(Invited) Practical Considerations of Si Vs SiC Technology in High POWER, High Frequency Inverters for Industrial Induction Heating Applications
Authors: Dede Enrique J. , Jordan Jose , Esteve Vicente 
Published: February 2020

Introduction

The paper aims to give practical technical considerations for choosing Si vs SiC technology in practical high frequency, high power inverters for industrial induction heating applications.
Actual inverters, for this application, are resonant inverters, in a current of in a voltage–fed configuration. For current–fed inverters, the basic switch is a turn-on or turn-off controlled device (IGBT of MOS) in series with a diode, while in voltage–fed inverters the basic switch is a turn-on or turn-off controlled device with a diode in antiparallel.

Si vs SiC diodes in high power inverters

High power converters, both current-fed or voltage fed, require paralleling high frequency diodes. With Si technology, diodes have to be matched due to their negative thermal coefficient, making in this case, the use of SiC diodes a much more robust solution due to its positive thermal coefficient. Manufacturing as well as maintenance costs compensate the over cost of SiC technology.
But on the other hand, many of the hardening application require a large number of power cycles and considering power cycling (PC) capabilities, the use of SiC diodes offers worst PC capabilities as the higher increase of the junction Temperature ΔTj due to higher body resistance of the SiC diode in comparison with the Si counterpart. For that reason, long term reliability has to be when using SiC diodes instead Si ones. The following figure shows the increase of the die temperature of SiC and Si diodes under the same conditions (1).

Figure 1

Another aspect influencing the PC capacity is related the different material properties of both materials. Figure 2 shows that under the same junction temperature increase ΔTj, the power cycling capacity of SiC diodes is about one third of the equivalent Si Diodes (2).

Figure 2

As a conclusion the use of Si or SiC diodes is a trade-off between paralleling and power cycling capabilities and the decision on the optimal diode technology is dependent on the application.

Si IGBT vs SiC MOSFET in high power inverters

SiC MOSFETs are specially adapted to high frequency applications as per their lower switching losses. The following figure shows the required number of semiconductor devices of a 240 kW/400 kHz induction welder as well as the better overall efficiency of the inverter (3).

Figure 3

Figure 4 shows the 200 kW/ 400 kHz the developed all SiC inverter prototype for testing SiC technology capacity vs IGBT technology.

Figure 4

Based on the experimental results from the above prototype, the curves for the optimal technology, depending the frequency in voltage-fed inverters for induction heating are shown in the next figure.

Figure 5

Consequently, the above experimental results show that up to 100 kHz, IGBT is the optimal technology for induction heating inverters, while from 150 kHz until 500 kHz SiC technology offers better power capability. The range from 100 to 150 kHz can be optimal covered with IGBT technology and advanced control techniques (interleaved Pulse Density Modulation-PDM ) or with SiC technology with actual PDM controllers.

Conclusions

The replacement of Si diodes by SiC diodes improves robustness of high power inverters for industrial induction heating applications due to their ability to be paralleled, but in applications, as hardening, with a large number of Power Cycles, the direct replacement is questionable due to the lower Power Cycling capacity of SiC diodes vs Si ones.
The use of SiC MOSFETs is specially encouraged in very high frequency induction, high power induction heating inverters, as in the case of welders operating at frequencies above 150 kHz. On the opposite, SiC technology does not give an optimal solution at lower frequencies than 100 kHz, being in that case SI-IGBT technology the optimal one.
In the final paper a much more deeper view on the above considerations, based on our experimental results, will be described for the optimal use of Si vs SiC technology in high frequency, high power resonant inverters for industrial induction heating applications.

Bibliography

1.- Ortiz González, J., Alatise O., Nobee N., Hu J., Mawby P., “ Electrothermal Considerations for Power Cycling in SiC Technologies” CIPS 2016 Proc.- Nürnberg.
2.- Herold Ch, Schaeffer M., Sauerland F.,Poller T., Lutz J., Schilling O., “ Power Cycling Capability of Modules with SiC Diodes” Proc. CIPS 2014 Conference. Nürnberg
3.- Dede, E. “A 100kW Full SiC Inverter for Induction Heating Applications”, Proc. ISiCPEAW 2015 Workshop. Stockholm 2015.

Figure 1