Design of AC Inverter Power Supply Based on UC2525

1. Design requirements

This power supply is used in the front end of an AC voltage conversion. The input control signal is 4VAC (50HZ AC effective value range 2VAC-8VAC), and the input power is 350VDC (accuracy 0.5%). The output signal should be linearly proportional to the input signal (amplified 20 times, accuracy 0.5%), and the phase error between the input control signal and the output signal is less than 20', and the power load is not less than 30VA.

Special requirements: The control signal input impedance is required to be greater than 500M.

2. Design Analysis

The model of this power supply is an AC inverter power supply, but it provides a control signal and requires a high linear ratio with the input signal, and has a phase requirement. Therefore, this power supply is an AC signal amplifier in a certain sense.

The input power is 350VDC, which needs to be converted into an AC signal. The conversion method is to use SPWM to generate a square wave, and then convert it into a standard sine wave through LC. To generate SPWM, TI's UC2525 voltage-stabilized pulse width modulator is used, and then the on and off of the MOS tube is controlled to generate a square wave.

If the input signal requires high impedance, an amplifier can be used for isolation. Due to the output accuracy requirements, the amplifier's gain needs to be adjustable to meet the design requirements. The processed signal is input to UC2625 as a PWM duty cycle control signal to obtain the correct output.

The design requires accuracy and phase. In order to achieve the effect of closed-loop control, a small signal voltage transformer is added at the output end as feedback.

3. Partial Implementation Instructions

1 Control signal input processing

5. Design Regrets

There is one thing in the circuit that I still haven't calculated clearly, which is the duty cycle control of UC2525. This is why I brought this design to the TI blog contest. The circuit can have detailed design calculations from control signal output to feedback input and then to half-wave rectification, but due to the uncertainty of the reference triangle wave of UC2525 (for example, the uncertainty of the peak-to-peak value, the uncertainty of the starting voltage, etc.), the accuracy of the sine wave cannot be supported by more accurate design calculations, and can only be guaranteed by small full-scale adjustments and feedback adjustments, which brings great inconvenience to mass production. If TI staff see this design, I hope to help.