High side on-time control circuit and method for an asymmetrical half-bridge power converter

A primary controller for use in an asymmetrical half-bridge (AHB) power converter, the power converter comprising a high side switch, a low side switch, and a primary side winding, the primary controller comprising:

• a first terminal to be coupled to receive a first signal representative of a voltage across the primary side winding,
• a second terminal to be coupled to receive a feedback signal representative of an output of the AHB power converter;
• a third terminal to be coupled to receive a second signal representative of the voltage at a half-bridge node of the AHB power converter;
• a low side control circuit configured to respond to the feedback signal and the second signal and to generate a low side switch control signal in response thereto;
• and
• a high side control circuit configured to respond to the first signal and the second signal and to generate a high side switch control signal in response thereto;
• wherein the high side control circuit comprises:
• a maximum on-time circuit configured to generate a first high side control signal to control a maximum on-time of the high side switch;
• a proportional on-time circuit configured to generate a second high side control signal to control an on-time of the high side switch to be proportional to an on-time of the low side switch in a switching cycle of the AHB power converter;
• and
• a volt-second on-time circuit, configured to receive the first signal and to generate a third high side control signal in response thereto.

A method for controlling an on-time of a high side switch of an asymmetrical half-bridge power converter comprising the high side switch, a low side switch, and a primary side winding, the method comprising:

• turning on the low side switch;
• measuring a first signal representative of a voltage on the primary side winding during the on-time of the low side switch;
• integrating the first signal during the on-time of the low side switch to generate a first integrated value;
• turning off the low side switch and allowing a deadtime period to expire;
• turning on the high side switch;
• measuring a second signal representative of the voltage on the primary side winding during the on-time of the high side switch;
• integrating the second signal during the on-time of the high side switch to generate a second integrated value;
• comparing the second integrated value with the first integrated value;
• and
• turning off the high side switch when the second integrated value exceeds the first integrated value.

A method for controlling an on-time of a high side switch of an asymmetrical half-bridge power converter comprising the high side switch, a low side switch, and a primary side winding, the method comprising:

• generating a first high side control signal representing a maximum on-time of the high side switch;
• generating a second high side control signal by:
• integrating a first signal representative of a voltage on the primary side winding during the on-time of the low side switch to generate a first integrated value;
• integrating a second signal representative of the voltage on the primary side winding during the on-time of the high side switch to generate a second integrated value;
• and
• comparing the second integrated value to the first integrated value;
• generating a third high side control signal by:
• integrating a first reference voltage during the on-time of the low side switch to generate a third integrated value;
• integrating a second reference voltage proportional to the first reference voltage during the on-time of the high side switch to generate a fourth integrated value;
• and
• comparing the third integrated value to the fourth integrated value;
• and
• selecting among the first high side control signal, the second high side control signal, and the third high side control signal to control the on-time of the high side switch.