The isolated bias supply is required in many applications, such as the gate driver bias for the traction inverters, on board chargers in electrical vehicles. It is also used in other sensing and control circuits in the electrical vehicles to minimize the noise or provide safety isolation. The open-loop LLC converter based on the UCC25800-Q1 transformer driver provides a reliable solution for these applications. It uses the open-loop control to improve the noise immunity. The LLC topology is able to operate at a higher switching frequency with soft switching, achieve high efficiency and low EMI, reducing the transformer size. Furthermore, the LLC topology is able to absorb the transformer leakage inductance as part of the resonant circuit. This absorption allows the transformer to have extremely low primary side to secondary side parasitic capacitance, which reduces the system level common-mode noise. The LLC topology also helps to simplify the transformer construction and reduces the transformer cost.