A switching power supply can be classified as either an isolated or non-isolated power supply, based on the presence of electrical isolation. An isolated power supply uses devices in the power transmission and signal feedback paths to provide electrical isolation, thereby protecting against ground loops and power supply failures. Traditional isolated power supplies utilize linear optocouplers, which face limitations such as limited bandwidth and aging. To address these issues, this paper proposes the design of an isolation error amplifier based on a high-voltage capacitor to close the isolation feedback loop. PWM and OOK modulation are used to enable analog signal transmission across the isolation barrier, offering the advantages of high integration and CMTI. The operational principles of the proposed isolation error amplifier and the modulation/demodulation process are analyzed in detail. The proposed chip is designed using a 0.18 μm BCD process. Simulation results demonstrate that the isolation error amplifier can support error voltage transmission of 1x and 2.6x when the output voltage of the isolated power supply is between 4 and 40 V, which is used to drive the PWM controller.