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SiC switches are the velociraptors of motor control – but where are they?

Moving dinosaurs

Ever since stone-age man realized he could pull more weight of a dead dinosaur by rolling it on poles, we’ve been trying to perfect the technique of converting muscle, and now electrical power to rotary motion. Today, the market for medium and low voltage motors is over $20B and their associated electronic drives $13B1,2 . Some reports put the global motor market size vastly bigger, over $107B in 20163 . Whatever your opinion of the accuracy of market surveys, they’re big numbers and set to keep growing as factory automation continues its expansion, fuelled by ‘Industry 4.0’ and the ‘Industrial Internet of Things’ (IIoT).

Technology dinosaurs

As you can imagine, in such a large market for drives and with the constant imperative to save energy, cost and space while improving production efficiency, there has been extensive research over the years into new design technology. MOSFETs were introduced and are now common for low voltage/low power applications. More recently there has been a buzz of excitement about wide band-gap (WBG) silicon carbide (SiC) and gallium nitride (GaN) switches as possible replacements for those lumbering dinosaurs of switching: IGBTs. The theoretical speed and gains in efficiency, size, and cost reduction with WBG in power conversion are spectacular. It’s a conservative industry though, with high infrastructure capital cost that must reliably live out until payback. This means that the vast majority of installed equipment still uses 10-20-year-old technology, typically IGBTs, running at sometimes sub-10 kHz switching frequency. The market for IGBTs in motor control is even set to increase as system designers opt for proven, reliable solutions in new designs, especially when space is not a major issue. In 2016, use of SiC in motor drives was estimated at just $24.8 million value4 , about 10 percent of all SiC production while the value of IGBTs sold was over $4 billion in total, mostly in the motor and motion control markets (Figure 1 ). The value of GaN devices sold into the same market was near zero.

Figure 1

Proportion of SiC devices sold compared with IGBTs, 2016

Proportion of SiC devices sold compared with IGBTs, 2016

Evolving switches with wide band-gap technology

So, what’s so good about WBG that should be tempting drives buyers? A headline advantage compared with IGBTs is smaller-sized passive components when operated at higher frequencies. Magnetics are often cited as a key area, though in motors, the main inductor is the motor itself, which isn’t going to downsize with a higher drive switching frequency. There is an efficiency gain though when replacing IGBTs in new designs at low frequencies.

However, robustness and reliability are a concern; SiC MOSFETs, compared with IGBTs, have limited current ratings and don’t have good immunity to short circuits, which are inevitable in a motor control environment. They are also costlier than IGBTs like-for-like and with their sensitive gate drive requirements, are certainly not an easy drop-in replacement, even if available in the same package. IGBTs need parallel, fast commutation diodes in motor control; so are these still necessary with SiC MOSFETs with their integral fast body diode? Unfortunately, yes, as for high voltage types, the diode forward conduction losses are high and a fast, parallel diode is still needed for ideal performance. SiC MOSFETs are a viable replacement for Si-IGBTs in low voltage drives but typically don’t confer enough advantage to warrant the extra cost when the frequency is kept moderate for EMC reasons. Ironically, even with their promise of high power ratings, SiC MOSFETs do have an advantage over IGBTs at low power in that lower conduction losses can be achieved in the RDSON of the MOSFET than in an IGBT with its fixed, saturation ‘knee’ voltage.

The SiC cascode – a robust and user-friendly velociraptor

So, do WBG devices have a place in today’s motor control applications? There is a version of a SiC switch that is overturning the objections and deserves consideration, and it’s called the SiC cascode: The SiC cascode is a normally-ON SiC JFET, co-packaged with a low voltage Si-MOSFET (Figure 2 ). The combination is normally-OFF, has the high temperature and speed characteristics of WBG devices and features a fast, low-drop body diode and easy, non-critical gate drive. Packages include TO-247 making it a true drop-in replacement for IGBTs and Si-MOSFETs.

Figure 2

SiC cascode structure

SiC cascode structure

SiC cascodes have controlled avalanche for over-voltages and excellent short circuit behavior, making them extremely robust at motor start-up or stall when current can be high. A particular feature of the UnitedSiC cascode is the way edge rates can be simply controlled by tailoring gate drive resistors to mitigate EMI concerns. Often, gate resistor value changes and adjustment of snubbers are the only modifications needed to swap-out IGBTs or Si-MOSFETs for a SiC cascode. Other savings can be obtained as wished by perhaps removing parallel diodes and optimizing snubber components. SiC Cascodes are available from UnitedSiC5 , have been extensively qualified and are 100% production-tested for avalanche withstand. Standard rated voltages are 1200 V and 650 V but ‘super cascodes’ can be assembled from strings of a SiC cascode and SiC JFETs from the same manufacturer to give ratings of several kV with the same performance advantages.

Jurassic v. 21st century

The SiC cascode is sure to start making inroads into motor drives as efficiency becomes more of an issue. When drives are designed from the ground up with these devices, frequencies can be pushed up and cost/size driven down, maybe turning your ageing brachiosaurus design into a 21st century, speedy and light-weight velociraptor.

References:

[1] Global Market Insights: Electric Motors

[2] The Fastest Way to Identify New Revenue Opportunities

[3] Grandview Research: Electric Motors

[4] Yole Development 2016

[5] United SiC

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