In medium-voltage (MV) power electronic systems such as grid-connected converters, a compact, isolated, and reliable auxiliary power supply is critical for powering control logic, gate drivers, and sensing circuitry. This design uses a high- frequency flyback converter that prioritizes insulation strength and EMI performance. The converter is designed to operate from a 230 V AC input (rectified to ~325 V DC) and delivers a 24 V DC output at up to 30 W. The flyback topology is selected especially for its simplicity at low power levels and inherent galvanic isolation. A UCC28C42-Q1 current-mode PWM controller is used to drive the power MOSFET at a switching frequency of 100 kHz. This controller features primary-side sensing, eliminating the need for opto-couplers or secondary-side feedback circuits. This improves reliability, particularly beneficial in medium/high-voltage isolated systems. To handle voltage spikes from leakage inductance during switch turn-off, an RCD clamp circuit is placed across the primary switch. This limits peak voltage stress and minimizes radiated and conducted EMI. The flyback transformer uses a planar magnetic structure, offering the following benefits: (a) Reinforced isolation up to 30 kV, essential for use in MV converter; (b) Low inter-winding capacitance, reducing common-mode noise coupling; and (c) Compact and thermally efficient structure, using multilayer PCB windings for tight coupling and repeatability.
A tertiary winding is specifically included to power the UCC28C42-Q1 controller, allowing it to self-bias after initial start-up through a high-resistance bootstrap path or auxiliary circuitry. To suppress common-mode noise further, common-mode filters are incorporated at the input and output. These filters help attenuate common-mode currents generated due to high dV/dt switching and parasitic capacitive paths, thus ensuring compliance with EMI standards and protecting sensitive low-voltage circuits. Furthermore, an earth-connected shield is placed between the primary and secondary windings. This provides a low impedance path for the common-mode currents, which would otherwise entirely couple through the inter-winding capacitance between the primary and secondary. This work was done by Senthil Arumugam, Syed Rasheed, Rajvardhan Patil and Sagnik Banerjee under the guidance of Prof. Sandeep Anand.
Photo of the 30W auxiliary power supply.