The aim of the work is to analyze the stability of the battery-supercapacitor hybrid storage of power supply for resistance microwelding equipment, considering the possible variation of the system parameters and taking into account parallel series resistance of the circuit components. Methodology. The sufficient accurate mathematical model of the hybrid energy storage system to stability analysis has been obtained by the state-space average method. According to the state-space averaging method, PWM switching converters are described by separate circuit topologies for each switching period. The system of differential equations for each time interval has been derived by use of the Kirchhoff rules. The small-signal model transfer function of the SEPIC converter has been obtained by applying the Laplace transform to linear state equations averaged over one switching cycle. Finally, the Nyquist stability criterion has been considered to evaluate the stability of the proposed energy storage system. Results. Bode diagrams of an open-loop system for different values of the duty cycle, average load current, and input voltage have been obtained by using MATLAB software. The gain margin ranges from 14.6 dB to 26.4 dB and the phase margin ranges from 45.4 degrees to 54.8 degrees. From these results, it is obvious that the proposed system meets the stability criteria regardless of the aforementioned parameter fluctuations. Originality. The high-efficiency energy storage system for micro resistance welding technology has been proposed. Developing of the energy storage system according to the battery semi-active hybrid topology enables to control the Li-ion battery discharge current within the maximum allowable value. SEPIC converter utilization ensures the high-efficient operation of the power supply despite the battery charge state. Moreover, this topology allows implementing series and parallel configuration of both batteries and supercapacitors to obtain the required value of voltage and current. Practical significance. The mathematical model of the SEPIC converter has been developed by applying the state-space averaging technique. The stability analysis for parameter variation, such as duty cycle and the average load current, the input voltage has been performed by using Nyquist criteria. References 10, tables 1, figures 8.

Micro resistance welding is one of the most effective ways to obtain permanent joints of metal parts. The quality of welded joints strongly depends on the characteristics of the power supply of welding equipment. The power supplies for micro resistance welding based on Energy Storage topology have a softer impact on the network than the ones based on Direct Energy topology. The use of supercapacitors for Energy Storage type power supplies makes it possible to reduce the dimensions of welding equipment and to improve its technical parameters. However, the feature of the supercapacitors is low value of the nominal voltage, which usually does not exceed 3 V. To provide higher voltage, the modules of supercapacitors connected in series are designed. In order to extend the life time of such modules, a voltage balancing system is required. A circuit for balancing the voltage of a modular supercapacitor energy storage of a power supply for micro resistance welding is proposed. The fragments of calculation of control units of a supercapacitor module cell are given. The simulation of the balancing circuit operation is carried out and time charts of the supercapacitor charge process are obtained. The operability and effectiveness of the proposed solution is confirmed. The advantage of the proposed circuit is the possibility of obtaining the high efficiency because of returning the excessive energy of the module cell back into the power supply.

The paper represents resistance welding characteristics and construction features of power supplies for resistance welding. The authors give an overview of circuit topologies of converters for resistance welding and distinguish the most promising one — the transistor buck converter with a synchronous transistor. It is shown that in order to ensure acceptable energy efficiency of power supply for resistance welding machines, while maintaining sufficient accuracy of current regulation in a welding contact, special modes of pulse transistor converters are used. The analysis of resistance welding features — high currents, low voltages — makes it possible to presume that the evaluation of the power losses in semiconductor elements only is insufficient and needs to be complemented by taking into account the losses on the inductive element of the converter circuit. In this work, the authors propose the method of estimating the power losses in the pulse buck converter of the power supply of resistance welding machine, which allows for more accurate calculations at the design stage due to consideration of the power losses on the inductive element of the circuit. The methodology is to calculate the total power losses as the sum of power losses on all individual elements of the circuit. Power losses on inductance is calculated using the Steinmetz equation. The calculations carried out with this technique proved the advisability of taking into account the power losses on the inductive element, especially in the region of high frequencies. The obtained diagrams demonstrated the dependency of the power losses in the converter on the frequency at different values of current and voltage.