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The main function of the on-load tap changing (OLTC) regulators consists in maintaining a constant voltage in order to feed critical loads despite the load changes or voltage changes in the AC mains. The traditional regulators are still used nowadays, but present several disadvantages, like a slow response, which reaches from 100 ms to several seconds. These drawbacks can be overcome if the OLTC regulators would have shorter response time, commuting several times every cycle of the mains. There are two basic topologies for fast OLTC regulators, the first one consists of several taps and uses hard switching. The second one consists of two main switches commuting at high frequency, using soft-switching in order to reduce the power losses. The present topology is of the second type. This paper presents a mathematical model of the power stage of the proposed regulator. It includes the parasitic resistances and the leakage inductances in order to obtain a better comprehension of the regulator operation.