Design of Switching AC Stabilized Power Supply

　　At present, the power supplies in space technology, computers, communications, radars and household appliances are gradually replaced by switching power supplies. The series-adjusted voltage-stabilized power supply generally used now is a continuously controlled linear voltage-stabilized power supply. This traditional series voltage regulator and adjustment tube always work in the amplification area, and the current flowing through is continuous. The disadvantage of this voltage regulator is that the ability to withstand overload and short circuit is poor, and the efficiency is low, generally only 35% to 60%. Since the adjustment tube has to lose a lot of power, a high-power adjustment tube is required, and a large heat sink is installed. The adjustment tube of the switching power supply works in the switching state, with low power loss and an efficiency of 70% to 95%. The voltage regulator is small in size and light in weight, and the power loss of the adjustment tube is small, and the heat sink is also reduced accordingly. In addition, the switching frequency works at tens of kHz, and filter inductors and capacitors with smaller values ​​can be used, so the allowable ambient temperature can be greatly increased.

　　1 Circuit composition and working principle

　　The block diagram of the switching AC regulated power supply circuit is shown in Figure 1. Description of the working principle: The triangle wave generating circuit generates a 150 kHz triangle wave, and the low-frequency sine wave generating circuit generates a 50 Hz sine wave. The two signals are sent to the in-phase and inverting input terminals of the comparator at the same time, and a rectangular wave will be generated at the output terminal of the comparator. The frequency of the rectangular wave is the same as that of the 150 kHz triangle wave. After the pulse width of the rectangular wave is modulated by the real-time amplitude of the 50 Hz sine wave, it changes with the real-time amplitude of the 50 Hz sine wave, that is, a modulated rectangular wave. It is sent to one input terminal of the high-speed electronic switch, and is reversed by a first-level inverter and sent to the other input terminal of the high-speed electronic switch.

　　The DC high voltage obtained by the mains rectification and filtering is twice the input AC voltage (typical value is about 3.11 V) and is sent to the power input end of the high-speed electronic switch. The two output ends of the high-speed electronic switch are driven by two reverse input rectangular waves. After obtaining energy from the approximately 3.11 V DC power supply, they are connected to the primary of the high-frequency switch transformer through a short time constant LC filter circuit. The function of the LC filter circuit is to make the corners of the rectangular wave pulse entering the primary of the high-frequency switch transformer smooth to reduce its high-frequency harmonics. The primary and secondary of the high-frequency switch transformer also play a role in isolating the mains. The secondary of the high-frequency switch transformer obtains an alternating rectangular wave voltage with smooth corners. After passing through a multi-stage long time constant LC filter circuit, the 150 kHz high-frequency signal is filtered out and a 50 Hz sine wave modulation signal is restored and sent to the load for powering the load.

　　The voltage and current sampling circuit obtains the voltage and current signals from the load, and sends them to two A/D converters for conversion, and turns them into discrete digital signals. On the one hand, it is used for real-time display after being processed by the microprocessor; on the other hand, it is used for sending the D/A converter to convert it into analog quantity after being processed by the microprocessor, and the amplitude of the sine wave generator is controlled by the photoelectric isolation drive circuit, and then it is used to control the stability of the voltage or current on the load through the comparator, inverter, high-speed electronic switch, LC filter, high-frequency switch transformer, multi-stage LC filter and other circuits. The function of the voltage transformer is to obtain a standard sine wave with low harmonic distortion from the mains, and control its amplitude through the sine wave generating circuit; the keyboard is used to input the voltage or current value to be provided to the load.

　　2 Circuit Design Analysis

　　2.1 Controllable sine wave generating circuit

　　The circuit diagram of the controllable sine wave generating circuit is shown in FIG2 .

　　The source of the sine wave is a 220 V/50 Hz sine wave directly from the mains, which is converted into a lower voltage 50Hz sine wave (for example, 5 V) using a voltage transformer. The harmonic distortion of the sine wave depends on the harmonic distortion of the mains and the parameters of the transformer. Its output amplitude is achieved by the D/A converter controlling the photoelectric coupler drive circuit. The D/A converter output signal controls the conduction degree of the photoelectric coupler, and after voltage division with the voltage divider resistor, a voltage superimposed by AC and DC is generated. The DC component is isolated by a capacitor, and only the AC component is retained and sent to the operational amplifier for several times amplification, generating a pure AC signal amount controlled by the amplitude of the D/A signal.

　　The principle of D/A control signal generation is: comprehensive calculation is performed based on the voltage or current output to the load and the voltage amplitude of the AC power. The microprocessor provides the D/A converter with the digital value obtained through the comprehensive calculation, so that the output voltage (or current) provided to the load tends to be stable.

　　2.2 Pulse Width Modulator

　　The PWM generation circuit consists of three parts: a sine wave generation circuit, a triangle wave generation circuit and a comparator. The triangle wave is added to the inverting input of the comparator, the sine wave is added to the same-direction input of the comparator, and the comparator output generates a pulse width modulation wave that changes with the instantaneous amplitude of the sine wave.

　　Figure 3 is the working waveform of the voltage-type PWM comparator. The input triangle wave is connected to the inverting input terminal of the comparator, and the controllable sine wave signal is sent to the non-inverting input terminal of the comparator, and the PWM signal is output after amplification.

　　2.3 High-speed electronic switching

　　The high-speed electronic switch circuit is used to amplify the PWM wave power. With the high-frequency electronic transformer and filter circuit, it can realize the demodulation circuit that restores the parameter when the input signal is a rectangular wave modulated by a certain signal parameter and the output signal is a demodulation circuit. Its typical circuit diagram is shown in Figure 4, which is the waveform of PWM through the inverter. The whole circuit consists of a bridge switch circuit composed of 4 field effect tubes, a high-frequency switch transformer, and multiple groups of LC filter circuits (only one group L3, C3 is drawn in the figure).

　　The high-frequency switching transformer Tr also serves as the mains isolation. In the circuit, L1, C1 and L2, C2 form a filter circuit to change the corner of the rectangular wave input to the primary of the high-frequency switching transformer into a "slow-changing" shape, so as to reduce the harmonic components flowing through the transformer and reduce interference.

　　The voltage induced by the secondary of the high-frequency switching transformer is further filtered through L3 and C3 (actually multi-stage LC, such as three-stage) to filter out the high-frequency rectangular wave of PWM, and obtain the restored sinusoidal waveform of the original modulation wave on the load, as shown in Figure 5.

　　The modulated wave restored in Figure 5 actually has a certain degree of sawtooth wave component. If the waveform is stored in a digital storage oscilloscope and then locally enlarged for observation, it can be found that the sawtooth shape after local amplification is shown in Figure 5. The degree of sawtooth reflects the distortion of the signal and is related to the performance parameters of the multi-stage LC filter.

　　2.4 Microprocessor

　　The microprocessor part is used to realize the intelligence of the system device. The microprocessor part includes microprocessor chip, keyboard, LCD display, A/D and D/A converters. The microprocessor chip suitable for control often uses a single-chip microcomputer, which basically includes I/O interface circuit, ROM, RAM, timer and interrupt system, so these components basically do not need to be expanded.

　　The software design includes subroutines for functions such as A/D converter, D/A converter, LCD display, keyboard system, etc. It also includes system monitoring program and various interrupt service programs.

　　3 Conclusion

　　The switching AC regulated power supply introduced here is a relatively advanced AC power supply design. With the rapid development of the times, the integration and miniaturization of switching power supplies are becoming a reality. Currently, integrated modules with switches and control circuits integrated on the same chip are being developed. However, integrating the power switch and control circuit, including the feedback circuit, on the same chip requires solving the problems of electrical isolation and thermal insulation, which will be a major research topic in the future.