Diode is the most basic semiconductor device, usually used in many electronic design, such as rectifier, converter, inverter, protection circuit, regulator, etc. The three most common types of diodes are rectifier diodes (general purpose diodes), zener diodes and Schottky diodes. We have learned the basic knowledge and working principle of diodes. But there is another special diode called TVS diode, which plays an important role in dealing with the transient spikes in power electronic design circuits. In this paper, we will study the transient voltage suppressor and why it is needed in the circuit.
What does transient mean?
Transients are short-term spikes in voltage or current that can damage the circuit in many ways. Some transients occur only once, while others can be repeated. These transients range from a few millivolts to a few thousand volts and can last from nanoseconds to hundreds of milliseconds.
What causes transients in the circuit?
Transients can be caused by internal or external connections in the circuit. For example, transients may occur internally due to poor connections in inductive load switches or switches and connectors. On the outside it may be caused by lightning strikes or inductive switches.
Transient voltage suppressor (TVS)
Transient voltage suppressors or TVs are protective devices used to protect your circuit from sudden voltage or current surges. The main way to protect the circuit from overvoltage is to place these TVs devices in parallel with the circuit.
Types of Transient Voltage Suppressors
There are many types of TVs devices that can be used for transient voltage suppression, such as metal oxide varistor, TVS diode, zener diode or bypass capacitor. According to the operation conditions, Transient Voltage Suppressors can be divided into clamp and crowbar. The clamping device limits the voltage to a fixed level. In this way, they absorb the extra energy of overvoltage events. TVS diode is an example of clamp device.
On the other hand, once the crowbar device is triggered, it will short-circuit the protected circuit, thus redirecting the extra energy to the protected circuit. When the trigger voltage (spike) is detected, the crowbar device shortens the circuit, thus reducing the line voltage. After a period of time, when the crowbar device disconnects the circuit, the line voltage rises to a stable state again. Normal operation of the circuit.
Transient voltage suppression diode – TVS Diode
Transient voltage suppression diode is a kind of solid-state PN junction diode, which is specially designed to eliminate the influence of sudden or transient overvoltage on sensitive semiconductor and circuit. Transient voltage suppressor diode is a kind of clamp device, so whenever the induced voltage exceeds the avalanche breakdown voltage, it will absorb the extra energy of overvoltage event, and then reset automatically in the case of overvoltage. It is true that standard diodes and zener diodes can also be used for overvoltage / transient protection, but they are not as robust as transient voltage suppressor diodes, which are designed for rectification and voltage regulation.
Type of TVS Diode:
There are two types of transient voltage suppression diodes. One is one-way, the other is two-way.
The unidirectional transient voltage suppressor diode, like other avalanche diodes, acts as a rectifier in the forward circuit and enables the unidirectional diode to withstand a very large peak current. The following figure shows the symbol of unidirectional TVS diode, which is similar to zener diode.
On the other hand, the bidirectional transient voltage suppression diode can be represented by two avalanche diodes connected in series with each other. These diodes are connected in parallel with the device or circuit to be protected. Unlike the symbols, these diodes are manufactured as individual components. The following figure shows the symbols for the bidirectional TVS diode.
How to use TVS Diode
TVs diodes are connected in parallel with the equipment or circuits to be protected. TVs equipment is specially designed to breakdown at a specific voltage level and conduct a large amount of current without damage.
At normal voltage, the TVS Diode shows an open circuit, but there is a small leakage current. When the normal voltage exceeds a certain level, the TVS diode junction will avalanche, so the over-voltage will be transferred from the protected circuit and split through the TVS diode. After the overvoltage disappears, the device will reset automatically.
The following figure shows the VI characteristics of unidirectional and bidirectional TVs diodes. The characteristic diagram shows the relationship between voltage and current. The two-way diodes have the same characteristic curves in the positive and negative directions, so it doesn’t matter how they are connected to the circuit. Compared with the negative direction, the positive opening voltage of unidirectional diode is higher.
TVS diode parameters
There are many types of TVs diodes on the market that are designed for specific applications. When you choose a TVS tube you can look at the following terms in the datasheet to meet your design.
- Reverse isolation voltage (VR): the reverse isolation voltage is the maximum voltage that can be applied to the protector without actually activating the device. The device VR shall be equal to or higher than the peak operating voltage of the protected circuit. This is to ensure that the protection device does not limit normal circuit operation or signal voltage.
- Breakdown voltage (VBR): breakdown voltage is the voltage at which the diode will begin to protect and conduct current. In general, V BR is specified as 1mA.
- Clamp voltage (VC): clamp voltage is the highest voltage that the protected circuit bears during a test waveform event. In most data books, clamp voltage is given for 1A or 2A waveforms with 8 μ s rise time.
- Peak pulse current (IPP): the peak pulse current is the maximum current that the protection equipment can bear.
TVs diodes are commonly used for transfer / clamp in low-energy circuits and systems, as well as ESD protection in circuits. Applications of transient voltage suppression diodes can be found in data and signal lines, microprocessors and MOS memories, AC / DC power lines, and telecommunications equipment.
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