The parameter optimization and the experiment analyzes of main circiut of soft-switching inverter resistance spot welder Yu Ming1 Zhang Wei2 Cui Huaixiang3 Fang Chenfu3 1. MacMic Science & Technology Co., Ltd Changzhou 213022. 2. Councry Da tun 65370 Army Team 65 Changchun 130163. 3. Advanced Welding Technology Provincial key laboratory, Jiangsu University of science and Technology, zhenjiang 212003 Abstract: The inverter main circuit of resistance spot welder has been designed, which is used of soft-switching technology in this article. Full-bridge main circuit structure is adopted as well as having saturated inductance. Making use of saturated inductance effect, which appears so big inductive reactance that the current keeps zero condition because of hindered breaking, when removing from saturation, which provide condition for the lagging legs cutting off under zero current state. Frequency effect is analyzed in soft-switching inverter spot welder in detailed. The main circuit parameters are optimized, through analysis using the theoretical calculation and electric circuit simulation of Pspice. By the analysis of the experiment，we can conclude: The designed main circuit parameter is reasonable; the performance meets the designed requirements. Keys: soft-switching resistance spot main circuit simulation 1 Introduction Resistance spot welding is a widely used production technology. In recent years, because of the development of new materials and new technologies, the requirements of resistance spot welding technology are increasingly stringent. Resistance spot welding technology account for a large proportion of the aerospace, electronic and automobile production in the industrial fields, particularly it plays an irreplaceable position in the aviation and aerospace industry. Its Welding quality standards have unique requirements and the production process of high quality must be maintained stability. The development of resistance spot welder experienced the process of single-phase frequency exchange spot welder, DC pulse spot welder, three-phase low-frequency spot welder, sub-rectifier and capacitor discharge spot welder, in the middle of 1980s there was inverter spot welder. Japan, the United States and other countries have launched inverter resistance welders, which were applied to the automotive, appliance, electronics industry, and they set up a spot welding robot inverter consisting of automobile body assembly line to bring the inverter resistance spot welder into the practical application stage. The research of foreign inverter spot welder is much mature and there is relatively high degree of commercialization. Domestic inverter welding technology in the application is mature, and has formed arc welding inverter series. But in resistance welding areas, the advantages of this technology has not been exploited, remains to be further research and development. Therefore a soft switch controlling inverter resistance spot welder was developed. 2 The choice of main circuit topology The main circuits of inverter resistance welding power supply are dual Forward circuit and full-bridge circuit. The transformers in full-bridge circuit work in the two-way excitation model, which can reduce the quality and size of transformers. And the maximum duty cycle of full-bridge circuit transformer’s secondary side close to 1, compared to double-Forward circuit, the output current rate of increase has been greatly increased. This paper selected full-bridge inverter main circuit topology, as shown in fig 1. Vin is input DC voltage source. Q1 to Q4 are power devices-IGBT, D1 to D4 are the reverse parallel diodes of Q1 to Q4 respectively, C1 and C2 are the parasitic capacitances of Q1 and Q4, Cb are blocking capacitor. Lk is the transformer leakage, Tr is the transformer, DR1 and DR2 are output rectifier diodes, R is a load. When two IGBT on the cross of the full-bridge inverter circuit are lead at the same time, supply will input energy, at the same time Cb is charged, the voltage of Cb changes from one direction to another direction, the saturated inductor is saturated. In the leading leg (Q1 is turn off earlier than Q4, Q2 is turn off earlier than Q3, so Q1 and Q2 are leading leg, Q3 and Q4 are lagging leg), after the process of changing switch state, primary side is in continuous flow stage, the voltage of Cb maintains constant. And the voltage polar tries to reverse direction after the primary current polar drops to zero, while the saturated inductance has withdrawn from saturation, showing a lot of inductance to prevent the further flow of the current so that the current remains at zero, so as to create a zero-current switching conditions for the switching tubes on lagging leg. Fig.1 Main circuit topology 3 The choice and calculation of main circuit parameters 3.1 The determination of the inverter frequency The soft switch spot welding machine power used the control method of PWM bipolar limited. And it's power mediation is Fixed Frequency and Modulated Pulse Width, so the inverter frequency can influence direct the work performance and circuit response performance of the system. The choice of inverter frequency base the whole design of power, seeking high frequency blindly is skimble-skamble, and heightening the frequency should be for enhancing the performance of the power supply design. The influence of the inverter frequency to soft switch spot welding power is positive, but the improper choice of inverter frequency also brings the negative impact, so the determination of the inverter frequency is a choice - optimization process. The choice of inverter frequency should consider two aspects: 1. The active effects of inverter frequency The heightened frequency can affect the design parameters of spot welding machine transformer. the product of transformer turns and the transformer core cross-sectional areas in proportion to the work frequency, so heightening the frequency can reduce substantially bulk and weight of spot welding machine transformer. Increased frequency of welding machine can shorten the control cycle, so improve system performance of dynamic response, welding current control precision, control the waveform, and realize the non-splash, stable quality, and energy-efficient precision welding. 2. The negative effects of inverter frequency As spot welding machine transformer secondary windings generally only has one turn, so turns has not reduced; secondary windings passes the big current, and it's cross-sectional areas is big, and core window area can not be too small, further improving the inverter frequency has little value to reduce solder size and weight. And the increased frequency can increase hysteretic and eddy current of transformer, increase the sub-loop pressure-drop of inductance, limit the output current. Although the frequency of inverter uses the soft-switch, the effect of improving frequency to the switching loss is not so obvious, but rectifier diodes of the sub-loop are still in hard switching forced converter form, heightened frequency can increase switching losses and the heat of rectifier diodes, instead reduce the efficiency of welding machine. And, high-frequency circuit distribution parameters are big, so can produce high frequency oscillation and electromagnetic interference, the requirements of the system reliability are higher. Comprehensive considering these factors, with the actual circuit design, so we choose 2 KHz as switching frequency of the soft switch spot welding machine power. 3.2 The design of saturated inductance Realized lag arm ZCS, in the continued flow stage, when the voltages on the blocking capacitor Cb are approximately constant, the polar and current of the former edge down to zero then try to change direction, saturated inductance must withdraw from the saturated state, and show a big inductance. According to Faraday's Law，the formula of the smallest turns nmin can be drawn. n min = NBs L e μΙ f (1) N: Turns ratio of inverter transformer; Bs: Core saturation magnetic induction; Le: Magnetic core length; µ: The core rate of Magnetic; If: The minimum load current for soft-switching requirements. After a preliminary calculation and verification, the ONL-644020 nanocrystalline base-iron toroid core was selected as the core of saturated inductance. And it's core effective cross-sectional area Se=1.68cm2, Core length Le＝1.63cm, Core saturation magnetic induction Bs＝1.25T, Residual magnetic induction Br=0.2T, The core rate of Magnetic µ＝2×104Gs/Oe. New Soft-Switch spot welding power supply is used for the welding of precision pieces, rate current of welding is 1500A, in 10% of the load current soft switching was achieved, viz. If＝150A. According to the Formula-2, the minimum number of turns of saturated inductance: n min NBs L e 100 × 1.25 × 16.3 × 10 −2 ≈ 5.7 = = μΙ f 2 × 4π × 10 −3 × 150 (2) The turns of Saturated inductance n＝6. The Unsaturated value of saturated inductance L : L s = μn 2 Se 1.68 × 10 −2 = 2 × 6 2 × 4π × 10 − 3 ≈ 9.32 × 10 − 4 ( H ) le 16 .3 (3) 3.3 The design of the electric capacity value in leading leg The electric capacity value in leading leg was increased, and IGBT’s Shutdown losses significantly lower, but increasing the same capacitance values, loss reduced of the relative value is getting smaller and smaller. But, in order to consider the cost and practicality, using Pspice to analysis and Simulate the main circuit, the simulation results shown in Figure-2, Poff（Q1） for the commissioner breakpoint of IGBT (Q1). According to the simulation results, we use 50n. Fig.2 Simulation waveform of the influence of IGBT’s Shutdown loss under electric capacity function in leading leg 4 Experimental analyses As shown in figure 3, when the leading leg switch tube Q1 opens (the tube voltage drops to zero), the drive signal is zero, namely the leading leg tube opens under the condition of zero voltage; when the leading leg switch tube Q1 is off, the drive signal is zero, namely the leading leg tube closes under the condition of zero voltage. VGE-Q1（20V/div） iQ1（2V/div） VCE-Q1（100V/div） Fig.3 VGE, VCE of Q1 and current waveform As shown in figure 4, when the leading leg switch tube Q4 opens, its current is zero, namely the lagging leg tube opens under the condition of zero current; when the lagging leg switch tube Q4 is off, its current is zero, and namely the lagging leg tube closes under the condition of zero current. VGE-Q4（20V/div） iQ4（5V/div） VCE-Q4（200V/div） ip（20V/div） Fig.4 VGE, VCE of Q4 and current waveform 5 Conclusions (1) The main circuit converter improved the zero-voltage effectiveness of the leading leg through increasing the leg parallel capacitor. The lagging leg achieved a zero-current switching. The impact of the current Smearing has been eliminated. Shutdown loss and stress have been decreased obviously. Through designing the blocking capacitor and the maximum duty cycle rationally, we achieved soft switching of full load. (2) Through laboratory tests and analysis, designs of the main circuit parameters meet the requirement to achieve a soft switch, which reduces switching losses and stress. (3) The theoretical calculations and simulation results of this paper were used in the actual welding power supply.
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