Overview

In the previous article, I briefly introduced what kind of effects WBG semiconductors can produce in EV applications. In this article, we will introduce the merits of WBG semiconductors in more detail.

Please refer to the previous article from the link below.

Wide bandgap semiconductor primer

WBG semiconductors in SiC and recent GaN technologies require a very high energy of 3.2 eV (electron volts) in SiC to transfer an electron from the valence band to the conduction band, compared to 1.1 eV in silicon. and has fundamental advantages over silicon. This means higher field breakdown performance for a given material thickness and the ability to withstand higher temperatures before breakdown, depending on the device, but typically with momentary peaks above 600°C. temperature is allowed.

SiC also has approximately 3.5 times higher thermal conductivity than silicon, which results in a much smaller die for a given current and voltage rating. Smaller dies naturally have much lower capacitance, allowing WBG devices to switch faster with lower losses in high temperature power applications. The switching frequency can be kept low to keep switching losses as low as possible, but pushing it up reveals other benefits. The higher frequency also improves net efficiency and consumes less power, resulting in smaller and cheaper heatsinks.

The result is more efficient, lighter and smaller products, leading to significant cost savings. The high temperature capability of the WBG device allows the traction inverter to be integrated into the motor housing, significantly reducing cost, reliability and size over having separate drive electronics interconnected to the motor. can. Motor control is also improved at high switching frequencies due to smoother current waveforms with less noise and vibration and improved motor efficiency. In addition, IGBTs require a "freewheeling" diode in parallel in motor drive applications, while some schemes using WBG switches offer additional cost and assembly savings by using WBG switches with integrated body diodes. You can save time and effort.

WBG switch family of SiC FETs

Silicon carbide and gallium nitride are suitable materials for manufacturing various types of power semiconductor devices such as SiC diodes, MOSFETs, JFETs and GaN HEMTs. Both are on the market with their respective "sweet spot" applications. For switches, SiC MOSFETs and GaN HEMTs are high performance, but have very demanding gate drive requirements. GaN HEMTTs do not have an effective body diode, whereas SiC MOSFET diodes are fast but have a large forward voltage drop, often requiring the use of an external diode.

SiC and GaN JFETs are useful in that they do not have fragile gate oxides. These can be constructed as normally-on or normally-off types. The normally-on type has the lowest chip resistance per unit area, but requires a negative gate voltage to keep the switch off. Since most circuit topologies are developed for normally-off, there is no provision governing the shoot-through condition that occurs when both normally-on JFETs in the bridge lose gate power. Also, there is no body diode. A SiC FET is a composite or “cascode” of a SiC JFET and a Si MOSFET that is normally off with no bias and capable of nanosecond switching.

Driven by SiC FETs installed in electric vehicles

So why hasn't such a miracle device made its way into EV motor control, given the demand for higher performance solutions? Apart from the natural conservatism of automotive system designers, WBG devices are expensive compared to comparably rated IGBTs, the drive inverter's load is the motor, and its inductance is similar to that of a DC-DC converter. There are practical reasons such as higher switching frequencies not being attractive because they do not scale down. Fast switching speeds mean high dV/dt rates that stress the insulation of the motor windings.

High dV/dt can also cause motor bearing wear due to EMI and "EDM" or EDM, causing common-mode noise currents to find a path through the motor bearings to ground. Importantly, since this technology is relatively new, has the reliability of WBG devices been demonstrated under the harsh conditions of automotive motor drives, such as short circuits, back EMF, and typical high temperature environments? The question remains as to what.

However, the potential for increased efficiency is still an attractive prospect. For EV motor drives, this means more available energy and better range. A smaller heatsink reduces cost and weight, which also helps increase range. Using SiC FETs is particularly efficient under typical operating conditions. Compared to IGBTs with a “knee” voltage, SiC FETs are simply resistive when on, effectively minimizing the ubiquitous power loss under all drive conditions. This is a comparison of a 200A, 1200V IGBT module using two 1cm x 1cm IGBT dies and a 200A, 1200V SiC FET module using two 0.6 x 0.6cm SiC stacked cascode dies Figure 2 is shown.

SiC FETs come standard with ratings of 650 V, 1200 V and higher, a good match for today's popular EV battery voltages of around 400 V and upcoming 750 V versions. SiC FETs have the unique property of being able to offer the lowest conduction losses in a given module footprint. Indeed, in a ground-up design, WBG motor drives are capable of switching at higher frequencies than IGBTs and are designed with sufficient EMI control to enjoy all the benefits of WBG. SiC FET dies are much smaller than similarly rated IGBTs and SiC MOSFETs, resulting in higher yields per wafer. Considering the cost savings from smaller heatsinks and filters, plus the energy savings from increased time and convenience, it makes economic and practical sense.

In the next article, we will discuss the reliability of WBG semiconductors in more detail.

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