Flyback power converter having emulated demagnetized signal and switching control circuit and control method thereof

A switching control circuit for controlling a flyback power converter which is configured as a resonant flyback power converter, wherein the flyback power converter includes:

• a first transistor and a second transistor which are connected to a switching node to form a half-bridge circuit;
• and
• a transformer and a resonant capacitor which are connected in series forming a resonant tank, wherein the resonant tank is connected to the switching node and is switched by the half-bridge circuit;
• wherein the first transistor is configured to switch the resonant tank for generating a primary switching current and an output voltage;
• wherein the second transistor is configured to switch the resonant tank for generating a circulated current to achieve ZVS (zero voltage switching) of the first transistor, and for generating a demagnetized current;
• the switching control circuit comprising:
• a control unit, configured to operably generate a first signal for controlling the first transistor and a second signal for controlling the second transistor according to a feedback signal which is related to the output voltage;
• and
• a cycle counter, configured to operably count a number of switching cycles operating in a non-DCM operation according to the primary switching current;
• wherein the control unit performs at least one switching cycle of an actively forced DCM (discontinuous conduction mode) operation when the primary switching current is determined and counted to have been operating in the non-DCM operation for a predetermined number of switching cycles, wherein during the actively forced DCM operation, an OFF time between two neighboring ON periods of the first transistor is actively forced to be longer to an extent that the flyback power converter is forced to operate in DCM;
• wherein the control unit calibrates a period of a demagnetized signal based on a demagnetized time during the actively forced DCM operation by detecting a knee point related to the switching node;
• wherein the period of the demagnetized signal, calibrated during the actively forced DCM operation, is configured to emulate the demagnetized time of the transformer for controlling the second transistor during the subsequent non-DCM operation.

A control method for controlling a flyback power converter, wherein the flyback power converter which is configured as a resonant flyback power converter includes:

• a first transistor and a second transistor which are connected to a switching node to form a half-bridge circuit;
• and
• a transformer and a resonant capacitor which are connected in series forming a resonant tank, wherein the resonant tank is connected to the switching node and is switched by the half-bridge circuit;
• wherein the first transistor is configured to switch the resonant tank for generating a primary switching current and an output voltage;
• wherein the second transistor is configured to switch the resonant tank for generating a circulated current to achieve ZVS (zero voltage switching) of the first transistor, and for generating a demagnetized current;
• the control method comprising:
• generating a first signal for controlling the first transistor and a second signal for controlling the second transistor according to a feedback signal which is related to the output voltage;
• counting a number of switching cycles operating in a non-DCM operation according to the primary switching current;
• performs at least one switching cycle of an actively forced DCM (discontinuous conduction mode) operation when the primary switching current is determined and counted to have been operating in the non-DCM operation for a predetermined number of switching cycles, wherein during the actively forced DCM operation, an OFF time between two neighboring ON periods of the first transistor is actively forced to be longer to an extent that the flyback power converter is forced to operate in DCM;
• calibrating a period of a demagnetized signal based on a demagnetized time during the actively forced DCM operation by detecting a knee point related to the switching node;
• and
• emulating the demagnetized time of the transformer for controlling the second transistor during the subsequent non-DCM operation based on the period of the demagnetized signal which is calibrated during the actively forced DCM operation.