The double swing fluxes of the ohmic field must be utilized in order to extend the flat top of plasma current in the SUNIST spherical tokamak. This requires an upgrade of the power system of the ohmic field. An H-bridge bidirectional discharge circuit is introduced, which is based on four high-current low-frequency switches composed of multiple parallel IGBTs. The key components of the circuit, 3kV/10kA class IGBT switches has been developed. In addition, both the single-phase and three-phase quasi-constant current charging circuit for the energy storage capacitor banks of the upgraded power s...