This letter presents an isolated bidirectional DC-DC converter for small power supply access to the low voltage bus of the energy router with high bidirectional gain and optimal operating efficiency. The idea of magnetizing and demagnetizing the primary and secondary sides of a high-frequency transformer is adopted, with a large capacitor-assisted excitation circuit on the primary side and a triple-voltage gain circuit on the secondary side. The converter has a small number of switching devices, the voltage gain can be determined using the transformer ratio as well as having a wide duty cycle for control, and the input current ripple is small at the low voltage side. The proposed converter is analyzed in depth and the effectiveness of the converter is verified by experimental results.