Several devices can be used to protect power, signal, and communication connections from damage due to destructive transient voltages. Embedded electronics, IoT, and M2M connections have become widespread in diverse areas so it is vitally important to protect interface connections from transient overvoltage events.
Overvoltage conditions and transient voltage events are essentially different. Overvoltage conditions can last seconds to minutes, or longer. Transients occur over a much shorter period of time – ten to a hundred microseconds from, for example, an industrial switching or lightning transient, down to a few nanosecond ESD transient.
One distinction of protection devices is they can operate as a clamp versus a crowbar. If a device is a clamp, it will conduct enough current to maintain the voltage across it at a safe, specified value when the transient is above the conduction voltage. It is rated for the power it can dissipate for a specific time. The protection device needs to be rated for the power to be dissipated during the transient event. The voltage across the load rises only slightly above the rated clamp voltage during the maximum surge voltage.
A crowbar device is triggered by a voltage above the protection voltage. It protects by becoming a low impedance across the protected load when it is triggered by the overvoltage condition. The device stays in the low impedance state until the current is reduced below the holding current and the device is returned to the high impedance state. In the low impedance state, the device conducts current which is limited by the external circuit. In this way, the voltage across the protected device stays low. The crowbar protection device acts as the low impedance device on the low voltage side of a voltage divider, keeping the high voltage in the external circuit and protecting the sensitive load on the low voltage side. The most popular overvoltage protection devices are defined below.
The TVS (Transient Voltage Suppressor) is a silicon clamp device with breakdown voltages from 4 to 500 volts in various power ratings to provide different surge protection capabilities. A TVS device is a bipolar junction similar to a rectifier diode but designed to survive reverse breakdown.
The MOV (Metal Oxide Varistor) is a ceramic clamp device with breakdown voltages from 14 to 1400 Volts capable of 6.5 kA for the 8/20 µs lightning surge waveform.
TSPs (Thyristor Surge Protectors) are silicon crowbar devices. Their breakdown voltage can be set precisely and they are available in many package types to dissipate various levels of surges. These are semiconductor thyristor (PNPN) structures.
GDTs (Gas Discharge Tubes) are ceramic crowbar devices enclosing a spark gap that conducts a high current arc when triggered by a high voltage, high current source, such as lightning.
In common applications, clamp devices are mainly used to protect current limited power connections. Crowbars can protect current limited power lines as well as data and signal lines.
When selecting the optimum protection device for power and signal connections, it is important to consider what type of device best fits the application requirements in terms of operation as a clamp or crowbar, or the desired material needed; between silicon or ceramic.