As electric vehicles (EVs) designs start to ramp up and proliferate, they are also shifting from 400 V to 800 V battery architectures. This raises new challenges in safety and complexity. To feed these needs, today, Texas Instruments has announced two new solid-state relay ICs designed to address the safety and reliability hurdles facing today’s EV design engineers.  

In this article, we discuss why isolation is critical in EVs, why solid-state relays provide an attractive approach, examine the details of the new relay products, and we share insights from our interview with Priya Thanigai, Manager of Power Switches at Texas Instruments.

EV Safety Issues Are at Stake

To put things in perspective, Thanigai explains that a car is basically the biggest example of a human-machine interface (HMI). In other words, the biggest kind of equipment that a human will touch, operate, and work with every day. For EVs this makes for an inherently dangerous situation.

Isolation becomes ever more critical in EV systems, especially as engineers move from 400 V to 800 V battery systems. Image used courtesy of Texas Instruments

“On one side you have the average very fragile human—not well trained, not well equipped—and then on the other side you have an 800 V battery. This adds up to vast implications to life and safety issues,” says Thanigai.

With all that in mind, power isolation is extremely important in EVs, and even more so as the EV industry transitions from 400 V batteries to 800 V batteries. As mentioned, to address these challenges, TI today introduced a pair of solid-state relay ICs. The company claims these provide the industry’s smallest solution size and shrink the bill-of-material (BOM) cost of powertrain and battery management systems (BMS) for 800 V-based systems.

From Mechanical to Solid-state Relays

The automotive industry has a long history of using mechanical relays to provide isolation. Solid-state photo relays (optoelectronic-based relays) have also been used. Additionally, when the first EVs were built, mechanical and photo relays were used because that was what was available. With its new purely solid-state relay products, TI says it’s able to offer longer operating life than mechanical relays, while at the same time offering higher isolation capability.

The two solid-state relays TI released today are the TPSI3050-Q1 and the TPSI2140-Q1. The TPSI3050-Q1 isolated switch driver integrates a 10 V gate supply and the TPSI2140-Q1 is a 1,400 V, 50 mA isolated switch. Both devices provide integrated power and signal isolation across a single barrier.

The TPSI3050-Q1 offers reinforced isolation up to 5 kVRMS, while also providing an operating lifetime that’s 10 times higher than electromechanical relays, which can degrade over time. Additionally, the TPSI2140-Q1 offers basic isolation up to 3.75 kVRMS, enabling it to achieve more than four times higher time-dependent dielectric breakdown reliability than solid-state photo relays.

According to TI, the TPSI3050-Q1 integrates the functions of an isolated power supply, digital isolator, and gate driver. TI claims that this reduces the solution size up to 90% compared to mechanical relay solutions. Meanwhile, the TPSI2140-Q1 reduces solution size by as much as 50% compared to traditional solid-state photo relays. It does so by embedding a signal field-effect transistor (FET) and resistors and eliminating the need for a reed relay.

These devices are the first in a new solid-state relays portfolio from TI that will also include ICs designed for high-voltage industrial applications. Along with this news, TI has released a technical article explaining the benefits of solid-state relays.

The On-chip Isolation Barrier

The two solid-state relay ICs are essentially power switches with an isolation barrier that's built into the chips. The chips separate the high-voltage and low-voltage sides. Control signals come from a microcontroller—or some other kind of low voltage domain—and pass through the isolation barrier to control some kind of high voltage path on the other side. The isolation technology at the barrier is a fabricated process that is highly resilient, says TI.

The new TI solid-state relays embed an isolation barrier separating the low voltage side from the high voltage side. Image used courtesy of Texas Instruments

“The working voltage for these devices is actually 1,400 V, so they are specified to work at voltages much higher than is needed for 400 V and 800 V EV battery technologies because we see that pushing further beyond 800 V in the future," says Thanigai.

Speed is another key facet of the new solid-state relays. TI believes that the new solid-state relays can disconnect and connect loads through a single isolation barrier much faster than electromechanical relays.

“Mechanical relays typically take tens to hundreds of milliseconds to turn on and off,” says Thanigai. “But in the case of solid-state relays, it's hundreds of microseconds.”

Solid-state relays can provide critical isolation features that keep humans safe from harm. They can prevent problems from happening, but also deal with them instantly if they do. To illustrate the significance of the speed increase, Thanigai uses the example of a typical EV function: the safety cut off.

An EV runs an “isolation check” function that runs in the background the entire time the car is on—checking to make sure that the 400 V battery is isolated from the chassis so that, if somebody pops open the hood and touches the chassis, they don't get electrocuted.

Thanigai explains that TI's solid-state relays can be used for their switching mechanisms to check whether the isolation barrier is working.

"Hopefully nothing ever happens. But, if there is something catastrophic, you want the voltage cut off to be really fast, really quick, and highly reliably. That's when these solid-state relays come in. They provide that first check to make sure that there is no ground connection between the high voltage battery and essentially the human side, which in this case the vehicle chassis."

More Isolation Products

Along with the two new solid-state relay ICs, TI also announced other new products as part of its isolation portfolio. These include UCC14240-Q1, an isolated DC/DC bias-supply module aimed at improving EV driving ranges.

Another new product is the AMC23C12, which TI claims are the "industry’s first" reinforced isolated comparator. Overall, the AMC23C12 marries the functions of standard comparators with a galvanic isolation barrier and provides isolated bidirectional overcurrent and overvoltage detection in less than 400 ns.

TPSI3050Q1EVM (shown) and TPSI2140Q1EVM evaluation boards are available for the TPSI3050-Q1 and TPSI2140-Q, respectively. Image used courtesy of Texas Instruments

TI says the TPSI3050-Q1 and TPSI2140-Q1 are available now in pre-production 1,000-unit quantities on TI.com. Also available are TPSI3050Q1EVM and TPSI2140Q1EVM evaluation boards for the two ICs.

As EV system designers plan their transition from 400 V to 800 V batteries, there are new technology choices to consider. These new isolation products from TI address many of the challenges ahead and facilitate a movement away from mechanical relays to solid-state alternatives.