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On Equalization of Voltages on the Series Distributed Power Semiconductor Devices in a Low Conductivity State


N. N. Bespalov, S. S. Kapitonov

Electric converters based on power semiconductor devices (PSDs) for rated currents up to a few ten thousand of amperes and voltage up to a few ten of kilovolts are used in various industries. Modern power converter blocks are developed on the basis of the group connection PSDs for commutation of power. In particular, the series distribution group connection of PSDs in the valve shoulder is one of the ways to build high-voltage converters power blocks. Difference between the values of parameters of their characteristics and their temperature variation of semiconductor structures (SS) during the operation determines unevenness in the distribution of voltage on them in a low conductivity state. This results to differences in the distribution of energy in the PSDs during commutation, which may affect the reliability during operation. To ensure equitable distribution of voltage valve shoulder on PSDs in a low conductivity state determines lining of the thermal regime of PSDs in operation, this provides increase of their reliability in operation. Snubber capacitors are used for voltage equalization on the PSDs in a low conductivity state. It is necessary to choose the minimum possible value of capacitance to minimize the weight and size of valve block and reduce losses in the SS of PSD, when it switches to a high conductivity state. Modern methods of calculating the values of capacitance of snubber capacitors do not take into account many factors, affecting the uniformity of voltage distribution on the PSDs. It results to unjustified overestimation of the value of capacitance of snubber capacitors. The article deals with voltage equalization on the PSDs in a low conductivity state, when they are series distributed in group connection by selection of the snubber capacitors capacitance, taking into account possible variations of the values of current-voltage characteristic’s (CVC) parameters of the PSDs in a low conductivity state and values of SS temperature of PSD during the operation. Analytical models of CVC of PSDs in a low conductivity state and the valve shoulder with series distributed group connection of PSDs was developed for the determination of the voltage amplitudes on them in a low conductivity state and for study this problem. These models were taken into account the effect on the voltage distribution on the values of CVC’s parameters and the SS temperature. Computer programs for investigation of electrothermal processes in SS of PSDs in a low conductivity state were developed based on models. Way of choosing the snubber capacitors’ capacitance, which provides uniform distribution of voltage on PSDs in the valve shoulder, was proposed. The program was developed in the LabView, which is implemented by the proposed method. CVC of high voltage standard PSDs and voltage distribution on them in the valve shoulder, consisting of four series-connected devices, was investigated, this is typical schematics of a valve shoulder without snubber capacitors. It was shown a significant influence of even minor variations in the values of SS temperature of PSD on voltage distribution. Snubber capacitors introduction selected by the proposed method the capacitance provides uniform voltage distribution on the valve shoulder of PSDs. We calculated the dependence of the snubber capacitors’ capacitance in the valve shoulder of the SS temperature of most heated device for basic types of PSDs. The values obtained for these tanks are several times lower then the values obtained by typical calculations. This shows the possibility of minimizing the snubber capacitors’ capacitance, designed to equalize the voltage values on series-connected PSDs in the valve shoulder converter.

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