Have you ever wondered how your electronic devices can survive sudden surges of voltage or current that could potentially damage or destroy them? How do they cope with the effects of lightning strikes, electrostatic discharge (ESD), inductive switching, and other sources of voltage transients?
The answer lies in a small but powerful component called a transient voltage suppressor (TVS) diode.
A TVS diode is a protection device that is designed to protect electronic circuits from voltage spikes induced on connected wires. It works by shunting excess current when the induced voltage exceeds a certain threshold, known as the breakdown voltage. It is a clamping device, suppressing all overvoltages above its breakdown voltage. It automatically resets when the overvoltage goes away, but absorbs much more of the transient energy internally than a similarly rated crowbar device.
TVS diodes are essential for ensuring the reliability and safety of electronic systems, especially those that are exposed to harsh environments or sensitive to voltage fluctuations. They are widely used in applications such as power supplies, data lines, serial communication, Ethernet, and many others.
In this article, you will learn everything you need to know about TVS diodes, including:
- What are the sources and effects of voltage transients, and why do they pose a threat to electronic circuits?
- What are the different methods of transient suppression, and how do they compare with TVS diodes in terms of performance and suitability?
- What are the main types and characteristics of TVS diodes, and how do they operate under normal and transient conditions?
- How do you select and use TVS diodes for different applications and scenarios, and what factors do you need to consider?
- How do you design and test TVS diode circuits for common devices and interfaces, and what tools and resources do you need?
By the end of this article, you will have a comprehensive understanding of TVS diodes and how to use them effectively for transient voltage suppression. You will also be able to apply your knowledge to your own projects and experiments involving electronic circuits.
So let’s get started!
Sources and Effects of Voltage Transients
Before we dive into the details of TVS diodes, let’s first understand what voltage transients are, where they come from, and how they can affect electronic circuits.
Voltage transients are temporary spikes or surges in voltage or current that can occur in any electrical circuit. They are usually caused by a sudden change in the electrical conditions of the circuit, such as switching on or off a load, opening or closing a switch, connecting or disconnecting a cable, etc. They can also be induced by external sources, such as lightning strikes, electrostatic discharge (ESD), electromagnetic interference (EMI), etc.
Voltage transients can be classified into two main types: impulsive and oscillatory.
- Impulsive transients are sudden, unipolar (either positive or negative) voltage spikes that last for a very short duration (less than 1 ms). They have a fast rise time and a slow decay time. They are typically caused by lightning, ESD, arcing, etc.
- Oscillatory transients are alternating, bipolar (both positive and negative) voltage surges that last for a longer duration (up to several ms). They have a slower rise time and a faster decay time. They are typically caused by inductive or capacitive switching, transformer energization, etc.
The magnitude of voltage transients can vary from a few millivolts to thousands of volts, depending on the source and the circuit parameters. The frequency of voltage transients can range from a few hertz to several megahertz, depending on the rise time and the decay time.
Voltage transients can have serious consequences for electronic circuits and devices. Some of the effects of voltage transients are:
- Damage or destruction of components: Voltage transients can exceed the breakdown voltage or the maximum rating of components such as diodes, transistors, ICs, etc., causing them to fail permanently or partially.
- Degradation of performance: Voltage transients can cause unwanted noise, distortion, interference, or errors in signals and data transmission, affecting the functionality and accuracy of electronic systems.
- Malfunction or reset of devices: Voltage transients can trigger false or unwanted operations in logic circuits, controllers, sensors, etc., causing them to malfunction or reset unexpectedly.
- Safety hazards: Voltage transients can create sparks or arcs that can ignite flammable materials or cause electric shocks to humans or animals.
Therefore, it is essential to protect electronic circuits from voltage transients using appropriate devices and techniques. One of the most common and effective devices for transient suppression is the TVS diode.
Methods of Transient Suppression
Now that we have seen what voltage transients are and how they can harm electronic circuits, let’s look at some of the methods that can be used to suppress or divert them. There are many devices and techniques that can be used for transient suppression, but they can be broadly classified into two categories: attenuation and diversion.
- Attenuation is the process of reducing the magnitude or energy of a transient by inserting a filter or a resistor in series with the circuit. The filter or resistor acts as a low-pass filter that blocks or attenuates the high-frequency transient while allowing the low-frequency signal or power to pass through. Attenuation is useful for suppressing low-energy transients that do not exceed the breakdown voltage of the circuit components.
- Diversion is the process of redirecting the transient away from the sensitive circuit by placing a clamping or a crowbar device in parallel with the circuit. The clamping device limits or clamps the transient voltage to a safe level by shunting the excess current through itself. The crowbar device short-circuits or grounds the transient voltage by triggering a switch or a thyristor. Diversion is useful for suppressing high-energy transients that can damage or destroy the circuit components.
Some of the common devices and techniques used for transient suppression are:
- Bypass capacitor: A capacitor placed in parallel with the circuit that acts as a low-pass filter and shunts high-frequency transients to the ground.
- Resistor: A resistor placed in series or parallel with the circuit that acts as a low-pass filter and reduces the energy of transients.
- Inductor: An inductor placed in series with the circuit that acts as a low-pass filter and blocks high-frequency transients.
- RC snubber: A resistor-capacitor network placed in parallel with a switch or a relay that acts as a low-pass filter and reduces the voltage spikes caused by inductive switching.
- Gas discharge tube (GDT): A sealed glass tube filled with an inert gas that acts as a crowbar device and conducts when the transient voltage exceeds its breakdown voltage.
- Metal oxide varistor (MOV): A ceramic disc made of metal oxide grains that acts as a clamping device and changes its resistance depending on the applied voltage.
- Transient voltage suppressor (TVS) diode: A semiconductor diode that acts as a clamping device and conducts when the transient voltage exceeds its breakdown voltage.
- Thyristor TVS diode: A semiconductor diode that acts as a crowbar device and triggers when the transient voltage exceeds its breakdown voltage.
Each of these devices and techniques has its own advantages and disadvantages in terms of protection time, voltage level, power dissipation, reliability, and performance.
Types and Characteristics of TVS Diodes
Among the various methods of transient suppression, TVS diodes are one of the most popular and effective devices for clamping voltage transients. TVS diodes are semiconductor diodes that operate by the avalanche breakdown principle. They have a nonlinear voltage-current characteristic that allows them to switch from a high-impedance state to a low-impedance state when the transient voltage exceeds their breakdown voltage. By doing so, they shunt the excess current through themselves and limit the residual voltage across the circuit to a safe level.
TVS diodes are similar to Zener diodes, but they are designed and tested to handle much higher peak currents and power dissipation. They also have lower clamping voltage, higher operating voltage, lower capacitance, lower leakage current, and faster response time than Zener diodes. TVS diodes are suitable for protecting circuits from both positive and negative transients.
TVS diodes can be either unidirectional or bidirectional, depending on the polarity of the transient they can handle.
- Unidirectional TVS diodes are designed to protect circuits from positive transients only. They act as normal rectifier diodes in the forward direction and as avalanche diodes in the reverse direction. They have a single breakdown voltage rating and a single clamping voltage rating. They are typically used for DC circuits or AC circuits with a known polarity.
- Bidirectional TVS diodes are designed to protect circuits from both positive and negative transients. They consist of two avalanche diodes connected in series with opposite polarities. They have two breakdown voltage ratings and two clamping voltage ratings, one for each direction. They are typically used for AC circuits or DC circuits with an unknown or changing polarity.
The schematic symbols for unidirectional and bidirectional TVS diodes are shown below:
TVS diodes are characterized by several parameters that determine their performance and suitability for different applications. Some of the key parameters are:
- Leakage current: The amount of current that flows through the TVS diode when the applied voltage is below the maximum reverse standoff voltage. It indicates the power dissipation and the noise level of the TVS diode under normal conditions. It should be as low as possible.
- Maximum reverse standoff voltage: The maximum DC or peak AC voltage that can be applied to the TVS diode without causing significant conduction. It indicates the operating range of the TVS diode under normal conditions. It should be higher than the normal circuit voltage.
- Breakdown voltage: The minimum reverse voltage that causes significant conduction through the TVS diode. It indicates the threshold of the avalanche breakdown phenomenon. It should be slightly higher than the maximum reverse standoff voltage.
- Clamping voltage: The maximum reverse voltage that appears across the TVS diode when a specified peak pulse current flows through it. It indicates the level of protection provided by the TVS diode under transient conditions. It should be as low as possible but higher than the breakdown voltage.
- Peak pulse current: The maximum peak current that can flow through the TVS diode for a specified pulse duration and waveform without causing damage or degradation. It indicates the surge handling capability of the TVS diode under transient conditions. It should be as high as possible.
In addition to these parameters, other factors such as capacitance, response time, temperature coefficient, junction area, package type, etc., may also affect the performance and selection of TVS diodes.
In summary, TVS diodes are versatile and reliable devices for transient suppression that offer fast response time, low clamping voltage, high operating voltage, low capacitance, low leakage current, and high peak pulse current ratings.
How to Select and Use TVS Diodes
After learning about the types and characteristics of TVS diodes, let’s see how to select and use them for different applications and scenarios. The selection and usage of TVS diodes depend on several factors, such as the normal operating voltage, the expected transient voltage, the peak pulse current, the clamping voltage, the capacitance, the package type, etc. Here are some general steps and guidelines for selecting and using TVS diodes:
- Select a TVS diode with a maximum reverse standoff voltage that is higher than the normal operating voltage of the circuit. This ensures that the TVS diode does not conduct under normal conditions and does not affect the circuit performance.
- Select a TVS diode with a breakdown voltage that is slightly higher than the maximum reverse standoff voltage. This ensures that the TVS diode has a margin of safety and does not conduct at small voltage fluctuations.
- Select a TVS diode with a clamping voltage that is lower than the maximum allowable voltage of the circuit components. This ensures that the TVS diode provides adequate protection and prevents damage or degradation of the circuit components.
- Select a TVS diode with a peak pulse current that is higher than the expected peak current of the transient. This ensures that the TVS diode can handle the surge current without overheating or failing.
- Select a TVS diode with a capacitance that is suitable for the application. For high-speed or low-noise applications, choose a low-capacitance TVS diode to avoid signal distortion or attenuation. For low-speed or high-noise applications, choose a high-capacitance TVS diode to provide additional filtering or damping.
- Select a TVS diode with a package type that is compatible with the circuit layout and design. For surface-mount applications, choose a small and flat package type to save space and reduce parasitics. For through-hole applications, choose a large and robust package type to withstand mechanical stress and thermal cycling.
- Place the TVS diode as close as possible to the source of the transient or to the input or output terminals of the circuit. This minimizes the lead inductance and resistance and improves the response time and clamping performance of the TVS diode.
- Connect the TVS diode in parallel with the circuit to be protected. For unidirectional TVS diodes, connect the cathode to the positive terminal and the anode to the negative terminal or ground. For bidirectional TVS diodes, connect either terminal to either terminal of the circuit.
The following figure shows an example of how to connect a unidirectional TVS diode to protect a DC circuit from positive transients:
In summary, selecting and using TVS diodes requires careful consideration of various factors and trade-offs. In general, it is advisable to choose a TVS diode that has a high standoff voltage, a low clamping voltage, a high peak pulse current, a low capacitance, and a suitable package type for the application. In addition, it is important to place and connect the TVS diode properly to ensure optimal protection performance.
Examples of TVS Diode Circuits
To illustrate how to design and test TVS diode circuits for common devices and interfaces, let’s look at some examples. We will use the following TVS diode models from Littelfuse as references:
- SMAJ5.0A: A unidirectional TVS diode with a maximum reverse standoff voltage of 5 V, a breakdown voltage of 6.4 V, a clamping voltage of 9.2 V, a peak pulse current of 38.7 A, and a capacitance of 100 pF.
- SMBJ5.0CA: A bidirectional TVS diode with a maximum reverse standoff voltage of 5 V, a breakdown voltage of 6.4 V, a clamping voltage of 9.2 V, a peak pulse current of 38.7 A, and a capacitance of 100 pF.
- SMAJ12A: A unidirectional TVS diode with a maximum reverse standoff voltage of 12 V, a breakdown voltage of 13.3 V, a clamping voltage of 19.9 V, a peak pulse current of 22.6 A, and a capacitance of 35 pF.
- SMBJ12CA: A bidirectional TVS diode with a maximum reverse standoff voltage of 12 V, a breakdown voltage of 13.3 V, a clamping voltage of 19.9 V, a peak pulse current of 22.6 A, and a capacitance of 35 pF.
Example 1: Power Supply Protection
In this example, we want to protect a power supply circuit that has an input voltage range of 4.75 V to 5.25 V and an output current rating of 1 A from positive transients up to 100 V and negative transients up to -100 V.
To select a suitable TVS diode for this application, we can follow these steps:
- Choose a bidirectional TVS diode to protect the circuit from both positive and negative transients.
- Choose a TVS diode with a maximum reverse standoff voltage that is higher than the normal operating voltage of the circuit (5 V). In this case, we can choose the SMBJ5.0CA model.
- Verify that the clamping voltage of the TVS diode is lower than the maximum allowable voltage of the circuit components. In this case, we can assume that the circuit components can tolerate up to 10% overvoltage (5.5 V). The clamping voltage of the SMBJ5.0CA model is 9.2 V, which is higher than the limit. Therefore, we need to add a series resistor to limit the current and reduce the clamping voltage.
- Calculate the value of the series resistor using Ohm’s law: R = (Vc – Vmax) / Imax, where Vc is the clamping voltage, Vmax is the maximum allowable voltage, and Imax is the maximum output current. In this case, R = (9.2 – 5.5) / 1 = 3.7 ohms.
- Verify that the peak pulse current rating of the TVS diode exceeds the expected peak current during transient conditions. In this case, we can assume that the transient source has an internal resistance of 10 ohms and can deliver up to 100 V in either direction. The peak current through the TVS diode can be calculated by Kirchhoff’s voltage law: Ip = (Vs – Vc) / (R + Rs), where Vs is the transient voltage, Vc is the clamping voltage, R is the series resistor, and Rs is the source resistance. In this case, Ip = (100 – 9.2) / (3.7 + 10) = 6.8 A for positive transients and Ip = (-100 – (-9.2)) / (3.7 + 10) = -8 A for negative transients. The peak pulse current rating of the SMBJ5.0CA model is 38.7 A, which is higher than both values.
Example 2: Data Line Protection
In this example, we want to protect a data line that has an operating voltage range of -0.3 V to +3.6 V and an impedance of 50 ohms from ESD events up to ±15 kV.
To select a suitable TVS diode for this application, we can follow these steps:
- Choose a bidirectional TVS diode to protect the data line from both positive and negative ESD events.
- Choose a TVS diode with a maximum reverse standoff voltage that is higher than the normal operating voltage range of the data line (3.6 V). In this case, we can choose the SMBJ12CA model.
- Verify that the clamping voltage of the TVS diode is lower than the maximum voltage.
Example 3: Serial Communication Protection
In this example, we want to protect a serial communication line that has an operating voltage range of -12 V to +12 V and a data rate of 115.2 kbps from ESD events up to ±15 kV.
To select a suitable TVS diode for this application, we can follow these steps:
- Choose a bidirectional TVS diode to protect the serial communication line from both positive and negative ESD events.
- Choose a TVS diode with a maximum reverse standoff voltage that is higher than the normal operating voltage range of the serial communication line (12 V). In this case, we can choose the SMBJ12CA model.
- Verify that the clamping voltage of the TVS diode is lower than the maximum allowable voltage of the serial communication line. In this case, we can assume that the serial communication line can tolerate up to 20% overvoltage (14.4 V). The clamping voltage of the SMBJ12CA model is 19.9 V, which is higher than the limit. Therefore, we need to add a series resistor to limit the current and reduce the clamping voltage.
- Calculate the value of the series resistor using Ohm’s law: R = (Vc – Vmax) / Imax, where Vc is the clamping voltage, Vmax is the maximum allowable voltage, and Imax is the maximum output current. In this case, R = (19.9 – 14.4) / 0.1 = 55 ohms, where we assume that the maximum output current is 0.1 A.
- Verify that the peak pulse current rating of the TVS diode exceeds the expected peak current during transient conditions. In this case, we can assume that the transient source has an internal resistance of 330 ohms and can deliver up to 15 kV in either direction. The peak current through the TVS diode can be calculated by Kirchhoff’s voltage law: Ip = (Vs – Vc) / (R + Rs), where Vs is the transient voltage, Vc is the clamping voltage, R is the series resistor, and Rs is the source resistance. In this case, Ip = (15000 – 19.9) / (55 + 330) = 43 A for positive transients and Ip = (-15000 – (-19.9)) / (55 + 330) = -43 A for negative transients. The peak pulse current rating of the SMBJ12CA model is 22.6 A, which is lower than both values.
- Choose a different TVS diode model with a higher peak pulse current rating or use multiple TVS diodes in parallel to increase the peak pulse current rating. In this case, we can choose two SMBJ12CA models in parallel, which will give us a peak pulse current rating of 45.2 A.
Conclusion
In this article, we have learned about TVS diodes and how they can be used for transient voltage suppression in various applications. We have seen:
- What are voltage transients and how they can damage or degrade electronic circuits.
- What are TVS diodes and how they compare with other methods of transient suppression in terms of performance and suitability.
- What are the types and characteristics of TVS diodes and how they operate under normal and transient conditions.
- How to select and use TVS diodes for different applications and scenarios and what factors to consider.
- How to design and test TVS diode circuits for common devices and interfaces and what tools and resources to use.
We hope that this article has given you a comprehensive understanding of TVS diodes and how to use them effectively for transient voltage suppression. You can apply your knowledge to your own projects and experiments involving electronic circuits.
If you want to learn more about TVS diodes or other topics related to circuit protection, you can check out these resources:
- Littelfuse: A leading manufacturer of TVS diodes and other circuit protection devices.
- All About Circuits: An online community and resource for electrical engineers and hobbyists.
Thank you for reading this article!
FAQs
A: TVS diodes are electronic devices used to protect sensitive electronics from voltage spikes and transient events.
A: TVS diodes divert excess voltage away from the protected circuitry, preventing it from reaching and damaging sensitive components.
A: TVS diodes can protect against voltage surges caused by lightning strikes, electrostatic discharge (ESD), and switching transients.
A: TVS diodes find applications in various electronic devices and systems, including power supplies, telecommunications equipment, automotive electronics, industrial control systems, and consumer electronics.
A: TVS diodes are typically made from semiconductor materials such as silicon or zinc oxide.
A: TVS diodes have a fast response time, allowing them to quickly absorb and dissipate the energy from transient events.
A: TVS diodes have a low clamping voltage, meaning they can limit the voltage to a safe level during transient events.
A: TVS diodes play a crucial role in protecting electronic components from voltage surges, ensuring the reliability and longevity of electronic systems.
