A switched-mode power supply comprising: a transformer with a primary winding (306, 308) and a secondary winding, the secondary winding having a first terminal and a second terminal; a rectifier with an input and an output, the input coupled to the first terminal of the secondary winding; an output capacitor with a first plate and a second plate, the first plate coupled to the output of the rectifier and the second plate coupled to the second terminal of the secondary winding; a power switch coupled in series with the primary winding, configured to be coupled between an output of a rectifier and ground; a signalling unit provided in parallel with the secondary winding, the signalling unit comprising a signalling capacitor and a signalling switch coupled in series with each other; a secondary controller configured to: detect a secondary side variable at the secondary winding, and control the signalling switch in accordance with the secondary variable, wherein closing the signalling switch is configured to generate a transformer signal by connecting the signalling capacitor to the secondary winding, in one of a single pulse and a plurality of pulses, during a ringing stroke between a secondary stroke of a first cycle and a primary stroke of a next cycle of the switched-mode power supply; a primary controller configured to: receive the transformer signal from the primary winding or a primary side auxiliary winding of the transformer, and control the power switch in accordance with the transformer signal. a transformer with a primary winding (306, 308) and a secondary winding, the secondary winding having a first terminal and a second terminal; a rectifier with an input and an output, the input coupled to the first terminal of the secondary winding; an output capacitor with a first plate and a second plate, the first plate coupled to the output of the rectifier and the second plate coupled to the second terminal of the secondary winding; a power switch coupled in series with the primary winding, configured to be coupled between an output of a rectifier and ground; a signalling unit provided in parallel with the secondary winding, the signalling unit comprising a signalling capacitor and a signalling switch coupled in series with each other; a secondary controller configured to: detect a secondary side variable at the secondary winding, and control the signalling switch in accordance with the secondary variable, wherein closing the signalling switch is configured to generate a transformer signal by connecting the signalling capacitor to the secondary winding, in one of a single pulse and a plurality of pulses, during a ringing stroke between a secondary stroke of a first cycle and a primary stroke of a next cycle of the switched-mode power supply; detect a secondary side variable at the secondary winding, and control the signalling switch in accordance with the secondary variable, wherein closing the signalling switch is configured to generate a transformer signal by connecting the signalling capacitor to the secondary winding, in one of a single pulse and a plurality of pulses, during a ringing stroke between a secondary stroke of a first cycle and a primary stroke of a next cycle of the switched-mode power supply; a primary controller configured to: receive the transformer signal from the primary winding or a primary side auxiliary winding of the transformer, and control the power switch in accordance with the transformer signal. receive the transformer signal from the primary winding or a primary side auxiliary winding of the transformer, and control the power switch in accordance with the transformer signal.