POWER
Wide Input Voltage Range and Optimised PCB Layout for GaN-based Motor Drive Reference Design
Federico Unnia, Applications Engineer – Motor Drives, Efficient Power Conversion
A
motor drive inverter reference design featuring a wide input range from 30 V to 140 V is suitable for battery systems of 80 V, 110 V, upwards.
The EPC91200 is a 150 V-rated, fully configured motor drive inverter board compatible with DC bus voltages from 30 V up to 140 V. The design features the EPC2305, a 150 V-rated GaN FET, with 2.2 mΩ typical ON-resistance, which can be used on a wide variety of applications using 80 V to 110 V batteries. The schematic principle of the board is shown in Figure 1.
The phase current is measured through current sense ICs of the ACS37003-family. The low internal resistance (265 µΩ) of such components helps reduce the heating on the board and minimise the losses on the inverter. The EPC91200 can be assembled with 5 V supplied current sense ICs of the same family since a conditioning circuit converting the signals from 5 V to the 3.3 V range is present on the board.
A full bidirectional overcurrent protection circuit is triggered from any of the three phase currents to prevent unwanted failures. The positive and negative overcurrent thresholds can be adjusted through resistors and the time constant of the overcurrent signal is set through an RC filter connected to the shutdown signal of the gate drivers. The generation of the 5 V power supply is made through a circuit that uses power integrations LNK306, making it suitable for DC voltage ranges from 30 V to 150 V. The DC link capacitors are 100 V ceramic and stacked to reach 150 V capabilities. Using two 100 V-rated stacked capacitors yields a better tradeoff than using 200 V-rated capacitors in terms of equivalent capacitance per area. The PCB layout of EPC91200 has been optimised, focusing on the DC resistance between the input DC connectors and the phase connectors, since in motor drive applications, the ON-resistance and the PCB resistance contribute significantly to the overall losses.
The Würth Redcube press-fit phase connectors used to connect the inverter to the motor make excellent ohmic contact. The male version of the connector is used to
Figure 1 - Block diagram of EPC91200
Figure 2 shows the main features of the EPC91200 on the top side and bottom side respectively
make the board ready for the use. Furthermore, the stackup of the board has been optimised to keep the heavier copper layers just below the top-layer; the first two internal layers are indeed 4 oz. (140 µm) thick, allowing for most of the DC current and heat to spread. Two central layers were used to route most of the analog and digital signals. The remaining layers are the mirror of the first three to maintain symmetry. Unlike DC-DC GaN applications, which
34 JULY/AUGUST 2024 | ELECTRONICS FOR ENGINEERS
necessitate HF loop decoupling capacitors on the same side of the GaN FETs and additional middle-frequency capacitors on the opposite side due to fast transitions on the switching node, motor drive inverters have slower transitions. Consequently, the number of decoupling capacitors can be reduced and relocated to the opposite side of the PCB. This adjustment makes more room for copper planes on the side of the FETs, aiding in the dispersion of heat.
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