This paper presents the design of a high-voltage (HV) single-inductor multiple-output (SIMO) fixed frequency DC-DC Buck converter for the power management unit of a Gate-Shaping digital gate driver for Silicon-Carbide power MOSFETs. Through shaping the switching waveform which drives the power MOSFET there are significant improvements regarding energy efficiency and electromagnetic compatibility achievable. Driving the gate of the power MOSFET the gate driver is provided with a galvanically isolated power supply of 20 V. Thus, the 0.4 W HV SIMO buck converter generates the internal 5 V and 1.8 V voltage rails required by the gate driver from the 20 V isolated power supply. It can provide up to 60 mA on each rail for analog and digital signal processing as well as driving the internal power MOSFETs. The design has been fabricated on an area of 0.6 mm² in a 0.18 µm HV BCD technology. In measurements it provides an average efficiency of 81.16 % over the whole operating range and a measured maximum efficiency of 87.44 %.