Figure 1: Understanding transients and its effect in electronics

Transients, also known as surges or power line transients, are short-duration temporary increases in voltage or current level that can damage or degrade sensitive equipment and systems connected to power lines or signal lines.  Typically, surges have a voltage range greater than 2000 volts and a current range exceeding 100 amps, with a rise time in the 1 to 10 microsecond range. Common causes of transients include lightning strikes, power line switching, inductive loads switching on and off, relay contact bounce, AC/DC switching operation, etc. 

 Transient protection refers to methods, devices, and circuits used to protect electrical and electronic systems connected to power or signal lines from transient overvoltage or overcurrent conditions. A transient protection circuit typically consists of both voltage-limiting devices and current-limiting devices to safeguard equipment connected to electrical circuits or power and/or signal lines.

Figure 2: Understanding single stage transient protection network and its function

What are Voltage-Limiting Devices?         

Voltage-limiting devices (VLDs) are essential components designed to limit transient overvoltages on power or signal lines to a safe range, thereby protecting connected equipment from high-voltage surges. The VLDs are also known as voltage clamping devices or surge protective devices. 

A voltage clamping device is connected in parallel (shunt) with the equipment being protected (figure 2). Under normal conditions, it remains non-conductive (invisible to the circuit), presenting high impedance. However, when a surge or voltage spike occurs, the VLD device switches to a conductive state, offering low impedance and diverting excess current to the ground. This action effectively clamps the transient voltage to a safe level, ensuring the protection of sensitive circuits and equipment. After the transients disappear, the voltage clamping device returns to a high impedance state (off state).  

Examples of Voltage-limiting devices (VLDs) or voltage clamping devices/surge protective devices include Transient Voltage Suppression (TVS) Diodes, Gas Discharge Tubes (GDTs), Metal Oxide Varistors (MOVs), and Thyristor surge protective devices (TSPDs). Let’s understand these devices.

Transient Voltage Suppression (TVS) Diodes:     

TVS Diode is like a Zener diode but has a wider PN junction area to handle surge current proportional to its power rating. Under normal circuit conditions, the diode does not conduct and presents high impedance (invisible to the circuit), see figure 2. However, when voltage spikes occur, it switches to a low-impedance state, diverting the surge current to the ground and clamping the voltage to a safe level. After the disappearance of transients, the device returns to a high impedance state (off state). 

Figure 3: Transient Voltage Suppression (TVS) Diode

TVS diodes (sometimes called silicon avalanche diodes) are available in both unidirectional and bidirectional configurations and offer a faster response time compared to GDTs and MOVs. These diodes are ideal for low-voltage electronic applications, such as DC lines and signal lines, where they protect equipment from surges (if the current is limited), electrical fast transients (EFT), and electrostatic discharge (ESD). 

Gas Discharge Tube (GDT):

A gas discharge tube (GDT) is typically a glass or plastic tube filled with an inert gas and consists of two pointed metal electrodes in close proximity. Under normal circuit conditions, the inert gas acts as a poor conductor. However, when voltage spikes occur, the gas becomes ionized, turning into a good conductor that allows the surge current to pass from the line to the ground until the circuit voltage returns to normal. Once the surges are cleared, the device returns to a high impedance state (off state).

Figure 4: Standard resistor based lightning protection circuit, comprises both GDT and TVS diode

GDTs are crowbar devices, meaning that once they turn ON, the voltage across the device drops to a very low value. They are known for their reliability and robustness, offering high surge current-handling capabilities. They are widely used in telecom applications to protect devices from surges and transients. 

Metal Oxide Varistors (MOVs): 

A Metal Oxide Varistor (MOV) is a voltage-dependent resistor that alters its resistance when applied voltage changes. It is a non-linear, bidirectional surge protection component made up of 90% zinc oxide (ZnO) and 10% other metal oxides sandwiched between two metal plates. Under normal circuit conditions, it presents high impedance (off state). However, when a surge occurs, the MOV conducts, partially absorbing and partially diverting the surge energy to the ground, thereby protecting sensitive devices. Once the overvoltage transients are cleared, the device returns to its high impedance state (off state). 

Figure 5: Metal Oxide Varistor (MOV) in surge protection

MOVs are known for their large power-handling capabilities and are commonly used in both AC and DC power lines to protect equipment from surges or power line transients. They can protect against both positive and negative surges. Additionally, surface-mount multilayer MOVs are available for PCB applications, providing protection against ESD, EFT, and surge events. 

Thyristor Surge Protective Devices (TSPDs):

A Thyristor Surge Protective Device (TSPD) is a type of voltage-limiting device that uses a thyristor to clamp over-voltage transients. It is connected across the equipment to be protected, and its operation is similar to that of a TVS diode. Under normal conditions, the thyristor presents high impedance and remains non-conductive. However, when voltage spikes occur, the TSPD switches to a low-impedance state, diverting the surge current to the ground and clamping the voltage to a safe level. As the transient energy dissipates, the current through the thyristor decreases and falls below the holding current (IH), resulting in TSPD returning to its high impedance (off) state.

Figure 6: Thyristor Surge Protective Device (TSPD) in transient protection

TSPDs are crowbar devices capable of handling currents up to 100 A and are available in both bidirectional and unidirectional configurations. Key benefits of TSPDs include low on-state voltage, no degradation in surge capability after multiple events within specified limits, no aging effects, and turn-on without parasitic oscillation or ringing. However, drawbacks include vulnerability to high di/dt excursions, wide breakover voltage tolerance, the requirement for a minimum holding current to turn off, and potential premature turn-on from excessive dv/dt.

TSPDs are ideal for providing surge protection in power supplies, telephony equipment (in both infrastructure and residential installations), and telecom network applications such as Ethernet, xDSL, T1/E1, EPON, and GPON.

Comparison between various voltage-limiting devices:

The following table provides comparison between various voltage-limiting devices (Referenced from Book: Electromagnetic Compatibility Engineering, Author: HENRY W.OTT). 

What are Current-Limiting Devices?

A current-limiting device is a component or equipment connected in series with the power or signal lines to protect the circuit or equipment from excessive current caused by overloads, short circuits, and overvoltage transients. This protective element limits the transient current through the voltage-limiting device when in an ON state (figure 5).

Examples of current-limiting devices include fuses, circuit breakers, inductors or ferrites, and Positive Temperature Coefficient (PTC) thermistors. These devices provide a low-impedance path when normal current flows through the electrical circuit. But it opens (becomes a high impedance) when excessive current flows through the circuit. Thereby protecting the circuit/equipment/system from over-current conditions.

• A fuse is a metal wire that melts (opens) when the current exceeds a predefined threshold, preventing further current flow and protecting the circuit/device.
• A circuit breaker (CB) is a device that automatically opens when excessive current flows through the circuit or equipment, protecting it from damage.
• A Positive Temperature Coefficient (PTC) thermistor is a device that heats when excessive current flows, causing its resistance to increase, which limits the current and protects the circuit or equipment from overcurrent.

A typical transient protection network or circuit comprises both voltage limiting device(s) and current limiting device(s) to protect electrical and electronic circuits, components, or devices from transients or surges in various fields, including communication systems, automotive, industrial equipment, medical, military, etc. 

Note: A hybrid transient protection network is a circuit consisting of more than one surge protective device. It is designed to protect equipment or circuits from multiple transient events like surges, EFT, and ESD. For example, a two-stage hybrid transient protection network is shown in the figure that consists of a TVS diode plus a MOV or a gas discharge tube, along with the current limiting device (resistors).

Figure 7: A two-stage hybrid transient protection network shown