TVS Diode Protection: Application Note for Transient Voltage Suppression

TVS Diode Protection Overview

TVS diode protection is widely used in electronic circuits to protect power lines, data lines, analog inputs, switching nodes, and sensitive semiconductor devices against short-duration voltage transients.

High-amplitude noise, electromagnetic interference, electrostatic discharge, inductive load switching, and surge events can cause the voltage on a line to rise above the safe operating range of the connected circuit. If these voltage peaks are not limited, they may disturb normal operation or cause permanent damage to unprotected components.

This application note explains the basic operating principle of TVS diodes, common protection configurations, differences between unidirectional and bidirectional TVS diodes, and practical selection points for transient voltage suppression in electronic hardware design.

1. General Definition

High-amplitude signal noise, electromagnetic interference (EMI), electrostatic discharge (ESD), and similar effects can increase the peak voltage level on a transmission or power line. These voltage transients may affect the normal operation of unprotected circuit components and may eventually cause permanent damage.

Different types of diode-based protection methods are used to reduce this risk. One of the most common protection components used for this purpose is the TVS diode, also known as a Transient Voltage Suppression diode.

TVS diodes are designed to react very quickly to voltage transients and limit the voltage across the protected circuit to a safer level. Depending on the application, they can be used on DC power lines, AC lines, digital interfaces, analog signal paths, and communication lines.

Figure 1: Symbols of unidirectional and bidirectional TVS diodes. (A) Unidirectional TVS diode symbol. (B) Bidirectional TVS diode symbol.

2. Protection Options Against Voltage Transients

There are different diode-based circuit configurations that can be used to protect a system against voltage transients. Each configuration can suppress overvoltage events, but the protection behavior, clamping level, capacitance, polarity response, and application area may differ.

Figure 2: Representative protection configurations for protected systems and Line 1 / Line 2 connections. (A) Unidirectional TVS diode protection. (B) Bidirectional TVS and diode array protection. (C) Bidirectional TVS diode protection. (D) Unidirectional TVS and Schottky diode protection.

The correct protection structure should be selected according to the protected line type, signal voltage, normal operating range, expected transient energy, data rate, capacitance requirement, and grounding/reference conditions of the system.

3. TVS Diodes

TVS diodes are protection components used in applications that require immunity against voltage transients on power lines or data lines. They are also widely used for ESD protection at PCB level.

A TVS diode protects the circuit by clamping the transient voltage to a safer level. During a transient event, it behaves like a voltage-dependent low-impedance path and redirects the transient current away from the protected circuit.

Although the V-I characteristic of a TVS diode is similar to that of a Zener diode, their design purpose is different. Zener diodes are generally designed for voltage regulation, while TVS diodes are specifically designed to suppress short-duration transient voltage events.

In other words, TVS diodes are selected to protect components connected to a line from the harmful effects of voltage spikes, ESD events, surge pulses, and other transient disturbances.

Figure 3: Basic operating principle of a TVS diode during a transient event.

TVS diodes operate as parallel protection elements. Under normal operating conditions, the TVS diode presents a high impedance to the protected circuit. Since it is not an ideal open circuit, a small leakage current may still flow, but for most practical purposes it behaves like an open circuit during normal operation.

When the voltage on the protected line exceeds the intended protection threshold, the TVS diode enters conduction and provides a low-impedance path for the transient current. As a result, the transient current is diverted away from the protected system and toward the source, return path, or a path where the transient energy can be safely dissipated.

The voltage across the protected circuit is then limited by the clamping voltage of the TVS diode. After the transient event disappears, the TVS diode returns to its high-impedance state.

The transient power that a TVS diode can absorb depends on its package, silicon die size, PCB connection geometry, copper area, thermal path, and pulse duration. For this reason, both the TVS diode datasheet and the PCB layout should be evaluated together.

One of the most important advantages of TVS diodes is their fast response time. In theory, the transition to a low-impedance state can occur at extremely short time scales. Since such response times are difficult to measure directly in many practical conditions, TVS diodes are often described as having an almost instantaneous response.

Their fast response and relatively low clamping voltage make TVS diodes suitable for protecting sensitive components at PCB level.

4. Diode Arrays

Diode arrays are protection structures formed by combining switching diodes, avalanche diodes, Schottky diodes, or similar diode elements in a package or circuit configuration. They are commonly used for ESD and EMI protection on data lines because they can provide low capacitance and compact multi-line protection.

In many applications, diode arrays redirect transient current to the supply rail or reference rail rather than absorbing all transient energy directly at the signal line. Their effective minimum clamping behavior may be limited by the forward voltage of the internal diodes and the voltage level of the rail to which the current is redirected.

Figure 4: Diode array protection example showing transient current redirected toward the supply/reference rails. 

For low-voltage signal applications, Schottky diodes may also be used because of their low forward voltage. This can help reduce the voltage level at which the protection path starts conducting.

However, when diode arrays redirect current into a supply rail, the supply rail must be able to absorb or dissipate the transient energy safely. Otherwise, the supply voltage may rise and create additional stress on other parts of the circuit.

5. Differences Between Unidirectional and Bidirectional TVS Diodes

TVS diodes are available as unidirectional and bidirectional devices. Diode arrays are usually used for unidirectional rail-clamping protection, although more complex configurations can be created depending on the application.

Both unidirectional and bidirectional TVS diodes can protect against positive and negative transient events. However, their clamping behavior is different.

Figure 5: V-I characteristics of unidirectional and bidirectional TVS diodes.

In a unidirectional TVS diode, the clamping behavior is different for positive and negative polarity conditions. In one direction, the device behaves similarly to a forward-biased diode. In the opposite direction, it operates in breakdown mode.

In a bidirectional TVS diode, the device provides similar breakdown behavior in both polarities. This makes bidirectional TVS diodes suitable for AC lines, bipolar signal lines, and interfaces where the signal can swing above and below the reference level.

In many applications, both unidirectional and bidirectional TVS diodes may appear usable at first glance. However, one type may provide a clear advantage depending on the circuit.

For example, in DC power supply protection or logic-level circuit protection, a unidirectional TVS diode is often preferred. This allows the circuit to benefit from a lower clamping level during negative transients.

Bidirectional TVS diodes are generally preferred in AC power lines, differential interfaces, long-distance data lines, and systems where the ground reference between transmitter and receiver may shift. They can also help in applications affected by common-mode offset voltage.

In some cases, bidirectional TVS diodes are selected as replacements or alternatives for metal oxide varistors (MOVs) in circuit schematics.

As a practical summary:

• Unidirectional TVS diodes are commonly used on DC power lines, short-distance data lines, and logic-level protection circuits where the reference potential is well defined.

• Bidirectional TVS diodes are commonly used on AC lines, bipolar signal lines, long-distance data lines, and systems where reference potential differences may occur.

6. Selecting TVS Diodes

Selecting the correct TVS diode is not only a matter of choosing a voltage rating. The selected device must remain inactive during normal operation, respond quickly during a transient event, clamp the voltage below the damage threshold of the protected circuit, and survive the expected transient energy.

The following points should be considered when selecting a TVS diode:

6.1. Working voltage and breakdown voltage

The TVS diode should not conduct during normal operation. Therefore, the working stand-off voltage should be higher than the maximum normal operating voltage of the protected line.

The breakdown voltage should be selected so that the TVS diode begins to conduct during an abnormal overvoltage event, but not during normal operation.

6.2. Clamping voltage

The clamping voltage must be low enough to protect the connected circuit. It should be compared with the absolute maximum ratings of the protected components.

If the clamping voltage is higher than the damage threshold of the protected IC or circuit block, the TVS diode may conduct but still fail to provide sufficient protection.

6.3. Peak pulse power dissipation

The TVS diode must be able to absorb or redirect the expected transient energy. For this reason, the peak pulse power dissipation rating should be checked carefully.

It should also be remembered that this rating may decrease with temperature. Thermal derating curves in the datasheet should be reviewed for real operating conditions.

6.4. Capacitance

In high-speed data lines, RF lines, communication interfaces, and precision analog circuits, the capacitance of the TVS diode is an important parameter.

A high-capacitance TVS diode may distort fast signals, increase rise/fall times, reduce bandwidth, or affect signal integrity. For similar signal lines, protection devices with similar capacitance values should be selected to preserve channel-to-channel matching.

6.5. Common-mode voltage and reference differences

In systems where transmitter and receiver grounds are separated by distance or connected through different ground references, common-mode voltage can become important.

For such applications, bidirectional TVS devices or protection structures suitable for common-mode voltage differences may be required.

6.6. Leakage current

Leakage current should be considered especially in high-impedance sensor inputs, low-power circuits, battery-powered systems, and precision analog applications.

Even if the leakage current is small, it may create measurement error or increase standby current in sensitive designs.

6.7. PCB layout

The protection device must be placed close to the entry point of the transient source. Long traces between the connector and the TVS diode increase parasitic inductance and reduce protection effectiveness.

A good TVS diode layout should provide:

• a short path from the protected line to the TVS diode,

• a short and low-impedance return path,

• sufficient copper area for current and heat distribution,

• minimum loop area in the transient current path.

7. Diode Configurations and Clamping Voltages

Different TVS diode and diode-array configurations create different clamping voltage levels for positive and negative transients. These differences should be considered when selecting a protection method.

Table 1: Diode configurations and corresponding clamping voltage behavior.

This table compares unidirectional TVS diodes, bidirectional TVS diodes, low-capacitance diode arrays, Schottky diode structures, and combined TVS protection configurations.

In practice, the best configuration depends on the line voltage, signal direction, required capacitance, transient polarity, rail behavior, and acceptable clamping voltage of the protected system.

8. Sample Applications Using TVS Diodes

TVS diodes can be used in many different protection circuits. They may be used alone or together with MOVs, chokes, Schottky diodes, diode arrays, rectifiers, and input impedance elements.

Table 2: Sample applications using TVS diodes.

Typical application examples include:

• MOV and choke-assisted TVS protection,

• DC load protection,

• AC power supply protection,

• protection of DC circuits against electromagnetic interference,

• op-amp output protection,

• inverter, MOSFET, and IGBT module protection in variable-frequency drive applications.

In power applications, TVS diodes are often used as part of a wider protection strategy. They may work together with MOVs, common-mode chokes, fuses, current-limiting elements, snubber networks, and proper grounding or shielding structures.

In signal-level applications, TVS diodes are typically selected for fast response, low capacitance, low leakage current, and suitable clamping voltage.

9. Conclusion

TVS diodes are effective protection components for limiting short-duration voltage transients in electronic circuits. They can protect sensitive components against ESD, EMI-related disturbances, surge events, inductive switching transients, and other overvoltage conditions.

A TVS diode should be selected according to the normal operating voltage, expected transient energy, clamping voltage, capacitance, leakage current, polarity, and PCB layout constraints.

Unidirectional TVS diodes are generally suitable for DC lines and logic-level protection. Bidirectional TVS diodes are more suitable for AC lines, bipolar signals, and systems where reference potential differences may occur. Diode arrays and Schottky-based configurations can be useful where low capacitance and fast signal protection are required.

A correctly selected and properly placed TVS diode can significantly improve circuit robustness, protect sensitive components, and reduce the risk of damage caused by transient voltage events.

10. References

[1] AP-209 – Design Considerations for ESD Protection Using ESD Protection Diode Arrays, California Micro Devices, 1998.

[2] SI99-01 – PCB Design Guidelines for ESD Suppression, Semtech, 2002.

[3] AND8231/D – Circuit Configuration Options for TVS Diodes, Rev. 1, ON Semiconductor, 2017.

[4] SI9601 – TVS Diode Application Note, Rev. 9, Semtech, 2000.

[5] Transient Voltage Suppressors (TVS Diode) Applications Overview, Rev. 1, Littelfuse, 2015.