Bench Talk

PTC Thermistors for Automotive Inrush Current and Overload Protection

(Source: memorystockphoto/stock.adobe.com)

For many engineers, variations in critical parameters such as transistor beta or device resistance due to temperature changes are problems that need to be accommodated, compensated, or canceled. However, savvy engineers also “turn the tables” by using these changes to create functional devices. These include temperature sensors such as resistance temperature detectors (RTDs) and thermistors, using their defined resistance changes due to temperature shifts.

Engineers have employed this temperature-dependent relationship of thermistors beyond using resistance changes to measure temperature. How so? The thermistor's increase in resistance—whether due to a rise in ambient temperature or self-heating caused by current flow—allows the thermistor to be used as a reliable, passive component to inherently limit the current. Doing so can minimize system problems such as circuit and component stress and even prevent associated failures.

One question remains: “Why not just use a standard, thermally activated fuse to cut off the current when it rises too much?” After all, fuses are reliable, well-understood, and widely available. One reason to look beyond the fuse is that a fuse is an all-or-nothing device. It either allows full current flow without limiting the peak inrush current or cuts flow off entirely. Further, it must be replaced once it triggers and breaks the current path, which is often impractical or costly.

A better approach is to use a thermistor as a current-flow limiter. There are two types of thermistors, with the most common being the negative temperature coefficient (NTC) thermistor. The NTC thermistor resistance decreases as the temperature increases and vice versa. It is frequently used for temperature sensing and measurement applications due to its reliable and sensitive response to temperature changes.

Its counterpart is the positive temperature coefficient (PTC) thermistor, in which the resistance increases as the temperature rises, and as such, there's an inflection point at which the resistance increases sharply. It's typically used as a fuse-like component but with a higher resistance value and without the fuse’s abrupt and irreversible on/off action. The PTC thermistor’s type of material and fabrication determines how much the current will be limited while the resistance shifts with temperature changes.

PTC thermistors limit excessive current by increasing resistance as temperature rises, thereby reducing the risk of overheating and protecting the circuit from overcurrent conditions. They do not have to be replaced after elimination of the fault but instead resume their protective function immediately after a short cooling-down period.

Moreover, PTC thermistors respond to excessive currents and a temperature rise above a preset limit. These high-energy components can protect automotive functions such as onboard chargers (OBCs) and DC links, which are critical for ensuring stable and efficient power transfer against overcurrent, overtemperature, and electrical spikes.

Automotive Applications

The high-energy operating environment of all cars—whether EV, hybrid, or internal combustion engine (ICE)—requires special attention to current and associated thermal overload situations. For example, the EV's onboard charger function is subject to multiple challenges, which may be due to internal component failure, line spikes, internal and external transients, and even damage to connecting cables from user abuse (Figure 1).

Figure 1: The onboard charger (OBC) in EVs and hybrid vehicles has many potential points of stress and even outright failure. PTC thermistors can provide simple and reliable protection against excessive current flow and associated heating. (Source: blueringmedia /stock.adobe.com)

As a result of these high-energy environments, excessive current may flow through parts of the OBC system and cause further damage or even incite a vehicle or user safety situation. Fortunately, an appropriate PTC thermistor can automatically restrict any excessive current flow along critical paths.

The DC link is also critical in automotive power electronics systems, as it stabilizes the voltage between the rectification and inversion stages. In scenarios where there is excessive current due to motor stalling or restricted movement, the DC link capacitors can experience electrical stress. PTC thermistors can protect the DC link from overcurrent and voltage surges, helping maintain system reliability.

High-energy PTCEL Inrush Current Limiting PTC Thermistors from Vishay Intertechnology, Inc. provide safe, repetitive inrush current limitation and protection in various high-power applications that require a controlled capacitor charge or discharge function. Their extended PTC inrush current limits, resistance, and lead spacings offer increased energy handling in high-voltage applications.

This family of thermistors is AEC-Q200 qualified for automotive applications and UL-recognized, and significantly reduces board space requirements (Figure 2). They also trim component count because they absorb higher energy levels of up to 340 joules for a single PTCEL67 and operate at high ambient temperatures of up to +105°C.

Figure 2: Members of the Vishay PTCEL family of inrush current limiting PTC thermistors are small and easily placed on the printed circuit board where needed. (Source: Vishay)

The built-in, self-regulating safety mechanism prevents the PTCEL thermistor from overheating in any overload situation. PTCEL thermistors are suitable for controlled charging and discharging of high-energy capacitors with voltage levels up to 1200 V_DC. Equally important, they can support highly repetitive overcurrent events and are rated for minimum 100,000 charge or discharge cycles.

The self-protecting characteristics of ceramic PTC technology prevent any overheating or overloading of the protected component and limit the current in the circuit to a safe, low level (Figures 3 and 4). Charge and discharge energy can be applied to the PTC quickly, ranging from 10 milliseconds to a few seconds. The absorbed energy is dissipated within a few minutes so the PTC can cool to ambient temperature and be ready for another inrush current limiting operation.

Figure 3: This graph illustrates the energy capability of three different Vishay PTCEL thermistors (PTCEL13, PTCEL17, and PTCEL67) as a function of ambient temperature for pulse times greater than or equal to 10ms. (Source: Vishay)

Figure 4: This graph shows a typical capacitor discharge process through a network of PTC thermistors, with the key aspects being the initial peak current limitation followed by the increasing resistance as the thermistor’s voltage reduces during heat up, leading to a controlled and safe dissipation of the stored energy. (Source: Vishay)

Ametherm was acquired to complement Vishay’s Inrush Current Limiter product line.

Conclusion

Although thermistors may not have the apparent glamor of some passive and many active components, they are an excellent choice for dealing with inrush and overload currents, especially in stressful applications such as automobiles. They are offered in many electrical sizes to match application specifics, are easy to use, and wait quietly and almost invisibly, ready to do their job and spring into action when needed.

Author

Bill Schweber is a contributing writer for Mouser Electronics and an electronics engineer who has written three textbooks on electronic communications systems, as well as hundreds of technical articles, opinion columns, and product features. In past roles, he worked as a technical web-site manager for multiple topic-specific sites for EE Times, as well as both the Executive Editor and Analog Editor at EDN.