industry SiC transistors
BJT is free from this. Our npn SiC bipolar transistors are normally off devices that deliver lower conduction losses than any other SiC technology. The saturation voltage knee that plagues the IGBT does not impair them, so their gain in efficiency at partial load currents is even higher.
Figure 2. Turn on (to the left) and turn off (to the right) waveforms for the SiC BJT. The green signal is the control signal, the yellow is the collector current, and the magenta is the collector-emitter voltage of the BJT. The turn on is very fast, with a total switch time of approximately 60 ns and the turn off is even faster, with a total time of approximately 30 ns
What’s more, they combine the best properties from the unipolar and bipolar silicon world and then enhance them even further. The bipolar behaviour contributes low conduction losses and good utilisation of the relatively costly SiC material, yet at the same time the switching performance is very fast. IGBTs are optimised and balanced towards either low conduction losses or low switching losses – with the SiC BJTs there is no need to compromise.
BJTs are also easy to deploy in parallel thanks to the positive temperature coefficient of the collector-emitter saturation voltage, Vcesat
. With SiC BJTs, higher
temperature leads to a higher saturation voltage, but this leads to favourable balancing of the total current between the transistors. And in addition, it helps to prevent hotspots within each die.
Figure 3. The energies produced in the switch required to turn on and turn off events of the SiC BJT in black compared to the silicon IGBT in red. The figure also shows that the temperature dependence of the switching losses is very low for the SiC BJT
In this process, the voltage and current are constantly chopped by the switching frequency. In the PV inverter case, their waveforms are smoothed out by filter inductances.
Consequently, any comparison of the performance of inverters employing SiC transistors and those using silicon equivalents must include conduction losses and also losses caused by switching events. During turn on or off, every semiconductor device simultaneously carries current and voltage, which produces losses. Minimising loss requires a reduction in the rise and fall times of current and voltage.
In the case of the most widely used silicon technology for switching applications, the insulated gate bipolar transistor (IGBT), a tail of current is conducted after the transistor has been turned off, increasing losses. The SiC
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www.compoundsemiconductor.net June 2011
Figure 4. The schematic of the simulated 8 kW boost stage, with 400 V input voltage and 800 V output voltage. Both a SiC approach with the BJT and a silicon approach with an IGBT were simulated
We have compared the forward characteristics of our 50 A SiC bipolar transistor with a silicon IGBT, a high- speed 40 A device that contains a silicon IGBT and anti- parallel diode (see Figure 1). Plotting the performance of both devices reveals that the saturation voltage of the SiC BJT is significantly lower – approximately 40 percent less at 40 A compared to the IGBT. This gain in performance gets larger and larger as collector current decreases, and at 15 A it is 70 percent at 25 °C and 75 percent at 150 °C. The superiority of the SiC BJT stems from a
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