Surge protector basic elements

The type and structure of a surge protector varies according to the intended use, but it should include at least one non-linear voltage limiting element. The basic components used for surge protectors are: discharge gaps, gas discharge tubes, varistors, suppression diodes, and choke coils.

1. Discharge gap (also known as protection gap)

It is generally composed of two metal rods exposed in the air with a certain gap. One metal rod is connected to the power line L1 or neutral wire (N) of the protection equipment, and the other metal rod and the ground wire ( PE) is connected, when the instantaneous over-voltage hits, the gap is broken down, and a part of the over-voltage charge is led to the earth to prevent the voltage on the protected equipment from rising. The distance between two metal rods of such a discharge gap can be adjusted as required, the structure is relatively simple, and its disadvantage is poor arc extinguishing performance. The improved discharge gap is an angular gap, and its arc extinguishing function is better than the former, and it is extinguished by the action of the electric force F of the loop and the rise of the hot gas flow.

Gas discharge tube

It is composed of a pair of cold cathode plates separated from each other and enclosed in a glass tube or ceramic tube filled with a certain inert gas (Ar). In order to increase the trigger probability of the discharge tube, there is also a triggering agent in the discharge tube. This type of gas-filled discharge tube has a diode type and a three-pole type.

The technical parameters of the gas discharge tube mainly include: DC discharge voltage Udc; impulse discharge voltage Up (normally Up≈(2~3)Udc; power frequency withstand current In; impulse withstand current Ip; insulation resistance R (>109Ω ); Electrode capacitance (1-5PF)

The gas discharge tube can be used under DC and AC conditions, and its selected DC discharge voltage Udc is as follows: Use under DC conditions: Udc≥1.8U0 (U0 is the DC voltage for normal operation of the line)

Use under AC conditions: U dc ≥ 1.44Un (Un is the AC voltage rms for normal operation of the line)

3. Varistors

It is based on ZnO as the main component of the metal oxide semiconductor nonlinear resistors, when the voltage across its ends reaches a certain value, the resistance is very sensitive to voltage. Its working principle is equivalent to the series and parallel of multiple semiconductor PNs. The varistors are characterized by good nonlinearity (I=coefficient α in CUα), high through-flow capacity (～2KA/cm2), low normal leakage current (10-7 to 10-6A), low residual voltage (depending on In the varistor operating voltage and flow capacity), the instantaneous over-voltage response time is fast (~10-8s), no freewheeling.

The technical parameters of varistors are: varistor voltage (ie, switching voltage) UN, reference voltage Ulma, residual voltage Ures, residual voltage ratio K (K=Ures/UN), maximum flow capacity Imax, leakage current, response time .

Varistor use conditions are: varistor voltage: UN ≥ [(√ 2 × 1.2) / 0.7] U0 (U0 is the rated voltage of the power frequency)

Minimum reference voltage: Ulma ≥ (1.8 ~ 2) Uac (used under DC conditions)

Ulma ≥ (2.2 ~ 2.5) Uac (used under AC conditions, Uac is AC working voltage)

The maximum reference voltage of the varistor should be determined by the withstand voltage of the protected electronic device. The residual voltage of the varistor should be lower than the level of the damage voltage of the protected electronic device, ie, (Ulma)max≤Ub/K. Where K is the residual pressure ratio, Ub is the loss voltage of the protected equipment.

4. Suppression diode

The Suppression Diode has a clamp-limiting function. It operates in the reverse breakdown region. Because it has the advantages of low clamping voltage and quick response, it is particularly suitable for use as the last few protection elements in multistage protection circuits. Suppression diode volt-ampere characteristics in the breakdown region can be expressed by the following formula: I = CUα, where α is a nonlinear coefficient, for the Zener diode α = 7 ~ 9, in the avalanche diode α = 5 ~ 7.

Suppress diode technology parameters

Breakdown voltage, which refers to the breakdown voltage at a specified reverse breakdown current (usually 1ma), which is generally within the range of 2.9V to 4.7V for the Zener diode rated breakdown voltage, and the rating of the avalanche diode The breakdown voltage is usually in the range of 5.6V to 200V.

(2) Maximum clamp voltage: It refers to the highest voltage appearing at both ends of the tube when it passes a large current of a specified waveform.

(3) Pulse power: It refers to the product of the maximum clamp voltage at both ends of the tube and the equivalent current in the tube at a specified current waveform (eg, 10/1000 μs).

(4) Reverse displacement voltage: It refers to the maximum voltage that can be applied to both ends of the tube in the reverse leakage zone, at which voltage the tube should not break down. This reverse-displacement voltage should be significantly higher than the peak operating voltage of the protected electronic system, ie, it cannot be in a weak conduction state when the system is operating normally.

(5) Maximum Leakage Current: It refers to the maximum reverse current flowing through the pipe under the action of reverse displacement voltage.

(6) Response time: 10-11s

5. Choke coil

The choke coil is a common-mode interference suppression device with a ferrite core. It consists of two coils with the same size and the same number of turns wound symmetrically on the same ferrite toroid to form a four-terminal coil. The device has an inhibitory effect on exhibiting a large inductance in the common mode signal, and has little effect on a differential leakage signal exhibiting a small leakage inductance. The use of a choke coil in a balanced line can effectively suppress common-mode interference signals (such as lightning interference), but has no effect on differential mode signals that are normally transmitted on the line.

Choke coils should meet the following requirements when they are manufactured

1) The wires wound on the core of the coil should be insulated from each other to ensure that no short circuit occurs between the turns of the coil under transient overvoltage.

2) The core should not saturate when a large current flows through the coil.

3) The core in the coil should be insulated from the coil to prevent breakdown between the two under transient overvoltage.

4) The coil should be wound as a single layer as much as possible. This will reduce the parasitic capacitance of the coil and enhance the ability of the coil to transient overvoltage.

6. 1/4 wavelength short circuit

The 1/4-wave short-circuiter is a microwave signal surge protector based on spectrum analysis of the lightning wave and the standing wave theory of the antenna line. The length of the metal shorting bar in this protector is based on the frequency of the operating signal (eg, 900 MHz or 1800 MHz. ) The size of the 1/4 wavelength is determined. The length of the parallel shorting bar has infinite impedance for the frequency of the working signal, which is equivalent to an open circuit and does not affect the transmission of the signal, but for the lightning wave, since the lightning energy is mainly distributed below n+KHZ, the shorting bar For lightning wave impedance is very small, equivalent to a short circuit, lightning energy level is discharged into the ground.

Since the diameter of the 1/4 wavelength shorting bar is generally a few millimeters, the impact current resistance is good and can reach 30KA (8/20μs) or more, and the residual voltage is very small. This residual voltage is mainly caused by the short-circuit rod's own inductance. The disadvantage is that the frequency band is about 2% to 20% due to the narrow working frequency band. Another disadvantage is that DC bias cannot be added to the antenna feeder facilities, which limits certain applications.