Touch current suppression for semiconductor-based galvanic isolation

Abstract

This paper presents the touch current (TC) suppression for the semiconductor-based galvanic isolation (SGI) solution. The solution achieves galvanic isolation by utilizing semiconductor switches to deliver differential-mode (DM) power during their ON states, while sustaining common-mode (CM) isolation voltage and blocking CM leakage current during their OFF states. In this solution, TC is resulted as the switch output capacitance is charged by the CM voltage at every switching event, and the TC rises as the switching frequency increases, which limits the system power density. This paper reviews multiple TC-suppression approaches for the SGI to meet the safety standard IEC60950. These approaches include preventing the TC generation, increasing CM impedance, bypassing the TC, and partially reduce the TC. Among them all, it is practical to reduce the TC via partial compensation for the switch output charge, using distributed auxiliary current-source circuits. To verify this approach, a simplified off-line power supply with a SGI-based dc/dc stage is built and tested. The results show the TC can be reduced effectively. It also allows the converter to operate at higher switching frequency to achieve higher power density, while meeting the IEC60950 TC requirement.