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Posted on 22 August 2019

Silicon Carbide Boosts Power Module Performance

 

 

 

 

 

 

 

Silicon Carbide offers new approaches for the design of power semiconductors. In conventional power Silicon technology, IGBTs are used as switches for voltages higher than 600 V, and Silicon PIN freewheeling diodes are state of the art. The design and soft switching behavior of Silicon power devices cause considerable power losses. With the larger bandgap of Silicon Carbide, high-voltage MOSFETs can be designed with blocking voltages up to 15 kV, while providing extremely low dynamic losses. With Silicon Carbide, the conventional soft turn-off Silicon diodes can be replaced by diodes in Schottky design, also offering extremely low switching losses. As an additional benefit, Silicon Carbide has a 3 times higher thermal conductivity as compared to Silicon. Together with small power losses, Silicon Carbide is an ideal material to boost power density in power modules.

Volker Demuth, Head of Product Management Components, SEMIKRON, Nuremberg, Germany

It is been a long way for Silicon Carbide (SiC). I remember purchasing 2-inch SiC wafers for crystal defect analysis at the university in the mid 90’s at unbelievable prices. In fact, those were “science grade” wafers, as “industrial grade” wafers were even more expensive. The defects were a nightmare for crystal growers and device manufacturers: high defect density and hollow tubes in crystal growth direction, that would show as holes in the SiC wafers after cutting. More than 15 years later, the SiC industry moves to 6-inch wafer production as a standard, and by intelligent growing techniques, the crystal defects are reduced to a minimum, increasingly making SiC an alternative to standard Silicon devices in power electronics. And the future looks promising. In some markets, such as solar, SiC devices are already in operation despite the fact that the prices for those modules are still higher by factors as compared to conventional Silicon devices. Current designs are available in SiC hybrid modules (IGBT + SiC Schottky diodes) and full-SiC modules.

SiC hybrid modules

In SiC hybrid modules, a conventional IGBT is switched together with a SiC Schottky diode. Although the main benefits of SiC devices are clearly related to low dynamic losses, the static losses of SiC Schottky diodes are discussed first.

Oftentimes, the static losses of SiC devices seem to be higher compared to conventional Silicon devices. Figure 1 shows the forward voltage drop Vf of a conventional soft switching 600 V SEMIKRON CAL HD freewheeling diode, a Fast Silicon diode optimized for low switching losses and a SiC Schottky diode, all rated at 10 A.

Freewheeling diode voltage drop

Figure 1. Forward current and forward voltage drop of different 10 A freewheeling diodes at 25°C and 150°C (SiC Schottky diode, soft switching Silicon diode (CAL HD), and fast Silicon diode (Si-Fast))

At rated current of 10A, the Silicon freewheeling diode reveals the lowest forward voltage drop, the Vf of the SiC Schottky diode is higher, whereas the Fast Si diode shows the highest forward voltage drop. The temperature dependence of the forward voltage is quite different: the Fast Silicon diode has a negative temperature coefficient, as Vf is lower at 150°C than it is at 25°C. The temperature coefficient of the CAL is positive for currents above 12 A and the SiC-Schottky diode, as characterized by positive temperature coefficient even for currents as low as 4 A.

Since diodes are often paralleled to achieve high power devices, a positive temperature coefficient is required to avoid unbalanced current sharing and inhomogeneous operation temperature of the paralleled diodes. Here the SiC Schottky diode shows the best behavior. But compared to the conventional Silicon diode, the static losses are higher for the SiC Schottky. As the diodes were compared based on the nominal current of 10A, it is important to take into account that the definition of nominal currents is sometimes unequal between devices by different suppliers. To gain better insight to device performance, it is useful to the plot the current density (forward current divided by chip area) as a function of the forward voltage drop, which takes into account the chip area.

Forward voltage drop and current density

Figure 2. Forward voltage drop and current density (forward current divided by chip area) of the diodes in Figure 1

Figure 2 shows that for equivalent current densities, the conventional Silicon and SiC Schottky diode have quite similar forward voltage drops, whereas the Vf of the Fast Si diode is still highest. In other words: Silicon and SiC diodes have comparable static losses, as identical chip areas are used. Usually SiC chips exploit smaller chip sizes, as the nominal current rating is done considering static and dynamic losses, causing low total losses, and therefore allow a chip size shrink. Looking at the dynamic losses, the main benefit of SiC devices becomes clear (see Table 1).

Freewheeling diode dynamic parameters

Table 1. Dynamic parameters of a conventional Silicon freewheeling diode (CAL HD), a SiC Schottky diode and a Fast Silicon diode (1200 V, 10 A rating)

Compared to the conventional Silicon diode, the reverse recovery current IRRM is more than 50 % lower for the SiC Schottky diode, the reverse recovery charge QRR drops by a factor of 14 and the turn-off energy Eoff is lower by a factor of 16. The Si Fast diode shows better characteristics than the conventional Silicon diode, yet it does not reach the superior dynamic characteristics of the SiC Schottky diode. Due to the low dynamic losses of SiC Schottky diodes, the inverter losses can be reduced significantly, saving expenses for cooling and increasing the power density of the inverter. In addition, the low dynamic losses make the SiC Schottky diodes ideal for high switching frequencies.

On the other hand, fast-switching freewheeling diodes may come with a drawback, as the very steep decrease of the reverse current may lead to a current cut-off and oscillations. In the case of Silicon diodes, the current cut-off is controlled by soft turn-off characteristics. Figure 3 compares the turn-off characteristics of the CAL HD and the SiC Schottky freewheeling diodes.

Freewheeling diode turn-off characteristics

Figure 3. Turn-off characteristics of Silicon and SiC freewheeling diodes. The turn-off energy of the SiC diode is barely visible. As the turn-off energy of the SiC diode is small, the reverse current drops rapidly, leading to small oscillations in reverse current and voltage

With the Silicon-based CAL HD diode, the well-known soft turn-off behavior is observed. As the reverse current is reduced smoothly, no over-voltage peaks nor oscillations are visible. On the other hand, the soft turn-off behavior causes a significant turn-off energy, because a considerable reverse current is flowing as the voltage at the diode rises. The SiC Schottky diode basically does not show any reverse recovery charge, and consequently an extreme low turn-off energy. Due to the rapid decrease of the reverse current, small oscillations are induced, visible as ripples in the reverse current and in the voltage drop. In our case, the fast turn-off behavior of the SiC Schottky diode was addressed by an optimized chip layout on the DCB and a low stray inductance of the module. Consequently, the voltage oscillations are small and do not lead to significant overvoltage peaks. It is therefore possible to manage the disadvantages of fastswitching diodes and fully exploit the benefits of SiC Schottky diodes by optimized module designs. In Figure 4 the advantage of the SiC diodes is shown by comparison of a conventional Silicon module and a SiC hybrid module equipped with fast Silicon IGBTs and SiC Schottky diodes.

6-pack module output current

Figure 4. Output current of a conventional Silicon 6-pack module (1200 V, 450 A trench IGBT + CAL freewheeling diode) and a SiC hybrid 6-pack module (1200 V, 300 A Fast IGBT + SiC Schottky). Thermal loss calculation of SKiM93 modules on water cooler

As expected, the superior dynamic behavior of the SiC Schottky diodes increases the output power of the module significantly. With the given chip setup, which was chosen for optimum performance at higher switching frequencies, the usable output current can be increased by more than 70 % at 30 kHz. As the switching frequency rises further, the benefit of the hybrid SiC module is even larger.

The lower losses and the resulting higher power output on module level can be exploited on several ways. Weight and volume of inverters can be reduced significantly, important for example for automotive and aerospace applications. Exploiting high switching frequencies smaller LC-filters are possible, reducing size and inverter cost. Last but not least, the lower losses cause also an significant advantage in energy efficiency, important for solar, UPS, and automotive applications.

Full SiC modules

Using SiC switches like SiC MOSFETS the overall losses of power modules can be decreased even further. In Table 2, static and dynamic losses of a 1200 V, 25 A 6- pack IGBT module are compared to a 20 A Full-SiC module.

Static and dynamic losses

Table 2. Static and dynamic losses of a 1200 V, 25 A IGBT module (trench IGBT + CAL diode) in comparison to a 20 A full SiC module (SiC MOSFET + SiC Schottky)

The static losses of the full SiC module are higher by 17 %, while the dynamic losses are significantly lower: turn-on losses are lower by a factor of 3, the turnoff losses by a factor of more than 6. Consequently, the usable output power of a full SiC module much higher compared to conventional Silicon technology, especially at higher switching frequencies (see Figure 5).

Output power of 6-pack module

Figure 5. Output power Pout of a 1200 V, 20 A 6-pack Full- SiC module and a conventional 1200 V, 25 A 6-pack IGBT module

At switching frequencies higher than 20 kHz, the output power of the full SiC module is more than 100 % higher as compared to the IGBT module. In addition, there is little dependency of the output power on the switching frequency. In turn, the full SiC power module can be used up to very high switching frequencies, as the output power is only  28 % lower at 40 kHz compared to the output power at 10 kHz. At switching frequencies lower than 5 kHz, the IGBT modules shows higher output power. This is a result of the SiC chipset used in the full SiC module, which was optimized for very high switching frequencies.

Output power per chip area

Figure 6. Output power divided by chip area showing the power density of the used power semiconductors. Thermal loss calculations on aircooled heat sink, 40°C ambient temperature

An optimization for lower switching frequencies is also possible. Again, it is useful to address the power density of both modules by considering the chip areas used for the Silicon and SiC chips. In Figure 6 the output power is divided by the chip area, showing the power density. The power density of the full SiC module is much higher as compared to the IGBT module, even at switching frequencies lower than 5 kHz. An optimization of full SiC modules for low switching frequencies is therefore possible by using larger chip sizes. SiC devices can provide higher output currents and output power over a broad range of switching frequencies, as the SiC chip sizes are adapted.

Conclusions

At module level, SiC has two main benefits: smaller chip sizes and lower dynamic losses. At system level, these advantages can be exploited in several ways. Lower dynamic losses lead to a significant increase of the output power, offering the chance to save weight and volume. Worth mentioning is the fact that this power increase can be achieved without additional cooling power. Since SiC leads to an actual loss reduction compared to Silicon devices, higher output power is possible with the same cooling efforts. Small power losses improve the energy efficiency, enabling the design of high efficient inverters, e.g. for solar and UPS applications. In addition, low dynamic losses make SiC devices ideal for higher switching frequencies above 20 kHz.

Exploiting high switching frequencies allows for reducing the cost and size of LC filtering. Depending on the chip area used, SiC shows advantages also at switching frequencies as low as 4 kHz. Other advantages of SiC are related to the enhanced thermal heat dissipation and the positive temperature coefficient, important for paralleling SiC chips. All of that makes SiC a very attractive material for a wide range of possible applications. However, the price of SiC power devices is still higher by factors, causing the prices of hybrid and full SiC modules to be much higher compared to conventional Silicon solutions. These higher costs constrain the market entry, and SiC solutions have found their way into niche applications mainly. Cost evaluations show that in many applications, the price of SiC modules needs to be 2-3 times higher in order to achieve positive business cases. In some applications, higher prices may be affordable, as benefits like small size, low weight, high efficiency, etc. can outweigh the higher cost. Much more than with conventional Silicon solutions, the total cost of ownership needs to be carefully considered with SiC.

Some benefits are not related directly to higher power output or higher efficiency. For example, reduced size and weight of wind inverters not only saves space inside the wind installation, but also reduces transportation and installation efforts. SiC offers lots of benefits and forces designers of power systems to think differently, to review conventional designs and to find new ways to take full advantage of the SiC technology. At module level, low inductive designs, optimized DCB layouts for extensive chip paralleling and of course new assembly technologies like sinter technology for high reliability and high operating temperatures are needed. SEMIKRON supports the SiC solutions by intensive research, SiC devices can be assembled in all standard packages; optimized solutions and topologies are evaluated in close relationship with the customers, which is indeed necessary. Tailored solutions are the way to be cost competitive.

However, the price issue persists and needs to be resolved for a broad breakthrough of SiC devices in the power electronics market. And the outlook is positive: according to market studies, the prices of SiC Schottky diodes are expected to drop by ~30 %, the prices for SiC MOSFETs are expected to drop by about 40 % within the next years, significantly increasing the competitiveness of SiC. It can be expected that the prices of hybrid and full SiC modules will be suitable not only for niche applications, but also for standard solutions within the next 3 years. Indeed, it’s been a long way for SiC into power electronics, and the quest for a broad market entry is not yet finished. But the signs are positive for SiC to become a mature technology for power electronic applications within the next years.

 

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One Response

  1. avatar kiran pisal says:

    Good information for selection of IGBT\Diodes for new designs

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    Rating: 5.0/5 (2 votes cast)

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