Silicon Carbide (SiC), a wide bandgap material with markedly superior characteristics to silicon in high-voltage circuits, has struggled to gain widespread market acceptance. In some ways, this is surprising: SiC components – diodes and MOSFETs – are ideal for high-voltage applications in which energy efficiency is a critical parameter.
For example, in solar inverters switching losses may be reduced by more than 30% through the use of SiC MOSFETs. Solar inverters with both SiC MOSFETs and diodes have been shown to be capable of achieving overall system efficiency of greater than 99%.
Similar efficiency benefits can be achieved in other applications that require high blocking voltages in combination with fast, efficient switching: industrial motor drives, DC power systems in data centres, power factor correction circuits, and high-frequency DC-DC converters in industrial, computing and communications power systems. Benefitting from low switching losses, SiC MOSFETs and diodes can enable operation at switching frequencies up to four times higher than those using conventional silicon IGBTs.
So why are SiC components not in widespread use in these applications? The answer is simple: component cost. An expensive manufacturing process means that the cost of a SiC MOSFET is far higher than that of a comparable silicon component. Simply replacing a silicon MOSFET or IGBT in a conventional power circuit with a SiC MOSFET normally makes little financial sense.
But this is the wrong way to approach power-system design with SiC components. In fact, the proper use of SiC components can result in lower total system costs, despite the relatively high cost of the SiC components. But if design engineers are to realise cost savings from the use of SiC technology, they must thoroughly review and modify their existing circuits, and possibly even abandon an existing design entirely and start afresh. This article shows why.
The cost contributors in a high-voltage power circuit
The reason the power-system designer should not focus on a simple cost comparison of silicon and SiC components is that these components make up only a small proportion of the total system cost. In fact, the main cost contributors to a high-voltage circuit are:
- Power semiconductors
In addition, in some end-product types there might be noticeable costs associated with transport or handling of the end-product. In these cases, the weight and size of the power circuit can have a marked impact on the manufacturer’s costs.
When a circuit is designed from the start with the intention of using SiC MOSFETs and diodes, savings can be made in every one of the cost contributors listed above. As a result, design teams that have designed new power circuits to take full advantage of SiC technology are gaining a distinct competitive edge. For example, motor-system manufacturer Kollmorgen (www.kollmorgen.com) has developed a prototype of a new SiC-based inverter, as shown in Figure 1, for use in heavy-duty Hybrid Electric Vehicles (HEVs) such as city buses.
Specified for operation at 750V DC and 400Arms, the SiC version of the inverter offers the following advantages over the equivalent product that uses silicon IGBTs:
- 1% superior system efficiency, the equivalent in a typical city bus to an annual reduction in fuel consumption of between 600 litres and 1,000 litres of diesel fuel
- A much cheaper thermal design using air cooling rather than water cooling
- Higher-frequency switching, enabling the use of smaller passive components
According to Lux Research (www.luxresearchinc.com), the savings in fully Electric Vehicles (EVs) look equally promising. Attempts to extend the performance of silicon devices in high-voltage applications are hitting the physical limits of the material’s characteristics. In an August 2014 paper, the research company found that the use of Wide Bandgap (WBG) materials such as SiC offers economic benefits because of the large batteries in EVs, as shown in Figure 2.
‘Efficient power electronics is key to a smaller battery size, which in turn has a positive cascading impact on wiring, thermal management, packaging and weight of electric vehicles,’ said Pallavi Madakasira, an analyst at Lux Research and the lead author of the report titled Silicon vs WBG: Demystifying the Prospects of GaN and SiC in the Electrified Vehicle Market.
On the Tesla Model S for example, a 20% reduction in power use can make the battery system $6,000 cheaper – some 8% of the vehicle’s total cost.
According to Lux Research, a power saving of just 2% makes the use of SiC diodes essential in EVs, on the assumption that battery costs fall below $250/kWh. For plug-in HEVs, the threshold for the introduction of SiC components is a 5% power saving.
To illustrate the way that SiC components enable cost savings across the whole of a power circuit, let us study an example, an application that has been developed by a research team at STMicroelectronics. The prototype developed by ST is a 5kW boost converter, a functional block in a solar inverter, as shown in Figure 3. The prototype uses the following SiC components:
- The SCT30N120 is a 1,200V SiC N-channel power MOSFET. It is rated for operation across a junction temperature range of -55°C to 200°C. On-resistance is rated at a typical 80mΩ.
- Two STPSC6H12 1,200V SiC Schottky diodes functioning as a rectifier.
The system is designed to boost a 400-600V DC input to 800V DC in continuous-current mode, supporting an output power of 5kW. The SiC MOSFET’s maximum junctiontemperature rating is some 25°C higher than that of a comparable silicon IGBT, which means that a smaller heat-sink can be used, as shown in Figure 4.
The boost inductor is rated for a maximum 25A current, with low parasitic capacitance and a 25A saturation current.
STMicroelectronics evaluated comparable systems at switching frequencies of 25kHz (the limit of a silicon IGBT’s performance in this application) and 100kHz (with a SiC MOSFET), to examine the trade-off between cost and performance.
Increasing the switching frequency allows for the use of a smaller inductor and/or a smaller output capacitor. Technically, given that the maximum current ripple occurs at Vin=Vout/2, the higher the switching frequency the lower the inductance required, according to the formula:
Figure 5 shows the reduction in the size and weight of the inductor achieved by operating at the high 100kHz frequency supported by the fast SiC components.
A summary of the superior performance achieved by the SiC-based design is shown in Figure 6: with a SiC MOSFET, the system switches four times faster and offers higher efficiency, and uses smaller and lighter magnetics and heat-sink.
Even more interesting, Figure 7’s cost comparison shows that, even though the SiC MOSFET is nine times more expensive than a silicon IGBT, the total system cost is lower in a design that takes full advantage of the SiC MOSFET’s superior characteristics. This is because the inductor, capacitor and heat-sink are expensive components. Moreover, the benefits of the reduction in weight and size provided in a design based on SiC components come in addition to the bill-ofmaterials cost savings.
Another example of the benefits that SiC components will bring to the fast growing EV market was presented by car manufacturer Toyota at the Automotive Engineering Exposition (May 2014, Japan). Toyota estimates that 20% of total electrical power losses in HEVs are attributable to power semiconductors. Improving the efficiency of the power semiconductors directly reduces fuel consumption.
Toyota has set a goal of achieving a 10% improvement in HEV fuel efficiency: SiC MOSFETs supplied by Microsemi are now playing a role in its strategy for achieving this goal.
Results from development work carried out by Toyota show that power losses when using SiC MOSFETs are around 10% of the power loss suffered by Si IGBTs. In addition, the switching frequency can be increased by a factor of ten, which enables a reduction in the size of the power control unit of around 80%.
There is a commercial as well as an engineering benefit to the use of SiC MOSFETs in HEV power supplies. Toyota presented a cost comparison for a three-phase 225kW inverter for an electric vehicle using a 350V battery. Toyota’s current solution uses 84 Si IGBTs. The goal was to replace these with SiC MOSFETs in order to improve system performance at a total cost no higher than that of the current design.
The design uses 60 SiC MOSFETs supplied by Microsemi, rated for a maximum voltage of 700V and with typical on-resistance of 40mΩ. It reduces the cost of the battery by 6%, and the magnetics by almost 50%, while the passives and other components have almost the same cost as the IGBT solution. Although the cost of the semiconductors is three times higher than that of the IGBT design, the total system cost is 5% lower.
And, as Figure 8 shows, the SiC MOSFET system has a particularly marked efficiency advantage over the IGBT system at low loads.
A fast-changing market The examples described above suggest that the cost/performance battle between SiC MOSFETs and IGBTs is finely balanced today. As the months go by, however, the balance will continue to tip further and further in the SiC components’ favour, because of expected steep falls in the price of SiC MOSFETs. This price drop is due to increased competition among wafer suppliers and the transition to a 6” wafer fabrication process.
As the basic price of SiC components falls, the benefits to be gained from designing systems around them become even more attractive. For manufacturers of solar inverters, as well as many other types of highpower equipment, a tipping point might now have been reached, at which silicon carbide becomes the favoured material for switching components.