Surge voltage suppressor and its application

1 Surge voltage

When a circuit is struck by lightning and when an inductive load or a large load is connected or disconnected, a high operating overvoltage is often generated. This instantaneous overvoltage (or overcurrent) is called surge voltage (or surge current), which is a transient interference: for example, when a DC 6V relay coil is disconnected, a surge voltage of 300V to 600V will appear; when an incandescent lamp is connected, a surge current of 8 to 10 times the rated current will appear; when a large capacitive load such as a compensation capacitor bank is connected, a large surge current impact often occurs, causing the power supply voltage to drop suddenly; when an unloaded transformer is cut off, an operating overvoltage of up to 8 to 10 times the rated voltage will also appear. The surge voltage phenomenon is increasingly endangering the safe operation of automation equipment. Eliminating surge noise interference and preventing surge damage have always been the core issues related to the safe and reliable operation of automation equipment. The degree of integration of modern electronic equipment is constantly improving, but their ability to resist surge voltage is decreasing. In most cases, surge voltage will damage the circuit and its components. The degree of damage is closely related to the withstand voltage strength of the components and is related to the energy that can be converted in the circuit.

In order to prevent surge voltage from destroying sensitive automation equipment, the conductor where this surge voltage occurs must be short-circuited with the potential equalization system (introduced to the ground) in a very short time. During the discharge process, the discharge current can be as high as several thousand amperes. At the same time, people often expect the protection unit to limit the output voltage to the lowest possible value when the discharge current is large. Therefore, components such as air spark gaps, gas-filled overvoltage arresters, varistors, avalanche diodes, TVS (Transient voltage suppressor), FLASHTRAB, VALETRAB, SOCKETTRAB, MAINTRAB, etc. are applied to the protected circuit alone or in the form of a combined circuit, because each component has its own different characteristics and has different performances: discharge capacity; response characteristics; arc extinguishing performance; voltage limiting accuracy. According to different application scenarios and equipment requirements for surge voltage protection, an overvoltage protection system that meets the application requirements can be combined according to the characteristics of various products.

2Surge voltage absorber

Surge noise is often suppressed by surge absorbers. Commonly used surge absorbers are:

(1) Zinc oxide varistor

Zinc oxide varistor is a varistor made of zinc oxide as the main material. It has high voltage nonlinear coefficient, large capacity, low residual voltage, small leakage current, no freewheeling, symmetrical volt-ampere characteristics, wide voltage range, fast response speed, small voltage temperature coefficient, simple process and low cost. It is currently a widely used surge voltage protection device. It is suitable for surge absorption of AC power supply voltage, surge voltage absorption and arc extinguishing between various coils and contacts, and surge voltage protection of power electronic devices such as triodes and thyristors.

(2) R, C, D combined surge absorber

The R, C, D combination surge absorber is more suitable for DC circuits. The devices can be combined in different ways according to the characteristics of the circuit. For example, Figure 1 (a) is suitable for high-level DC control systems, while Figure 1 (b) uses Zener diodes or bidirectional diodes, which are suitable for circuits that need forward and reverse protection.

Figure 1 R, C, D surge protector

(a) One-way protection (b) Two-way protection

Figure 2 TVS voltage (current) time characteristics

(3) Transient Voltage Suppressor (TVS)

When the two poles of TVS are subjected to reverse high-energy impact, it can change the impedance between the two poles from high to low at a speed of 10-12s, absorb up to several kW of surge power, clamp the potential of the two poles to a predetermined value, and effectively protect the components in the automation equipment from damage by surge pulses. TVS has the advantages of fast response time, high transient power, low leakage current, small breakdown voltage deviation, easy control of clamping voltage, and small size. It is currently widely used in electronic equipment and other fields.

① Characteristics of TVS

Its forward characteristics are the same as those of ordinary diodes, and its reverse characteristics are typical PN junction avalanche devices. Figure 2 is the current-time and voltage-time curves of TVS. Under the action of surge voltage, the voltage between the two poles of TVS rises from the rated reverse shutdown voltage VWM to the breakdown voltage Vbr and is broken down. With the appearance of breakdown current, the current flowing through TVS will reach the peak pulse current IPP, and the voltage at both ends of it will be clamped to below the predetermined maximum clamping voltage VC. Afterwards, as the pulse current decays exponentially, the voltage between the two poles of TVS also continues to decrease, and finally returns to the initial state. This is the process of TVS suppressing possible surge pulse power and protecting electronic components.

②Comparison between TVS and varistor

At present, it is common to use varistors on many devices in China that require surge protection. The performance comparison between TVS and varistors is shown in Table 1:

Table 1 Comparison of TVS and varistor

3Comprehensive Surge Protection System Combination

3.1 Three-level protection

The surge protection required by the automatic control system should be comprehensively considered in the system design. According to the characteristics of the automatic control device, the surge protector used in the system can basically be divided into three levels. For the power supply equipment of the automatic control system, lightning current arresters, overvoltage arresters and terminal equipment protectors are required. The interface circuits of data communication and measurement and control technology are obviously much more sensitive than the power supply system circuits of each terminal, so the data interface circuits must be carefully protected.

The first level of protection for the power supply equipment of the automation device uses lightning current arresters, which are either installed at the entrance of the building or in the main distribution box. In order to ensure that the subsequent equipment does not bear too high residual pressure, overvoltage arresters must be installed in the lower-level distribution facilities according to the nature of the protected range as a secondary protection measure. The third level of protection is to protect the instruments and equipment. The method adopted is to install the overvoltage arrester directly at the front end of the instrument. The three-level protection layout of the automatic control system is shown in Figure 3. Between arresters of different levels, the minimum length of the wire must be observed. The distance between the lightning current arrester and the overvoltage arrester in the power supply system shall not be less than 10m, and the wire distance between the overvoltage arrester and the instrument and equipment protection device shall not be less than 5m.

3.2 Three-level protection device

(1) The inert gas-filled overvoltage arrester is a widely used primary surge protection device in automatic control systems. The inert gas-filled overvoltage arrester, which is generally constructed in this type of arrester, can discharge transient currents within 20kA (8/20μs) or 2.5kA (10/350μs). The response time of the gas arrester is in the ns range and is widely used in the field of telecommunications. One disadvantage of this device is that its triggering characteristics are time-dependent, and the transients of its rise time intersect with the triggering characteristic curve in a range that is almost parallel to the time axis. Therefore, the protection level will be close to the rated voltage of the gas arrester. Particularly fast transients will intersect with the triggering curve at an operating point that is ten times the rated voltage of the gas arrester. That is to say, if the minimum rated voltage of a gas arrester is 90V, the residual voltage in the line can be as high as 900V. Another disadvantage is that it may generate subsequent currents. When the gas arrester is triggered, especially in a circuit with low impedance and voltage exceeding 24V, the following situation may occur: the short circuit state that was originally expected to be maintained for several milliseconds will continue to be maintained because of the gas arrester, and the consequence may be that the arrester explodes within a fraction of a second. Therefore, a fuse should be connected in series in the overvoltage protection circuit using the gas arrester so that the current in this circuit can be quickly interrupted.

Figure 3 Arrester distribution diagram

(2) Varistors are widely used as secondary protection devices in systems because they have a faster response time in the ns time range and will not cause subsequent current problems. In the protection circuit of measurement and control equipment, varistors can be used for intermediate protection devices with a discharge current of 2.5kA to 5kA (8/20μs). The disadvantages of varistors are aging and high capacitance problems. Aging refers to the P-N part of the diode in the varistor. Under normal overload conditions, the P-N junction will cause a short circuit, and its leakage current will increase as a result. The value depends on the frequency of the load. Its application in sensitive measurement circuits will cause measurement distortion and the device is prone to heat. The large capacitance problem of varistors makes it impossible to use them in high-frequency information transmission lines in many occasions. These capacitors will form a low-pass link together with the inductance of the wire, thereby causing serious damping to the signal. However, in the frequency range below 30kHz, this damping effect can be ignored.

(3) Suppression diodes are generally used in highly sensitive electronic circuits. Their response time can reach ps level, and the voltage limit of the device can reach 1.8 times of the rated voltage. Its main disadvantage is that the current load capacity is very weak and the capacitance is relatively high. The capacitance of the device itself changes with the rated voltage of the device, that is, the lower the rated voltage of the device, the greater the capacitance. This capacitance will also form a low-pass link with the inductance in the connected wire, and produce a damping effect on data transmission. The degree of damping is related to the signal frequency in the circuit.