POWEREFFICIENCY
total SiC BJT switch energy (turn on plus turn off) is only 28 percent of that for the IGBT switch energy at 50 A and 800 V (see Figure 3).
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
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 voltage offset at zero current in the IGBT.
Another strength of our SiC bipolar transistor is that it does not go into the ‘hard’ saturated state – previous generations of silicon bipolar transistors were notorious for this behavior. This makes the SiC BJT switch as a unipolar device, without the need for special precautions such as baker clamps. Thanks to the small die size and lack of parasitic components, they can operate at high frequencies with negligible turn off delays and no current tail at turn off.
Switching times below 20 ns are possible for a 800 V and 6 A SiC BJT. For the 50 A device the switching time is longer. That’s because the current rise and fall time is governed by the mutual stray inductance in the emitter path, and switching a higher current takes longer.
The turn on and turn off waveforms for our 50 A SiC BJT are very fast: turn on from 800 V to 50 A takes 60 ns, and turn off is even faster, requiring just 30 ns (see Figure 2).
Switching energies can be determined by integrating the product of voltage over and current through the device during the transitions. We have compared electrical losses with those from the datasheet for the IGBT. The difference is huge: the
We have carried out higher-level system simulations to reveal the impact that the lower conduction loss and switching energies can have on a typical system. Two different topologies have been investigated, one boost and one inverter. Both are widely used in photovoltaic inverter designs. In such an inverter it is typical for the voltage from the solar panels to be initially increased by the boost stage (see figure 4). This boost voltage is then fed to the DC bus of the inverter stage (see figure 5), which causes a sinusoidal AC current flow through the output filters and into the network.
The details of our simulations are that a 8 kW boost converter stage was fed with 400 V and this voltage was increased up to 800 V. Different switches and diodes were compared at 16 kHz and 64 kHz, under the same cooling conditions. For the inverter a DC link voltage of 800 V was used, and a regular 230 V AC network connection on the output was used.
For the 8 kW inverter stage the maximum output current amplitude was set to 50 A (33 A RMS). The same alternations of semiconductors and switching frequency were made, and the cooling conditions were throughout these alternations kept the same in this case as well. The results of our simulations are presented in Figure 6 and Figure 7. These simulations not only highlight the lower conduction losses of the SiC BJT compared to the
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Figure 5. A DC input is transformed into a sinusoidal AC output with this typical inverter circuit. Both a SiC solution with BJTs and a silicon solution with IGBTs were simulated
www.solar-pv-management.com Issue VII 2011
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