This paper presents the design, modelling, and control of a front-end boost power factor correction converter for a medium-power, high-voltage supply used in non-thermal plasma-based electrostatic precipitator air purification systems. The PFC feeds a downstream inverter, high-voltage transformer, and dielectric barrier discharge, forming a nonlinear and time-varying load. After plasma initiation, the load exhibits capacitive behavior with a nonlinear resistive component, imposing constraints on control bandwidth and stability. To ensure robust operation and tight DC bus regulation, a deliberately slow outer voltage loop is required. A frequency-differentiated modelling approach is proposed, where a state-space averaged model represents the fast inner current loop, and a capacitor charge transfer model captures low-frequency outer-loop dynamics. A grid-aligned resonance tuning method is introduced for DC-link capacitor selection by aligning the plant resonance with the grid frequency, improving current shaping, and reducing harmonic distortion with a simple PI controller. In addition, a generalised PI tuning method is derived directly from passive component values and desired crossover frequencies, avoiding detailed small-signal modelling and iterative frequency-domain design. Experimental validation on a 900 W prototype demonstrates effectiveness. The proposed methodology is well suited for application driven PFC designs that require simplicity, robustness, and compliance with power quality standards.