Why your next relay or fuse could be a transistor

By Philip Ling   |   October 8, 2024   |   Provided By Avnet Silica

Making and breaking connections is fundamental to applying electrical power to do mechanical work. The invention of devices that could be switched electronically changed everything. And when we worked out how to put multiple devices on a single substrate, everything changed, all over again. In fact, things haven’t stopped changing since.

The transistor’s early departure from being a simple on/off switch to becoming the building block of modern life overshadowed its inherent ability to also be a simple and effective on/off switch. High-voltage power distribution branched off, dominated by other types of technology, with the transistor playing only a relatively small part in that world.

We now have semiconductor substrates that can handle extremely high voltages and currents. But do transistor-based switches still have a place in the high-power domain? Of course, the answer is yes. We see them in power conversion and management in every market. The world couldn’t run without power transistors because most of the time there isn’t an alternative solution available for their job in these applications.

But what about applications where there are alternatives? What about circuit breakers, as an example? Most circuit breakers today are electromechanical in nature. They involve two metal contacts forced together or driven apart by electromagnets controlled by transistors. Circuit breakers are safety devices, normally supplementary to the last line of defence, which is typically a fuse.

Automotive manufacturers are moving toward solid-state fuses in vehicle architectures, specifically because they are controllable. They are also inherently resettable, not forgetting they offer faster switching speeds, quieter operation and, without any moving parts, higher reliability. For those reasons, transistor-based fuses and circuit breakers are displacing regular wire fuses and electromechanical devices in EVs and other vehicles.

Circuit breakers come with their own requirements that set them apart from fuses. They can be used more than once, for one thing. That’s something the transistor is very good at. Another is they normally carry large currents and deal with high voltages for prolonged lengths of time. Most transistors can’t do this without getting hot. And when they get too hot, they break.

The key figure of merit is normally the on-resistance or R_DS(on). As a circuit breaker spends most of its lifetime allowing current to flow, the on-resistance is critical. IGBTs are low cost but are unsuitable for use in solid-state circuit breakers because of a “knee voltage” that causes high conduction loss regardless of the number of devices in parallel.

Wide bandgap materials have led to silicon carbide and gallium nitride FETs. Both have good features, most significantly low on-resistance, making them good candidates for solid-state circuit breaker (SSCB) applications.

One of the main growth areas for SSCBs is in DC circuits. This includes the now familiar applications where DC is the norm, such as EVs and renewable energy. Newer applications include those in high-voltage DC (HVDC) infrastructure, including distribution grids. One example is the Viking Link between the UK and Denmark, a distance of 765 km. The cables joining the two nations can carry 525 kV DC up to 1,400 MW of power.

Qorvo recently announced a new 750 V SiC JFET designed for SSCBs. Its industry-leading low R_DS(on) of just 4 mΩ is the lowest for any device in this voltage range. Looking at the part number, we can work out that the UJ4N075004L8S is a Gen 4 normally-on, 750 V 4.3 mΩ device. It has a peak current, I_DM, of 588 A and a junction temperature, T_Jmax, of 175°C, and thus can be used in high-power circuit breaker and power relay applications. It can also be used to provide surge protection and inrush current control.

The market for DC SSCBs is expected to grow at a CAGR of around 8% through 2030. With devices like the new UJ4N075004L8S now available, OEMs can develop solutions that take advantage of this growth. To learn more or request samples, contact your Avnet Silica representative today.

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Philip Ling

Philip Ling is a senior technology writer with Avnet. He holds a post-graduate diploma in Advanced M...

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