Communicating fault indications between primary and secondary controllers in a secondary-controlled flyback converter

An alternating current to direct current (AC-DC) flyback converter comprising:

• a primary-side field effect transistor (FET);
• a flyback transformer of the AC-DC flyback converter, the flyback transformer coupled to the primary-side FET;
• and
• a primary-side controller coupled to the flyback transformer, wherein the primary-side controller is configured to:
• receive a control signal from a secondary-side controller across a first galvanic isolation barrier;
• convert the control signal into a first pulse signal with a first pulse width;
• apply the first pulse signal to the primary-side FET in response to the control signal to turn on and turn off the primary-side FET;
• and
• communicate information to the secondary-side controller across a second galvanic isolation barrier of the flyback transformer by varying the first pulse signal with the first pulse width to generate a second pulse signal with a second pulse width and applying the second pulse signal with the second pulse width to the primary-side FET, the second pulse signal to turn on or turn off the primary-side FET, wherein varying the first pulse signal with the first pulse width comprises converting the control signal into the second pulse signal with the second pulse width, and wherein the secondary-side controller, responsive to the primary-side controller communicating the information, communicates information about a fault condition across a second galvanic isolation barrier of the flyback transformer by applying one or more additional pulses with the second pulse width that causes the secondary-side controller to detect a voltage on a secondary drain of a secondary-side FET going low using negative sensing (NSN).

An alternating current to direct current (AC-DC) power adapter device comprising:

• a power converter comprising a flyback transformer and a rectifier coupled between AC terminals and DC terminals, the rectifier to convert AC power on the AC terminals to DC power on the DC terminals, and the flyback transformer to provide galvanic isolation between the AC terminals and the DC terminals;
• a primary-side controller coupled to the flyback transformer;
• a secondary-side controller coupled to the flyback transformer;
• a primary-side power switch;
• and
• a secondary-side power switch, wherein the primary-side controller is configured to:
• receive a first signal from the secondary-side controller across a galvanic isolation barrier;
• convert the first signal into a second signal with a first pulse width;
• apply a second signal to the primary-side power switch in response to the first signal to turn on and turn off the primary-side power switch;
• and
• communicate information to the secondary-side controller across the flyback transformer by varying the second signal with the first pulse width to a third signal with a second pulse width and applying the third signal with the second pulse width to the primary-side power switch, the third signal to turn on or turn off the primary-side power switch, wherein varying the second pulse signal with the first pulse width comprises converting the second signal into the third signal with the second pulse width, and wherein the secondary-side controller, responsive to the primary-side controller communicating the information, communicates information about a fault condition across a second galvanic isolation barrier of the flyback transformer by applying one or more additional pulses with the second pulse width that causes the secondary-side controller to detect a voltage on a secondary drain of the secondary-side power switch going low using negative sensing (NSN).