TVS diodes 101

Electrical surges, or transient overvoltage, can cause irreversible damage to sensitive electronic components and malfunctioning equipment. Adding effective surge protection techniques to electronic systems will significantly enhance electronic products' reliability, longevity, and overall performance. This article explores how Transient Voltage Suppression (TVS) diodes shield electrical circuits from surge, Electrostatic Discharge (ESD), and overvoltage.

What are TVS diodes?

A Transient Voltage Suppressor (TVS) is an avalanche diode designed to clamp overvoltage and dissipate high transient power surges. It absorbs excess energy when the induced voltage exceeds its avalanche breakdown voltage and then automatically resets after the overvoltage condition passes.

How do TVS diodes safeguard against transient voltages?

The TVS diode is a p-n semiconductor junction that becomes conductive during a transient voltage spike. Under normal conditions, it has high impedance and minimal leakage current, effectively acting like an open circuit. When the voltage exceeds its threshold, the avalanche effect in the semiconductor activates, causing the p-n junction to conduct and create a low-impedance path that diverts excessive current away from the protected device.

TVS diodes respond extremely quickly ( in picoseconds), making them effective at diverting intense ESD pulses, even those with fast rise times. Compared to Metal-Oxide Varistors (MOVs), TVS diodes offer several advantages, such as the absence of aging effects, ensuring stability and reliability over time. They are more robust and of higher quality than Zener diodes, rated for low DC and not specified for handling surges.

Once the surge subsides, the diode resets to its high-impedance state, ready to counter the next transient. TVS diodes are available in unidirectional and bidirectional types and are suitable for both DC and AC applications. They are widely used in industrial, consumer, and medical electronics for their reliable, high-speed protection.

Figure 1: Working principle of TVS diodes (Source)

Key parameters and electrical characteristics of TVS diodes

To ensure reliable protection, TVS diodes must be sized appropriately to handle expected transient pulses without failing while effectively clamping surge voltages without disrupting normal line voltage. Key specifications and electrical characteristics to consider when selecting a TVS diode include –

• VRM (Maximum standoff voltage or peak reverse voltage): The maximum continuous voltage a TVS diode can handle without activating its protective response, which is known as the peak reverse voltage. Under normal conditions—when no surge is present—the line voltage should stay below this VRM level. At or below t is voltage, the TVS diode continues its high impedance and only minimum leakage current flows.

  For reliable protection, it is recommended that a VRM value be selected that's about 10-20% higher than the circuit's maximum operating voltage. This prevents the diode from triggering unnecessarily, ensuring it only activates when an actual transient event occurs.

• VBR (Reverse breakdown voltage): The reverse breakdown voltage (VBR) is the point at which a TVS diode activates its protective function during a transient event. When this voltage is reached, the diode enters breakdown mode and starts clamping to protect sensitive components from overvoltage.

  We should select a diode with a VBR a little above the circuit's maximum operating voltage for consistent performance. Such an action will keep the diode inactive during regular operation while ensuring it quickly responds to sudden transients. VBR is typically defined as the voltage at which the diode conducts a specified amount of current, usually measured at 1 mA, although this can vary between devices. Originally a specification for Zener diodes, VBR marks the threshold at which the TVS diode effectively begins clamping voltage surges.

• VCL (Clamping voltage): The clamp voltage (VC) is the maximum voltage across a TVS diode during a surge. To effectively protect sensitive components, VC should be below the absolute maximum voltage the component can handle. We must remember that absolute maximum ratings (AMRs) are based on DC voltage levels, usually lower than transient voltage peaks. Clamp voltage represents the diode's ability to limit transient voltage spikes, preventing damage to downstream components. It's measured at specific peak pulse current levels—often with an 8/20 µs waveform—indicating the ESD performance of the diode.

• IPP (Peak pulse current): IPP is the highest surge current a TVS diode can handle safely without damage. To choose the suitable diode, review historical surge data or conduct transient testing to identify the peak surge levels in your application. Always select a TVS with an IPP rating higher than your maximum expected surge to ensure reliable protection. Datasheets typically specify IPP under standard conditions, such as 8/20 µs and 10/1000 µs surge waveforms, which reflect real-world surge profiles.

• IRM (Leakage current): IRM is the small leakage current that flows through a TVS diode when it’s reverse-biased at a specified voltage. It is measured at the diode's peak reverse voltage (VRM) and is typically listed as a maximum value in the datasheet.

• RD (Dynamic resistance): A TVS diode acts like a voltage source in series with a dynamic resistance (RD) resistance. This resistance varies between different TVS diodes and changes with surge duration. The RD value is essential for calculating the clamping voltage during a surge event. Dynamic resistance represents the slope of the VF–IF curve between the breakdown voltage (VBR) and clamping voltage (VCL) as the diode enters reverse breakdown when the reverse voltage increases.

Figure 2: Voltage-vs-cu rent curve of TVS diodes (Source)

Types of TVS diodes

TVS diodes are available in unidirectional types, which protect against transients in one polarity, and bidirectional types, which handle transients in both directions, making them ideal for high-speed and data communication lines. Here's a closer look at some of the main types of TVS diodes-

• Zener diode: Zener diodes are a common type of TVS diode known for their effective ability to maintain a steady voltage through controlled breakdown, which occurs at a specified Zener voltage. This characteristic makes them perfect for high-frequency circuits and high-speed data lines, where they provide effective protection against overvoltage and ESD events. Zener diodes are widely used for voltage reference generation and clamping, ensuring stable operation in sensitive circuits by diverting excess current and protecting components from voltage spikes.
• Automotive TVS diode: TVS diodes play a significant role in protecting sensitive electronic components from transient spikes and ESD in the automotive industry. These diodes are near high-risk areas such as alternators, regulators, and electronic data buses. For instance, when installed across an automotive data bus, a TVS diode safeguards the vehicle's ECU (Electronic Control Units) from voltage spikes that could cause irreparable damage. TVS diodes are designed to provide reliable protection against high-energy events like field decay and load dump, ensuring the longevity and stable operation of automotive electronics, even in harsh environments.
• Clamping voltage diode: Clamping voltage diodes play a vital role in protecting sensitive devices from transient voltage spikes. They divert the excess voltage and clamp the residual voltage to a secured level. These diodes are much more effective in high-frequency circuits as they respond rapidly to voltage surges. Specialized camping diodes are designed to handle high-voltage events, offering fast response times and ensuring that vulnerable circuits remain safe from transient damage.
• ESD protection diode: ESD protection diodes are designed to shield sensitive integrated circuits (ICs) from damaging high-voltage electrostatic discharge (ESD) events. When an ESD event occurs, the diode rapidly creates a low-impedance path to divert excess current to the ground, preventing overvoltage from reaching critical components and causing malfunctions. These diodes are placed as close as possible to the ESD source to maximize its effectiveness, such as connectors or user interfaces, with minimal distance to the ground. This strategy placement allows ESD diodes to shunt transient energy smoothly, providing robust protection for sensitive circuits.
• Protection diode: These shield sensitive devices from potentially damaging reverse voltage. They allow current to flow in the forward direction but block it in reverse, protecting components that could be adversely affected by backward current flow. An important factor in selecting a protection diode is its peak reverse voltage rating, which determines the maximum reverse voltage it can withstand before breaking down. For example, a protection diode in a solar power system prevents reverse current from flowing back into the solar cells or power electronics, ensuring reliable operation and avoiding damage from unexpected reverse voltage conditions.
• Transient suppression diodes: Transient suppression diodes protect sensitive circuits from voltage spikes by quickly clamping and grounding excess energy. Due to their fast response and straightforward design, these diodes are particularly effective in moderate-frequency circuits, and they come in both unidirectional and bidirectional formats to suit various applications. A transient suppression diode prevents voltage surges from impacting measurement accuracy and device safety. While they offer reliable protection against transients like ESD events, their higher capacitance can limit use in high-frequency circuits.
• Transil diode: Transil diodes are specifically engineered to protect circuits and sensitive components from harmful voltage spikes and surges. These diodes can be unidirectional or bidirectional. The versatility of transit diodes allows them to be used in medical devices, wireless communication systems, and other applications. For example, in a wireless communication setup, a Transil diode shields the receiver circuitry from high-voltage surges, guaranteeing stable performance. Such an action also increases the life of the component. The special features of diode fitted systems are Quick response time, efficient surge handling capabilities, and reliable transient protection.

Application of TVS diodes

Protecting sensitive electronics from surges, spikes, and ESD is vital for reliable operation. Different TVS diodes—like ESD, clamping, and transient suppression diodes—are tailored to shield circuits across applications, from automotive ECUs (Electronic Control Units) to telecom systems. The chart below highlights various TVS diodes and their roles in safeguarding equipment and ensuring durability, accuracy, and uptime in demanding environments.

Conclusion

The world of electronics is complex and sensitive, making TVS diodes indispensable for shielding systems against damaging transients. Their rapid response and robust design make them key components for ensuring stability and longevity in telecommunications, automotive, and industrial control industries. When using TVS diodes, it is vital to consider several factors such as breakdown voltage, clamping voltage, and peak pulse current. Such an approach ensures that the diodes effectively protect circuits against voltage spikes.