Design of a Capacitive-based Isolated Signal Transmission Circuit

In response to the difficulty of integrating traditional optocoupler isolation in the secondary-side feedback of AC-DC converters, a feedback signal transmission circuit of AC-DC converter is designed based on SMIC 180nm BCD process in this paper. The circuit adopts capacitive isolation technology and consists of two parts: the transmitter and the receiver. The transmitter is responsible for modulating the input signal, and the receiver is used to receive the modulated signal through the isolation capacitor and demodulate it, and restore it to the input signal. In the transmitter, a non-overlap modulation signal generation circuit and a midpoint potential bias circuit are proposed. The former can avoid the simultaneous on-off of the upper and lower the dynamic power consumption. The latter can solve the error decoding caused by the DC potential attenuation at the receiving end. A pre-processing circuit is proposed in the receiver to avoid the effect of the comparator’s input capacitance on the signal transmission quality. Compared with the traditional optocoupler isolation scheme, the capacitive isolation transmission circuit designed in this paper can be better compatible with CMOS process, realize the integration of the original and secondary main control chips, and improve the power density of the system. Finally, simulation and verification are carried out in the AC-DC system. Simulation results show that the 5MHz digital signal can be transmitted correctly under the power supply voltage of 3.5 ∼ 5.5V, and the transmission delay is only 17.1ns in typical state, and the typical dynamic power consumption is 1.28mW.