Low voltage input DC/AC converter module

    Abstract: This article introduces the working principle, application scope and technical indicators of low-frequency AC modules working under low voltage input.

    Keywords: switching power supply, square wave, pulse width modulation

1 Introduction

As a relatively new type of regulated power supply, high-frequency switching power supply has the advantages of high efficiency, small size, and light weight. Therefore, it has been widely used in electronic instruments, meters and household appliances at home and abroad. Its products have developed rapidly and have broad market prospects. At present, high-frequency switching power supplies can be divided into three major categories: AC/DC converters are rectified power supplies, DC/DC converters are the conversion of various high and low voltage DC power supplies, and DC/AC converters are inverters and ring current generators. Among them, rectified power supplies and DC converters occupy a considerable market share. The main issues at present are how to further improve efficiency, reduce module size and make control and protection intelligent. Inverters have a low market share, while ring current generators are widely used in communication systems and sonar systems. The low-input DC/AC converter module introduced in this article is a ring current generator that operates at low voltage. It is used to develop new varieties of low-voltage ring current for electronic technicians to refer to when designing new products.

2 Module appearance structure and wiring

2.1 Module appearance

The module outline is shown in Figure 1.

2.2 Application wiring

See Figure 2 for application wiring.

3 Circuit Principle

The circuit principle is shown in Figure 3.

IC1, V1, L1, D1, etc. form a high-frequency oscillation circuit to generate high voltage for the pulse width modulation integrated circuit to work, pushing V3 to work in the switching state, boosting and rectifying the positive and negative high voltages through the transformer, R15, R16, R17, V7, D13, and IC4 form a sampling comparison amplifier circuit that is fed back to the pulse width modulator for voltage stabilization control. IC3, C13, R19, D12, and R20 generate low-frequency square waves to drive V5 and V6 to work, forming a continuous 25Hz square wave with an effective value of 75V at both ends of the output. D4, D5, L2, and C2 supply power to IC2 to ensure the normal operation of V3, and shut down IC1 through the circuit composed of V2, D3, R1, and R6 to stop oscillation. R11, R12, and C8 form an overcurrent detection circuit for overload protection. D10, D11, L5 and C12 form a low-voltage power supply to supply IC3 square wave generator and comparison amplifier to work. D14 and D15 can prevent V5 and V6 from being turned on at the same time.

The key points of circuit design are as follows:

(1) Low voltage working option

Since the current general pulse width modulation integrated circuit cannot work at around 5V, it must be at a working voltage greater than 8V to generate oscillation pulses. At the same time, V3 also requires a gate control voltage greater than 5V, so a series switching power supply is designed for IC2 For power supply, pin 3 of IC1 is the oscillation output terminal, which is amplified by R4, R5 and V1 to generate a voltage with an amplitude of 2Ui. After rectification and filtering by D1 and C2, a DC voltage of about 9V is formed. In order to save current when IC2 is working normally, a shutdown circuit is designed to shut down the oscillation of IC1 (pin 4 of IC1 is the switch control terminal, low level turns off and high level oscillates). When the entire circuit is working normally, the power supply voltage of IC2 is higher than 10V, which causes the zener diode D3 to work, triggering V2 to be fully conductive and placing pin 4 of IC1 at a low level.

(2) Output voltage stabilization design

Since the output effective value of the square wave output changes greatly from no load to full load, the peak value at no load is as high as more than 200V, which is easy to damage the load. In order to be safe and reduce the cost of components (reduce the back pressure value of V5 and V6), a voltage regulator is used The circuit control peak value does not exceed 150V when no-load. In order to reduce power consumption and reduce size, the positive voltage terminals of IC4 and R17 are connected to the power supply level of IC3. Due to the stability of the high voltage, the auxiliary power supply is also relatively stable, ensuring that IC3 is normal. Work.

(3) Square wave generation and frequency stabilization

The square wave generation of this circuit is composed of 555 clock circuit IC3, R19, R20, D12 and C13. D12 forms the discharge circuit. C13, R19 and R20 form the low-frequency oscillation circuit and determine the oscillation frequency. R19 and R20 determine the on and off. time ratio, select the appropriate resistance value by adjusting the waveform during design, so that the on-time and cut-off time are equal. During the frequency stabilization process, because the thermal stability of the resistor is relatively good, the 25Hz low frequency is greatly affected by C13. Because the capacitance value of C13 is large, it is easily affected by temperature and causes changes in capacitance (the temperature of the module is higher at full load than at no load). , through repeated selection tests, the polypropylene capacitor with good stability was finally selected to stabilize the output frequency within the allowable range.

4 Introduction to electrical properties

Input voltage +5V±5%

Output voltage 75V±10% (effective value)

Output frequency 25Hz±3Hz (square wave)

Output power 7.5W

Efficiency 70%

Input and output isolation voltage ≥500V

The electrical performance test meets the usage needs. If you need a more accurate low-frequency square wave, you can choose a better square wave generator.

5 Applications and Precautions

At present, this module has been produced in small batches and is used in vehicle command and communication systems under low-voltage operation to meet the requirements for on-board use. There is no need to connect an external heat sink during use. The external pins of the module should be welded close to the substrate and away from heat sources. The input power cord should be thicker. If the line is too long, a large electrolytic capacitor should be connected near the module output to avoid abnormal output and Produces high-frequency radiation that affects the operation of other circuits. A series of DC/AC converters can be developed based on this circuit model.