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In this paper, a quasi-resonant (QR) flyback dual-output LED driver with current-sharing transformer (CST) is analyzed thoroughly. Although the CST is utilized to balance the currents in two LED strings, from the theoretical analysis, it shows that the parasitic capacitance of CST and its magnetizing inductance have a significant effect on the output current deviation and diode's voltage stress. In order to achieve the optimal current sharing ability between two outputs with minimum auxiliary components, a design guideline for the CST in QR flyback is proposed according to the theoretical analysis. A 100-W dual-output dc-dc prototype is built up to verify the theoretical analysis.