To meet the requirements for wide-range output, current-limit protection, and stable closed-loop control in high-voltage capacitor charging, laboratory high-voltage sources, and pulse-power preamplifier systems, this paper presents a high-voltage constant-voltage/constant-current power supply based on a phase-shifted full-bridge (PSFB) topology. The supply operates from a typical 24 V low-voltage DC input and uses a 1:100 high-frequency transformer for isolated step-up conversion. Its output control range is 0–2200 V and 0–200 mA, with a rated power limit of 400 W. The controller uses an STM32G474CBT6, which generates four complementary phase-shifted gate-drive signals through the HRTIM module and samples secondary-side voltage, secondary-side current, primary-side current, auxiliary-supply status, and temperature feedback with an ADC synchronized to the switching cycle. The control algorithm calculates the constant-voltage, constant-current, and constant-power PI loops in parallel and selects the actual phase-shift duty cycle through minimum-duty-cycle arbitration. To reduce the efficiency fluctuation of fixed-frequency PSFB operation at very low and very high duty cycles, an automatic frequency-conversion strategy is introduced. Frequency increase or reduction within 11–45 kHz is selected according to the duty-cycle state, while score-counter hysteresis, a lockout window, and feedforward duty-cycle correction reduce the disturbance caused by frequency changes. Simulation and experimental results show that the prototype provides stable high-voltage output, constant-current charging, constant-voltage load operation, and ZVS turn-on, confirming its practical engineering value.