON-time after a current flowing through the secondary switch crosses zero in the first drive mode, and the ON-state of the secondary switch is extended for an extended ON-time in the second drive mode.

10 . The isolated switching converter of claim 9 , wherein:

if a valley number of the first voltage across the secondary switch is zero and the compensation signal is higher than a first threshold voltage, the mode signal becomes the second level;

if the valley number of the first voltage across the secondary switch is higher than zero, the mode signal becomes the first level; and

if the compensation signal is less than a second threshold voltage, the mode signal becomes the first level, wherein the first threshold voltage is higher than the second threshold voltage.

11 . The isolated switching converter of claim 9 , wherein in the first drive mode:

in a first time interval, the gate voltage of the secondary switch is controlled and is kept at a third voltage;

in a second time interval, the gate voltage of the secondary switch is pulled down;

in a third time interval, the gate voltage of the secondary switch is substantially zero; and

in a fourth time interval, the gate voltage of the secondary switch is pulled high to the third voltage.

12 . The isolated switching converter of claim 9 , wherein in the second drive mode:

in a first time interval, the gate voltage of the secondary switch is controlled and is kept at a third voltage;

in a second time interval, the gate voltage of the secondary switch is pulled down; and

in a third time interval, the gate voltage of the secondary switch is pulled high to the third voltage.

13 . The isolated switching converter of claim 9 , wherein the gate driver comprising:

a timer circuit configured to start timing in response to a valley number of the first voltage across the secondary switch being 0, if the timing duration reaches a time threshold and the current flowing through the secondary switch is still higher than zero, the ON-state of the secondary switch is extended.

14 . The isolated switching converter of claim 9 , wherein the gate driver is configured to provide a primary on enable signal at an end point of the second ON-time or at an end point of the extended ON-time, and the controller further comprising:

an isolation circuit having a first input terminal to receive the primary on enable signal and a first output terminal for outputting a first synchronous signal electrically isolated from the primary on enable signal;

a zero cross detection circuit configured to provide a voltage zero-crossing detection signal by detecting whether a second voltage across the primary switch crosses zero; and

a primary logic circuit configured to provide a primary control signal for controlling the primary switch based on the first synchronous signal and the voltage zero-crossing detection signal.

15 . The isolated switching converter of claim 14 , wherein further comprising:

a pulse signal generator configured to provide a pulse signal when a ramp signal increases to a difference between a second reference voltage and the compensation signal, wherein the ramp signal increases from zero with a slope when the primary on enable signal is activated;

the isolation circuit further has a second input terminal to receive the pulse signal and a second output terminal for outputting a second synchronous signal electrically isolated from the pulse signal; and

a threshold voltage generator coupled to the second output terminal of the isolation circuit to receive the second synchronous signal and configured to provide a current threshold voltage based on the second synchronous signal.

16 . A control method for an isolated switching converter, wherein the isolated switching converter has a primary switch coupled to a primary winding of a transformer, a secondary switch coupled to a secondary winding of the transformer, the control method comprising:

providing a compensation signal based on a difference between an output feedback signal representative of an output signal of the isolated switching converter and a reference voltage;

providing a mode signal to determine a first drive mode or a second drive mode based on a first voltage across the secondary switch and the compensation signal, wherein the mode signal has a first level indicating the first drive mode and a second level indicating the second drive mode;

turning on the secondary switch for a second ON-time after a current flowing through the secondary switch decreases to zero in the first drive mode; and

the ON-state of the secondary switch is extended for an extended ON-time in the second drive mode.

17 . The control method of claim 16 , further comprising:

if a valley number of the first voltage across the secondary switch is zero and the compensation signal is higher than a first threshold voltage, the mode signal becomes the second level;

if the valley number of the first voltage across the secondary switch is higher than zero, the mode signal becomes the first level; and

if the compensation signal is less than a second threshold voltage, the mode signal becomes the first level, wherein the first threshold voltage is higher than the second threshold voltage.

18 . The control method of claim 16 , wherein in the first drive mode:

in a first time interval, the gate voltage of the secondary switch is controlled and is kept at a third voltage;

in a second time interval, the gate voltage of the secondary switch is pulled down;

in a third time interval, the gate voltage of the secondary switch is substantially zero; and

in a fourth time interval, the gate voltage of the secondary switch is pulled high to the third voltage.

19 . The control method of claim 16 , wherein in the second drive mode:

in a first time interval, the gate voltage of the secondary switch is controlled and is kept at a third voltage;

in a second time interval, the gate voltage of the secondary switch is pulled down; and

in a third time interval, the gate voltage of the secondary switch is pulled high to the third voltage.

20 . The control method of claim 16 , further comprising:

starting timing in response to a valley number of the first voltage across the secondary switch being zero; and

if the timing duration reaches a time threshold and the current flowing through the secondary switch is still higher than zero, the ON-state of the secondary switch is extended for the extended ON-time.

21 . The control method of claim 20 , further comprising:

starting timing at a start point of the extended ON-time; and

if the timing duration reaches a time threshold and the current flowing through the secondary switch is still higher than zero, the ON-state of the secondary switch is extended.

22 . The control method of claim 16 , further comprising:

providing a primary on enable signal at an end point of the second ON-time or at an end point of the extended ON-time;

receiving a first synchronous signal electrically isolated from the primary on enable signal through an isolation circuit; and

providing a primary control signal for controlling the primary switch based on the first synchronous signal and a voltage zero-crossing detection signal indicating whether a second voltage across the primary switch crosses zero.

23 . The control method of claim 22 , further comprising:

increasing a ramp signal from zero with a slope when the primary on enable signal is activated;

providing a pulse signal when the ramp signal increases to a difference between a second reference voltage and the compensation signal;

receiving the pulse signal and outputting a second synchronous signal electrically isolated from the pulse signal through the isolation circuit; and

provide a current threshold voltage based on the second synchronous signal.