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Si to SiC: How to know when it’s time to switch

electricuwe
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Si to SiC: How to know when it’s time to switch

Over the last few years silicon carbide (SiC) switch devices, especially SiC MOSFETs, have evolved from a research topic to a significant commercial business. Initial adoption took place in photovoltaic (PV) inverters and battery electric vehicle (BEV) drive trains, but now, more and more applications are being addressed.  Everyone designing equipment and systems using power electronics has to assess what potential SiC may provide in his market and what the best strategy to exploit this potential might be. So, where do you start?

Experienced engineers might still remember how fast bipolar transistors have been replaced by MOSFETs in SMPS or how fast IGBT modules kicked the bipolar Darlington transistor modules out of early drives inverters.

The driving forces in power electronics have always been loss reduction, miniaturization, and reliability improvement. This is expected to continue. So, is there a need to hurry to convert every design from silicon (Si) to SiC as fast as possible? Will the IGBT completely disappear from the market like the bipolar Darlington some decades ago?

Today’s power electronics applications are much more diverse than those in the 80s and 90s and the market for power semiconductors is much larger. Therefore, a sequential and partial change from Si to SiC is much more likely than a disruptive change throughout all applications. How this plays out in specific applications, however, depends on the value SiC provides to this specific application.

But how does SiC provide value to a power electronics design?

Of course, in any system using inductors or transformers, SiC can enable an increase in switching frequency leading to smaller, lighter, and eventually less expensive inductive components. This was essential for SiC MOSFETs finding their way into PV inverters. Years ago already, the SiC-MOSFET – together with the ANPC topology – was crucial to designing high-power string inverters for 1500 V PV systems at reasonable cost. The topology enabled the increase in switching frequency with only one-third of the power devices being SiC MOSFETs.

But not all applications benefit from increasing switching frequency. In a general purpose drives (GPD) application, there are no inductive components which could benefit from higher switching frequencies.  Even at the moderate switching frequencies used today, the motor currents are already nearly perfectly sinusoidal.  But, SiC can also be used to reduce conduction loss. Contrary to IGBTs and des, SiC MOSFETs feature a knee voltage-free characteristic in both directions, as long as the usual PWM pattern is applied. So if sufficient active area is provided, a significant reduction in conduction losses can be achieved. This makes it possible to design drives that are integrated in the motor, into an enclosure with high ingress protection and natural cooling, or into smaller housings. Furthermore, applications running a significant share of time with only a partial load can utilize this feature to reduce energy consumption and total cost of ownership (TCO).

If the body diode can be used as freewheeling diode, as in case of the Infineon CoolSiC™ MOSFET, this will provide another benefit important for some drives. Power dissipation will always be in the same die, regardless of the direction of power flow, significantly reducing the temperature swing relevant for power cycling.

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Figure 1: Synchronous rectification in body diode.

Sometimes higher temperature operation is claimed as a benefit of SiC. This will be an important lever in applications where heat has to be dissipated in hot environment, but only as long as the package and other system components are also suitable for this environment.

To sum it all up: when an application could benefit from switching from Si to SiC, it is important to make the switch with the right strategy. A simple swap will rarely be able to exploit the full potential of the new semiconductor material, even if it works without problems. Minimizing stray inductance in the DC-link connection and on the gate driver should be considered, and in most cases the protection scheme also needs to be adapted. As this takes time, development projects should be started early, even if the supply situation for production quantities looks to be tight currently.

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