SiC Power Devices in a Soft Switching Converter, Including Aspects on Packaging

In many applications of power electronic converters efficiency and size are important figures of merit. Low losses in the power semiconductors and high frequency operation are important factors. The converters considered in this paper are off-line powered and in the power range of 50 – 150 kW with a high-voltage dc-output. The switching frequency is 20-40 kHz. With modern IGBTs an efficient operation (n> 95%) can be obtained when using soft-switching topologies. Fig 1(a) shows the schematic diagram of an SLR, series loaded resonant converter [1]. In Fig 1(b) typical waveforms of transformer current and voltage are depicted.

                In a recent paper, [2], frequency dependent IGBT losses are evaluated. Fig 2 shows the on-state voltage of an IGBT when conducting a sinusoidal current at different frequencies. At the higher frequency (30 kHz, Tr4) the increase of conduction losses is evident.

                Power switches made from SIC show excellent properties enabling; low conduction losses, high frequency operation and an ability to operate at high die temperatures [3]. Fig 3 (a) shows the on-state voltage of an unipolar SIC switch while Fig 3(b) shows the on-state voltage of a bipolar switch. Measurements have been performed up to 200 kHz without showing any high-frequency effects.

                High voltage SiC PiN diodes can replace several series connected Si diodes due to the higher critical field strength in SiC. This enables substantially reduced conduction losses in the output rectifier. In Fig 4 on-state voltages of 10 kV SiC and Si-diodes are shown, indicating a 50% reduction of losses.

References

[1]  V. Vlatkovic, M. J. Schutten, and R. L. Steigerwald, “Auxiliary Series Resonant Converter: a New Converter for High-Voltage, High-Power Applications,” in Proc. of PESC -96, 1996.

[2]  P. Ranstad, and H-P. Nee, “On dynamic effects influencing IGBT losses in soft switching converters,” IEEE Transactions on Power Electronics, vol. 26, No. 6, December 2010.

[3]  P. Ranstad, H-P. Nee, J. Linner, and D. Peftitsis “An experimental evaluation of SiC switches in soft-switching converter,” IEEE Transactions on Power Electronics, 2014