POWEREFFICIENCY
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
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applications, the higher operational temperatures of a SiC bipolar transistors is its primary asset. However, efficiency is an important property here as well, since losses contribute to additional device heating.
To cater for these differences, we have embarked on a two-pronged product development program. One of the directions that we have taken services the need for high efficiency and low cost within the industrial market, with components packaged in plastic TO-247 packages for use up to 175 °C. The other approach focuses on high temperature and has yielded devices in TO-258 metal packages operating at junction temperatures of up to 250 °C. All of these applications are based on switch-mode
power conversion. 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 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.
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.
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
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. 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
www.solar-pv-management.com Issue VII 2011
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