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Bipolar SiC Transistors Enhance Electrical Power Conversion

Today’s switch-mode power converters restrict the efficiency of solar systems and hybrid electric vehicles. One way to lift this barrier, while cutting the bill of materials at the system level, is to replace the silicon transistors with SiC bipolar equivalents that can deliver currents of up to 50A, argues Fairchild’s Anders Lindgren.

Transistors built from SiC are set to play a key role in improving the efficiency, while cutting size and weight, of various electrical products serving a diverse range of applications. These wide bandgap semiconductor devices will probably first make an impact in the renewable energy sector, increasing the efficiency of solar inverters and slashing their bill of materials, thanks to a reduction in heat sink requirements and size of the filter inductances.

SiC transistors also promise to improve the driving range of hybrid electric vehicles, by cutting the size and weight of the hybrid inverter systems. And these robust transistors are also attracting the attention of engineers in the geothermal, oil and gas industries, who are searching for devices that can operate at really high temperatures. SiC transistors will also enable down-hole tools to operate at even further depths, allowing for a greater and more widespread use of geothermal energy.

At TranSiC, now part of Fairchild Semiconductor, we are pioneering the development of bipolar SiC technology that can serve this broad range of applications. Our efforts account for the differing requirements of all these applications that look to exploit different properties of SiC. Industrial applications, such as PV inverters, can mainly benefit from the high efficiency of SiC transistors, and their ability to operate at very high switching frequencies. In comparison, for down-hole 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.

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 highspeed 40 A device that contains a silicon IGBT and antiparallel 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 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.

 



Figure 1 I-V-forward characteristics of the SiC BJT compared to the silicon IGBT. The BJT’s collector current, IC, is plotted as a function of the collector-emitter voltage at a range of base currents, IB, ranging from 250 mA to 1 A. The dotted red line represents the same parameters for the IGBT

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).

 



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

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 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). 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.

 



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

 



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

 



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



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.

 



Figure 6. The results from the electro-thermal simulations by engineers at Fairchild reveal the superiority of SiC BJTs in the boost stage. Losses are plotted versus the output current for the 8 kW boost converter in figure 4. It can be seen that the gain in efficiency using SiC compared to silicon is gets larger and larger as the load current is decreased (part load)

 



Figure 7 Fairchild’s SiC BJTs can reduce the losses in the inverter stage if they are used to replace silicon IGBTs, according to calculations by the company. The losses are plotted versus output current amplitude, in the 8kW inverter circuit in figure 5. It can be seen that the gain in efficiency using SiC compared to silicon is relatively getting larger and larger as the load current is decreased (part load)



These simulations not only highlight the lower conduction losses of the SiC BJT compared to the IGBT, but also show that the low switching losses in this wide bandgap device are greatly enhancing the system efficiency. If extremely low losses are the primary goal for the system designer, then the SiC BJT should be used at the same low switching frequency as the IGBTs, in this case 16 kHz.

Take that route, and according to our simulations, losses in the boost stage can be cut by 52 percent,

and losses in the inverter stage by 65 percent. However, if cost, size and weight are considered as important factors – cost and weight are normally viewed in this manner – then the system designer should increase switching frequency. This reduces both the size of the choke in the boost stage and the inductances in the output filters. Even at four times the original switching frequency, 64 kHz, the losses associated with the SiC BJT are lower than those for the IGBT running at only 16 kHz. That four-fold frequency increase can nearly halve the cost, size and weight of the switch inductances, while producing less loss in the semiconductors.

These simulations illustrate how SiC bipolar transistors can play a pivotal role in driving down the cost and size of power conversion systems in a wide range of applications, starting with the those containing switch inductances such as DC-to-DC converters and inverters with output filters. That, in combination with requirements on high efficiency, makes SiC BJTs an ideal choice in PV inverters and mobile equipment such as automotive DC-to-DC converters and traction drives.

Electrification within the automotive industry is proceeding at a rapid pace, and when this sector taps into the great set of attributes of the SiC BJT it will spur the development of smaller, easier to cool electrical systems.

© 2011 Angel Business Communications. Permission required.
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