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　　Technical principles of square wave inverter

　　The power supply is the power part of electronic equipment and is a highly versatile electronic product. It has been widely used in various industries and daily life. Its quality greatly affects the reliability of electronic equipment. Its conversion efficiency and load capacity are directly related to its application scope. Square wave inverter is a low-cost, extremely simple conversion method. It is suitable for various rectifier loads, but it is not very adaptable to the load of the transformer and produces large noise. Based on the basic principles of inverter power supply and analysis and derivation of existing data, this article proposes a method of making a square wave inverter and debugging it.

　　1. Basic principles of the system

　　The input terminal of this inverter power supply is a battery (+12V, capacity 90A·h), and the output terminal is a power frequency square wave voltage (50Hz, 310V). Its structural block diagram is shown in Figure 1.

　　At present, there are many new technologies that constitute DC/AC inverter. However, considering the specific usage conditions, cost and reliability, this power supply still uses typical two-level conversion, namely DC/DC conversion and DC/AC inverter. First, the DC12V voltage is inverted into a high-frequency square wave by DC/DC conversion. It is boosted by a high-frequency step-up transformer, and then rectified and filtered to obtain a stable DC voltage of about 320V. Then, the DC/AC conversion is used to invert the square wave. , invert the stable DC voltage into a square wave voltage with an effective value slightly greater than 220V; then through LC power frequency filtering, a 50Hz AC voltage with an effective value of 220V is obtained to drive the load.

　　2. DC/DC conversion

　　Since the primary side voltage of the transformer is relatively low, in order to improve the utilization rate of the transformer and reduce costs, the DC/DC conversion is shown in Figure 2. A push-pull circuit is used. The center tap of the primary side is connected to the battery, and both ends are controlled by switching tubes to work alternately. , which can improve conversion efficiency. The push-pull circuit uses fewer switching devices, and the transformer working at both ends is relatively small, which can increase the duty cycle and increase the output power.

　　The core area product formula of a square wave inverter transformer working at both ends is

　　AeAc=Po(1+η)/(ηDKjfKeKcBm) (1)

　　In the formula: Ae (m2) is the cross-sectional area of the iron core;

　　Ac(m2) is the window area of the core;

　　Po is the output power of the transformer;

　　eta is the conversion efficiency;

　　δ is the duty cycle;

　　K is the waveform coefficient;

　　j(A/m2) is the average current density of the wire;

　　f is the inverter frequency;

　　Ke is the effective coefficient of core section;

　　Kc is the window utilization coefficient of the core;

　　Bm is the maximum magnetic flux.

　　The switching tubes S1 and S2 on the primary side of the transformer each use IRF32055 and are connected in parallel. The reason for parallel connection is mainly because when the inverter power supply is connected to the load, the current on the primary side of the transformer is relatively large. Parallel connection can shunt the current, which can effectively reduce the number of switching tubes. power consumption without causing damage.

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