By Isaac Sibson, Automotive Applications Engineer, Diodes Incorporated
In automotive applications, it is essential to protect sensitive semiconductor components against power surges, transients and ESD events.
A single transient voltage spike could easily damage a component or disrupt its operation, while even moderate electrical noise energy can interrupt digital communications.
Transients can occur as momentary or continuous surges wherever high voltages are present, and can propagate through PCB tracks and cables. Momentary surges can easily reach a voltage of 3kV, and can often occur when inductive loads are switched, for example when a motor stops. As most modern ICs operate from low DC voltages, transients are a common threat both to these components and to the digital signals that they handle.
To stop these transients, a common solution is the Transient Voltage Suppressor (TVS). This article explains the characteristics of TVS devices, and discusses the way they may be used in automotive applications.
The TVS is a solid-state device which provides a low-impedance path to earth for a transient voltage surge, but a high-impedance path at all other times. This allows supply voltages and signals to operate as intended, with no current routed via the TVS, but almost instantly clamps any high voltages safely, protecting sensitive components and routing the excess surge to earth.
A TVS is effectively a PN junction diode designed to enter avalanche mode, in which it can pass very high currents, when the potential on its cathode exceeds a preset level. This level will depend on the application, but the important characteristic is that the junction should break down quickly - in less than 1ns - so that the transient surge is handled rapidly enough to avoid damage. The TVS would normally be placed either in parallel with the load across the 0V and supply rail, between ground and a single-ended signal, or across a differential pair of signals.
A TVS resembles a Zener diode, but is designed to operate slightly differently. Instead of handling a steady voltage for a sustained period of time, a TVS is designed to break down quickly and absorb a high amount of energy for a short time.
TVS in Automotive Applications
In most cars, the main voltage rail is at 12V DC, supplied by the rectified and regulated output of an alternator, as well as from the battery. The alternator is inductive, and therefore a potential source of transient voltages.
In addition, car manufacturers are adding more automatic features to their products. This means that new car designs include more electrical motors, for powering functions such as mirrors, windows and seats. The electrification of the drivetrain also means that mechanical systems such as water and oil pumps are driven by electric motors. These motors impose an inductive load on the system. They are therefore potential sources of transient voltages when loads are connected and disconnected.
Industry standards apply to the implementation of surge protection in the automotive environment, including ISO 7637-2 and ISO 16750. Figure 1 shows the shape of the test pulses on the 12V power rail specified by these two standards. Separately, national standards are also applied in some countries.
Fig. 1: TVS test pulses used in the automotive industry
The circuit protection system must be able to handle each of the pulses shown in Figure 1. This may call for several TVS devices to be distributed around the circuit. For example, there is likely to be a large main load-dump TVS located close to the alternator, as shown in Figure 2. The remaining energy would need to be dissipated by supplementary TVS protection devices at each module, and a system-level approach should be used to define the requirements for this.
Fig. 2: A typical automotive application using large devices around alternator and regulator; TVS and reverse-polarity protection around modules; and protection around data buses
In harsh environments there will be a markedly larger area under the curve of the pulse, corresponding to more energy to be dissipated, compared to other use cases.
This holds true for automotive applications. In fact, large vehicles such as trucks and vans might operate from a 24V supply rail, which means that the protection must be able to deal with much larger amounts of energy.
Protecting Safety-Critical Signals with TVS Devices
As modern vehicles include more and more data communications, the use of automotive-qualified TVS devices for CAN, FlexRay™ and LIN networks is becoming more common.
These networks are used to carry safety-critical communications between automotive modules. Their signals might be interrupted by noise at a lower energy than transient surges. The signals and modules must therefore be protected while avoiding any impact on signal bandwidth. As CAN and FlexRay are differential buses, they require a dual bidirectional TVS to protect both lines.
The simplest type of TVS is unidirectional, and is used when the area of the circuit to be protected is always positive, for example 0V to 5V. The unidirectional TVS is still able to protect against both positive and negative transients.
In contrast, a bidirectional TVS provides protection from transients across a split-rail system or differential signaling scheme, to protect against transients that are both positive and negative with respect to the signal. These devices can be either symmetrical, when the breakdown voltage is the same in both directions, or asymmetrical, when the reverse breakdown voltage is higher in one direction than in the other. A LIN bus, for example, would employ an asymmetrical bidirectional TVS because the signal line can easily fluctuate between -15V and 24V due to ground variance.
A vehicle might also include high-speed data buses such as USB and HDMI, which require low-capacitance TVS protection to preserve data integrity.
The Electrical Characteristics of a TVS Device
A TVS is specified based on the nominal working voltage. To protect a microcontroller operating at 3.3V, for example, a TVS of 3.3V could be used.
The actual parameter specified in this case is the reverse working voltage, VRWM, defined as the voltage at which the device is guaranteed not to pass more than a minimal specified leakage current, IR. This means that the reverse working voltage is the voltage at which the TVS has a negligible influence on the rest of the circuit.
The related breakdown voltage parameter, VBR, is the point at which the device will conduct at least a specified minimum current IT. Figure 3 shows that this is measured at the value of IT at which the TVS starts to conduct strongly.
Fig. 3: A TVS operates over a shorter period than a Zener diode
As shown by the dotted lines in Figure 3, the breakdown voltage can vary over a wide range, so it is important to consider this variation with respect to the circuit being protected. The device’s peak pulse power limit, PPK, can be found on the transfer characteristic of a TVS, as shown in Figure 4. The point at which the transfer curve intersects with the peak pulse power curve denotes the maximum clamping voltage, VC, and the maximum peak current, IPP, for the device.
Fig. 4: The transfer characteristics of a typical TVS diode
The length of the pulse determines the amount of energy that can be absorbed. Standard duration and magnitude pulses are defined in international standards. These include IEC 61000, which covers fast surges such as those caused by by electro-static discharges and lightning strikes, and defines two relevant points on the power curve: 8µs (t1) and 20µs (t2) which are commonly referred to as 8/20. Another relevant standard is IEC 6164, which deals with slower surges with higher energy levels, such as those that would occur from inductive loads. The time intervals for this standard are 10/1000, which equals 10µs (t1) and 1,000µs (t2). These figures are used to define TVS performance, as shown in Figure 5. Note that t2 is total elapsed time from 0.
Transient voltages are an ever-present threat to sensitive ICs, and in automotive applications they are unavoidable. The TVS is the right component, in many cases, to provide protection against surges, spikes and transients.
Diodes Incorporated offers a wide range of TVS protection products, enabling every part of a vehicle’s electronic systems to be protected from hazardous transients, and providing for higher reliability and safety.
Fig. 5: The top image shows how the points t1 and t2 are used to define the energy in the spike, while the bottom image shows the shape of the surges defined by the IEC standards. The shaded areas represent the energy of the pulses.