The 48 V power supply used in most automotive systems puts unique constraints on the components that design engineers choose to integrate.

For one thing, these systems require transistors that can handle higher currents, higher input voltages, and (typically) lower RDS(on) values. However, even transistors that are designed to handle these voltages may fail under certain conditions, namely high transient spikes that are common in automotive systems. 

Nexperia’s new 80 V RETs for EV systems. Image used courtesy of Nexperia

Nexperia is aiming to alleviate this problem with its newest release: an 80 V resistor-equipped transistor (RET). These RETs, also known as digital transistors, were designed to help future-proof automotive designs, giving the component more than enough voltage headroom to meet the needs of EV boards. 

What are Resistor-Equipped Transistors (RETs)?

A RET is a bipolar transistor that has a built-in biasing network that consists of a series base resistor and base-emitter resistor.

An NPN RET. Image used courtesy of Toshiba

The input resistor serves to convert the input voltage to current, while the base-emitter resistor helps to absorb leakage current and prevent malfunctions due to noise. The BJT operates just like a normal BJT with a VBE in saturation of about 0.7 V. This means that when input currents are small (the voltage across R1 is small), then the majority of the current will be sunk to ground through R2.

When the input current is large, the majority will go to the base of the transistor with only ~0.7/R2 amps of current flowing to ground. Hence this setup ensures safe operation, protecting from the effects of noise (assuming they are small voltages). 

Most of the time, RETs operate as switches. 

Why Designers May Opt for RETs in EV Systems

While the RET works to ensure safe operation, a designer could also just design a discrete transistor with two discrete resistors and achieve the same result. 

The reason that RETs are more popular is that all of the components are integrated into one package, saving board space and cost. While saving board space is always an important consideration for designers, EV engineers may be especially concerned with this goal to make the system as lightweight as possible.

Output vs. input voltage for a RET. Image used courtesy of ON Semiconductor

A downside, however, is that 48 V RETs tend to fail when large voltage spikes occur. As mentioned before, the setup is meant to prevent failures due to small noise currents. When a large voltage fluctuation occurs, the input base current is roughly (Vfluctuation-0.7V)/R1 amps. So, when these fluctuations get too high, the transistor could experience a base current that is too high and burn out as a consequence. 

It is worth noting that a highly popular, and widely used, transistor alternative in EV systems is high-power MOSFETs such as SiC FETs or other 48 V FETs. 

A RET Fit for EVs

With a new design that is meant to handle 80 V standard operation, Nexperia has provided a way to use RETs in EV systems without worrying about failures due to voltage spikes. Nexperia claims that this is the industry's first 80 V RET, making the news more significant, since this may be one of the first instances that RETs have been made truly feasible for EV systems.