Automotive to cool to protect to connect
Heat dissipating case & extruded heatsinks
Table 1: Design specifications for 10 kW EV charger
perform ZVS in both buck and boost mode. This makes them suitable for high frequency operation in both EV chargers and OBCs. By replacing the rectifier diodes of the Vienna Rectifier PFC with active switches, synchronous rectification and fully bidirectional power conversion is possible.
Design features
Higher power density was made possible by three key features of this design. Firstly, a novel 1/3-PWM synergetic modulation scheme operating at 560 kHz uses the DC-DC stage to control the DC- link voltage, so that in the AC/DC stage only one of the three phases are switching at any time. This has the advantage of reducing power dissipation caused by the switching losses in the AC/DC stage, and also allows the DC-link voltage to be reduced to the lowest possible for a boost topology, i.e., to the six-pulse envelope of the line-to-line voltages and/or the maximum line-to-line voltage. This also has an advantage for the DC/DC stage, by reducing the voltage transfer ratio required in subsequent DC/DC converters. For example, with a minimum system output voltage of 250 V, the DC-link voltage does not require boosting to the maximum line-to-line voltage - a six- pulse shape between 490-565 V, thereby lowering the voltage range of the buck converter.
Secondly, the DABs are modulated using the degrees of freedom of the duty cycles, phase shift and also the switching frequency to achieve ZVS over a wide input and output voltage range. A topology which includes two active full bridges, allows voltage to be controlled over a wide range. However, there is a challenge in determining the optimal balance between the switching losses and the conduction losses in the transformer and semiconductor devices. The most favourable operating point for
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the DABs is the one where the voltage transfer ratio between the primary and the secondary is the same as the turns ratio, where low RMS currents are present as well as ZVS transitions in all the bridge- legs. However, when modulating the DAB beyond the natural voltage transfer ratio, advanced modulation schemes can be used to minimize the losses, for a given set of operating parameters. Under these conditions, the degree of freedom is the phase shift between the primary and the secondary side, which then allows the power flow from primary to secondary (or vice versa) to be controlled.
Finally, when compared to silicon and silicon carbide devices, GaN GIT HEMT devices have smaller output capacitance for the same on-resistance thereby enabling full-ZVS at lower current levels. They
also have exceptionally low specific Rdson values and can operate in both partial-hard and hard-switched modes. By combining advanced control and modulation schemes with the superior behaviour of GaN devices under different switching condition, the desired level of power density required for next-generation OBCs for EVs could be achieved. The uncased power density of the final EV charger design (Figure 2), which measured 17.8 x 400 x 140 mm, was 10kW/L.
Conclusion
GaN HEMTs have inherent features that make them suitable for high frequency operation which enables them to operate in both hard- and soft-switching regimes. This allows them to be used in advanced modulation and control schemes which make it possible to achieve wide input and output voltage ranges in designs with high power density. Infineon have demonstrated that these devices will make it possible to realize the level of power density required for next- generation OBC chargers in EVs.
https://www.infineon.com/ Components in Electronics February 2023 19
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