| Spot welding has the advantage of high productivity, simple operating, easy realization of automatization. In recent years, with the development of new material and new technology, the process of spot welding is known more deeply, and the new demands for better spot welding quality are urgent. As a method of spot welding quality control, the constant current control is the most popular, and it is concerned in this article. The spot welding quality is controlled by the current average value and spot welding time.Several principal circuits are compared in this article. Two switch forward converter and full-bridge converter are the most popular in spot welding power. The full-bridge converter worked in two quadrants, so the volume and the weight of the magnetic core are much reduced, and the maximum duty of it almost equals to one. Compared to two switch forward converter, the speed of current ascending is higher in full bridge converter. So the full bridge topology is used in this article. There are two modes in full-bridge soft switching which are ZVS and ZVZCS.ZVSZCS mode has no primary loop current, so it has higher efficiency. But the transformer turn ration is so large that the leak inductance is large. When the maximum duty ratio sets to 0.85,and the primary current reach to 3A,the voltage on block capacitance should be greater than 20 percent of input voltage in order to realize lag bridge soft-switching. This large amplitude waveform distortion will make control difficult. So the phase shift FB ZVS DC/DC converter with auxiliary network was used in the end. The output current is large, so the primary duty loss is obvious.It will decay the performance of the spot welding power, and reduce the speed of current ascending as a result. The principle circuit in used need to minish the primary duty loss therefor. The saturation inductance has been got rid of from the principle circuit in this paper. The lag bridge soft-switching in all kinds of load is realized just by the leakage inductance and the auxiliary net work. Under the condition of having no experience formulation, I designed and optimized the parameter of auxiliary network by MATLAB programme. The power electronic principle circuits are nonlinear and time-variable system for involving switching .The analytical technique for linear model is not fit. At the present time, the linear model always be derived from the principle circuit under some hypothesis, and the control system is developed on it. In the design of high frequency switch power, the dynamic small-signal model is the most popular. This model was derived on the assumption that the frequency of switch is much higher than the crossover frequency of the principle circuit filter. But the spot welding dominant circuit is different .In order to realize the great current output, the switching frequency must not be high. Furthermore the crossover frequency of the filter is high for the demand of power dynamic response ability. As a result the dynamic small signal model base on high frequency power is not fit to spot welding power. A control model of the principle diagram of the spot welding principle power is derived thoroughly. The period of spot welding is short, so flux imbalance should be avoided. The peak current control mode is the best method to avoid flux imbalance. But there is theory average current error in it.Under the consideration of the input voltage fluctuation, the average current can not trail the programmable reference accurately even using ramp compensation. The double loop control method is introduce in this article. The primary side current is controlled by peak current mode, and the secondary side current is feedback by a single zero and pole compensator. The principle diagram is simulated using PESPIC language, and the simulation waveform is achieved. The soft switching realization is confirmed under all condition of load. The UC3875 model is constructed by PSPICE, and the closed-loop simulation system is made up on this foundation. The waveforms are obtained, which co...
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