In overvoltage protection circuits, the four commonly used overvoltage protection technology products are: semiconductor discharge tube (TSS) (TSS), ceramic gas discharge tube (GDT) (GDT), metal oxide varistor (MOV), transient Suppression Diodes (TVS); All four devices are connected in parallel with the circuit they are protecting and exhibit high off-state impedance when the bias voltage is below the corresponding off-state repetitive peak voltage.

Semiconductor discharge tube (TSS)
A semiconductor discharge tube (TSS) is a PNPN device, which can be regarded as a thyristor without a gate. Once its peak off-state voltage (VDRM) is exceeded, the semiconductor discharge tube (TSS) will clamp the transient voltage within the range of its rated switching voltage (VS). Once the current flowing through the semiconductor discharge tube (TSS) exceeds its switching current, the device will undergo a sharp short circuit, simulating a short circuit condition. When the current flowing through the semiconductor discharge tube (TSS) is less than its holding current (IH), the semiconductor discharge tube (TSS) will reset and return to its off-state high impedance state.

Advantages of semiconductor discharge tubes
The advantages of semiconductor discharge tubes (TSS) include fast response time (see Figure 1.1), stable electrical characteristics, good long-term stability, and low capacitance. In addition, because the semiconductor discharge tube (TSS) is a crowbar device, it will not be damaged by voltage.

Limitations of semiconductor discharge tubes
Since the semiconductor discharge tube (TSS) is an electric crowbar device, it cannot be used directly on the AC line and must be placed behind the load. Failure to do so will result in exceeding the rated on-state current of the semiconductor discharge tube (TSS), causing it to enter a permanent short circuit condition.

Applications of semiconductor discharge tubes
Although they can be used in other applications, semiconductor discharge tubes (TSS) are mainly used in telecommunications and data communication circuits as main overvoltage protectors.

Ceramic gas discharge tube (GDT)
Ceramic gas discharge tube (GDT) (GDT) is encapsulated in glass or ceramic, filled with inert gas inside, and sealed at both ends with electrodes. When the transient voltage exceeds the rated DC breakdown voltage of the device, the voltage difference will cause the electrodes of the ceramic gas discharge tube (GDT) to ignite, resulting in an arc. The arc will cause the gas in the tube to ionize and provide a barrier to the transient voltage. low impedance path through. When the transient value drops below the DC overholding voltage and current, the ceramic gas discharge tube (GDT) will return to the off-state.

Advantages of ceramic gas discharge tubes
Ceramic gas discharge tubes (GDT) have high surge current and low rated capacitance. Its rated current can be as high as 20 kA, and its rated capacitance can be as low as 1 pF when the bias voltage is 0 V.

Applications of Ceramic Gas Discharge Tubes
Ceramic gas discharge tubes (GDTs) are often used for base protection due to their high surge ratings. However, due to their low interference with high-frequency components, they are also an alternative for high-speed data links.

Metal Oxide Varistor (MOV)
Metal oxide varistors (MOVs) are through-hole components with two leads, usually in the shape of a disk. MOVs are made of sintered oxide and are roughly equivalent to two connected PN nodes, which can shunt transient currents by reducing resistance when voltage is applied.

Advantages of varistor
Since the MOV's surge resistance is determined by its dimensions, it can provide high surge current ratings. Additionally, because MOVs are clamping devices, they can be used as transient protectors in secondary AC power lines.

Applications of varistor
Although MOV cannot be used in many telecommunications equipment (except disposable equipment), it can be used in AC equipment that requires clamping devices and does not require small voltage tolerances.

TVS transient suppression diode
Transient voltage suppressor (TVS) diodes are clamping voltage suppressors composed of connected PN nodes. While conducting electricity, a TVS diode creates a low-impedance channel by changing its resistance when a voltage is applied to its terminals. Once the voltage is removed, the diode turns off and returns to its high off-state impedance.

Advantages of TVS transient suppression diodes
Because TVS diodes are solid-state devices, they will not experience fatigue failure and their electrical parameters will not change as long as they operate within the specified range. TVS diodes can effectively clamp rapidly generated transient voltages and are ideal for use in low-voltage equipment that does not require the shunting of large amounts of electrical energy.

Application of TVS transient suppression diodes
Due to their low power rating, TVS diodes cannot be used as primary interface protectors between the positive line (Tip) and negative line (Ring), but can be used as secondary protectors embedded in circuits.

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