www.epc-co.com POWER CONVERSION 21 www.power-mag.com Issue 2 2023 Power Electronics Europe converter, ideal for battery charger applications in energy storage systems, was built and tested as shown in figure 3. It can supply up to 60 A into a 48 V load. The experimental unit comprises two interleaved phases and each phase employs a 3-level flying cacpacitor converter topology designed using two EPC2215 FETs [4] connected in parallel for each switch position and operated at 150 kHz switching frequency. Each phase employs three half-bridge Figure. 3: The experimental unit image with identification of that various function circuits. Figure. 4: A zoomed in layout of the power stage area showing switches (Q1-Q4 located on the bottom side of the PCB), gate drivers (GD1-3) and capacitor placement. gate drivers using NCP51820 [5]. The lower FET are driven by a signle gate driver for Q3 and Q4. To drive the upper series of Q2 and Q1, two additional high-side gate driver outputs are used. The gate drivers are powered by a cascaded bootstrap circuit. A zoomed-in layout of the switch and capacitor placement is shown in figure 4, where the FETs are located on the bottom side of the baord. Low ESL ceramic capacitors are placed between switch pair Q1/Q4 and Q2/Q3 which act as high frequency decoupling paths to reduce the parasitic inductances associated with the switch positions. The experimental unit is digitally controlled using a dSPIC33CK controller on EPC9528. The experimental unit is tested with the input on the high voltage port and output voltage regulated down to 40 V. In this mode, the controller is comprised of one single output voltage loop, two current loops and two flying capacitor balancing loops as shown in figure 2. The two current loops share the same current reference generated by the output voltage side to ensure active current balancing. The two phases are interleaved (180° phase shift) to achieve ripple cancelation on the capacitors. A measured waveform is shown in figure 5 which shows the switch node of one phase at 24 A output with VOUT = 40 V and VIN = 200 V. Measured, efficiency and power loss at VOUT = 40 V and VIN = 200 V are given in figure 6. In single phase configuration, the converter achieves 97.8% peak efficiency around 14 A output. The experimental board is equipped with an aluminum heat-spreader and heatsink which can be attached to the back side of the PCB where the FETs are located. The heat-spreader is mounted to the PCB using SMD threaded spacers which are 1 mm tall and use 6 mm long M2 countersunk screws for a flat mounting surface and follow best practice [6]. High performance thermal interface material (TIM) from T-Global (part TG-A1780) is used between the FETs and heat spreader for improved heat conductance. A heatsink To receive your own copy of Power Electronics Europe subscribe today at: www.power-mag.com
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