Surge Signals and Protection Methods

1 Introduction

Surge is a spike pulse with a high rise rate and short duration. There are many reasons for its generation, such as: overvoltage of the power grid, sparking of switches, reverse power supply, static electricity, motor/power supply noise, etc. As we all know, electronic products often encounter unexpected voltage transients and surges during use, which lead to damage to electronic products. The cause of damage is that the semiconductor devices (including diodes, transistors, thyristors and integrated circuits, etc.) in electronic products are burned or broken down. It is estimated that 75% of the failures of electronic products are caused by transients and surges. Voltage transients and surges are everywhere. Power grids, lightning strikes, explosions, and even people walking on carpets will generate tens of thousands of volts of electrostatic induction voltage. These are all invisible and fatal killers of electronic products. Therefore, in order to improve the reliability of electronic products and the safety of the human body, protective measures must be taken against voltage transients and surges. One of the methods is to ground the whole machine and system. The ground (common terminal) of the whole machine and system should be separated from the earth. Each subsystem in the whole machine and system should have an independent common terminal. When data or signals need to be transmitted between subsystems, the earth should be used as the reference level. The grounding wire (surface) must be able to flow a large current, such as hundreds of amperes. The second protection method is to use voltage transient and surge protection devices in key parts of the whole machine and system (such as computer monitors, etc.), so that the voltage transient and surge are bypassed to the subsystem ground and the earth through the protection device, thereby greatly reducing the transient voltage and surge amplitude entering the whole machine and system. The third protection method is to use a combination of several voltage transient and surge protection devices for important and expensive whole machines and systems to form a multi-level protection circuit.

2 Surge protection methods

Surge protectors provide a simple, economical and reliable method for protecting electronic equipment from power surges. Through surge protection components (MOVs), they quickly transfer surge energy to the ground when lightning strikes or operating overvoltages occur, protecting the equipment from damage.

(1) Parallel surge protector connected in parallel to the power supply line

Under normal circumstances, the varistor in the lightning protection module is in a high-resistance state. When the power grid is struck by lightning or a transient surge overvoltage occurs during switch operation, the lightning arrester responds within nanoseconds, and the varistor is in a low-resistance state, quickly limiting the overvoltage to a very low amplitude.

When there is a long-lasting pulse or continuous overvoltage in the circuit, the performance of the varistor deteriorates and the temperature rises to a certain level, causing the thermal release mechanism to trip, thus avoiding fire and protecting the equipment.

(2) Series filter type surge protector connected in series to the power supply line

Provide safe and clean power for valuable electronic equipment. In addition to huge energy, lightning waves also have extremely steep voltage and current rise rates. Parallel surge protectors can only suppress the amplitude of lightning waves, but cannot change their sharply rising frontiers. Series filter power surge protectors are connected in series to the power supply line. In the case of overvoltage, MOV1 and MOV2 respond within nanoseconds to clamp the overvoltage; at the same time, the LC filter reduces the steep voltage and current rise rate of lightning waves by nearly 1,000 times, and the residual voltage by 5 times, thereby protecting sensitive user equipment.

(3) Install varistor-type limiting components between phases and lines of the power line to limit surge overvoltage.

The first method has a better protective effect on electrical equipment with a high level of impulse voltage resistance, such as lighting, elevators, air conditioners, and motors. However, for modern electronic equipment with high integration and compact structure, the actual protective effect is not so satisfactory. The reasons are as follows:

Taking the inductive lightning protection of single-phase 220V AC power supply as an example, the common method is to connect a suitable varistor between the neutral and ground wires to absorb and limit the peak voltage generated by the inductive lightning. The effectiveness of the lightning protection of the power line depends entirely on the selection of the parameters of the varistor and the reliability of the varistor. The selection of the varistor limit value is based on the peak value of 310V of the mains power, plus 20% of the influence of grid fluctuations, 10% of the device dispersion error and 15% of the reliability factors such as heating, moisture, and component aging caused by long-term work. The general value is 470V~510V. Various peak interference voltages such as inductive lightning are limited to 470V. For voltages below 470V, the varistor does not operate. The power frequency withstand voltage value of ordinary low-voltage electrical equipment (machine tools, elevators, lighting, air conditioning, etc.) is generally AC 1500V, and the instantaneous withstand voltage peak can reach more than 2500V, so the voltage of 470V is very safe. However, the working voltage of modern electronic equipment composed of large-scale integrated circuits is generally between ±5V and ±15V, and the maximum withstand voltage is generally not more than 50V. Therefore, the high-frequency peak voltage less than 470V superimposed on the mains will be directly sent to the load, and disproportionately transmitted to the switching power supply or integrated circuit chip through the spatial coupling capacitor, the transformer interlayer and inter-electrode capacitance, which can cause failure. Although high-frequency switching power supplies and electronic equipment have corresponding anti-spike interference measures, they are limited by cost and volume, and the intensity and spectrum of spike interference such as induced lightning strikes vary greatly, so the protection effect is not ideal. This is the effect obtained under the condition that the varistor is relatively ideal. In fact, due to the influence of the residual pressure of the varistor and the lead inductance, under strong induced lightning strikes, the actual limiting voltage peak may rise to more than 800V~1000V, which will threaten the subsequent electronic equipment.

(4) To enhance the protection of electronic equipment, an ultra-isolation transformer (also known as isolation method) is connected in series between the power supply and the load to isolate high-frequency spike interference, while facilitating secondary equipotential connection.

The isolation method mainly uses an isolation transformer with a shielding layer. Since common mode interference is a kind of interference relative to the earth, it is mainly transmitted through the coupling capacitor between the transformer windings. If a shielding layer is inserted between the primary and secondary and it is well grounded, the interference voltage can be shunted through the shielding layer, thereby reducing the interference voltage at the output end. In theory, a transformer with a shielding layer can achieve an attenuation of about 60dB. However, the quality of the isolation effect often depends on the process of the shielding layer. It is best to use a 0.2 mm thick copper plate, and add a shielding layer to the primary and secondary sides. Usually, the shielding layer of the primary side is connected to the shielding layer of the secondary side through a capacitor, and then connected to the ground of the secondary side. The shielding layer of the primary side can also be connected to the ground wire of the primary side, and the shielding layer of the secondary side can be connected to the ground wire of the secondary side. And the cross-sectional area of ​​the grounding lead should also be larger. Using an isolation transformer with a shielding layer is a good method, but it is larger in size.

This method has a limited market and few manufacturers because the transformer has a single function, is relatively large in size and weight, is not very convenient to install, and has poor protection against medium and low frequency spikes and surges. Therefore, it is generally not used unless in special occasions.

(5) Absorption method

The absorption method mainly uses absorbing devices to absorb surge spike interference voltage. Absorbing devices have a common feature, that is, they show high impedance below the threshold voltage, and once the threshold voltage is exceeded, the impedance drops sharply, so they have a certain inhibitory effect on the spike voltage. This type of absorbing devices mainly includes varistors, gas discharge tubes, TVS tubes, solid discharge tubes, etc. Different absorbing devices also have their own limitations in suppressing spike voltages. For example, the current absorption capacity of varistors is not large enough; the response speed of gas amplifier tubes is slow.

TVS (Transient Voltage Suppressor) is a new type of highly efficient device for absorbing spike pulses on the power supply line. TVS, also known as TVP, is a special voltage regulator diode in Chinese. When its two ends are subjected to a momentary high-energy impact, it can change the impedance value between the two ends from high impedance to low impedance at a very high speed, absorbing a momentary large current, thereby clamping the voltage between its two ends to a predetermined value, protecting the subsequent circuit components from the impact of transient high-voltage spike pulses. Because of this, its lightning protection effect is also very good. TVS tubes, like voltage regulator tubes, are applied in reverse. Unlike voltage regulator tubes, the maximum peak current that TVS tubes can withstand can even be as high as hundreds of amperes, the maximum pulse power can reach 5000W, and the clamping response time is only 10-12. Figure 1 is a schematic diagram of TVS for ordinary power supply lines. A bidirectional TVS tube is used here, which can effectively absorb the peak pulse voltage and lightning superposition voltage of the power grid. TVS has many uses. It can also be used as a lightning protection circuit and a protection and absorption circuit for various high-power devices.

3 Main technical parameters and requirements of surge protectors

The main technical parameters of surge protectors include the following aspects:

1. Protection level (residual level) Up

When a surge current passes through a surge protector, the voltage difference between the two ends of the protector is called residual voltage. The protection level refers to the residual voltage level at the protection end at the rated discharge current, that is, the clamping ability of the instantaneous clamping voltage. This is an important indicator for selecting a surge protector, because electrical and electronic equipment can only withstand a certain range of instantaneous overvoltage, such as telephone exchanges require less than 1000V, and host control parts require <700V, otherwise it may cause damage to the equipment.

2. Response time ta

The response time of the surge protector must be faster than the speed of the surge current. It is an important indicator of the surge protector and reflects the characteristics of the surge protector. The smaller the response time, the faster the surge transient voltage is suppressed. Generally, the response time of the surge protector of electronic equipment controlled by a computer should be <10ns, so as to achieve the purpose of protecting the electronic equipment.

3. Maximum surge current (discharge capacity) Imax

The maximum surge current refers to the maximum operating current of the surge protector to handle transient overvoltage. The larger the surge current Imax, the higher the reliability of the surge protector. Of course, the size of the capacity is determined by factors such as the strength of the surge in the minefield, the importance of the protective equipment, and the economic value.

4. Noise attenuation

Surge transient overvoltage generally causes microwaves and transient high-frequency noise. If the surge protector does not use a high-frequency filter module to filter microwaves and high-frequency noise, it will cause system disorder and aging of electronic components. This is an important indicator of the safety and reliability of the surge protection system.

5. Protection Mode

In order to ensure the safety of the protected system, a full-mode surge protection system must be used in the three-phase four-wire power supply with a ground wire.

6. Voltage

Nominal voltage Un: consistent with the rated voltage of the protected system, for example: 230/380V.

Working voltage: Ability to operate normally within the grid voltage fluctuation range.

Maximum continuous operating voltage Uc: The effective value of the maximum continuous operating voltage applied to the surge protector terminal. The Uc value must be consistent with the nominal voltage and within the specified range in the instructions for use.

7. Surge protection capability (lifespan)

The number of impacts a surge protector withstands under a certain waveform (usually 10kA 8/20μs 20kV waveform).

8. Automatic fault protection

Surge protectors must have fail-safe protection.

9. Insulation resistance: ≥1000MΩ

10. Auxiliary functions of surge protector

Status display, sound alarm, surge counting and remote monitoring functions.

11. Electromagnetic compatibility

Electromagnetic compatibility should comply with international and domestic standards.

4 Most commonly used components for surge protectors

The main ones are: Metal Oxide Varistor MOV, Silicon Avalanche Diode SAD, Filter Capacitors and Hybrid Types, etc.

(1) Metal Oxide Varistor MOV

It is a non-linear electronic component that allows large current to pass through to maintain a very low residual voltage at the terminal (specified end). When the metal oxide varistor encounters a voltage that exceeds its starting voltage instantaneously, it immediately changes from high impedance to low impedance, allowing the instantaneous huge surge to be discharged to the ground, keeping dangerous high voltage away from sensitive electronic equipment.

A typical example is the zinc oxide (ZnO) surge absorber, which is a polycrystalline semiconductor ceramic component made of ZnO material, added with a variety of transition metal oxides and processed by high temperature sintering. Due to the tunneling effect of the electrical microstructure, it has a nonlinear voltage-current characteristic curve similar to that of a Zener diode. In addition, the pulse energy of this component is almost dozens or hundreds of times that of a Zener diode. So far, this component has been widely used in power supply equipment or other low-frequency circuits to protect against lightning strikes and absorb switching surges.

(2) Filter capacitor

It can eliminate huge pulse hazards and filter high-frequency noise. When high pulses with amplitudes of tens to hundreds of volts enter the power supply, if they are not processed, these pulses will cause electronic system disorder and component degradation. Transient surges can be handled by metal oxide varistors and silicon avalanche diodes, but high-frequency noise cannot be eliminated, and filter capacitors can solve this problem.

(3) Hybrid

It is compatible with the large overcurrent capacity characteristics of metal oxide varistors, improving surge current conduction capabilities, and has the fast response and low clamping voltage characteristics of silicon avalanche diodes. It provides you with the lowest stable clamping voltage. At the same time, the filter capacitor eliminates high-frequency noise.

(4) NTC thermistor

Negative temperature coefficient thermistors for suppressing surge current can effectively suppress the surge current at the moment of power on to less than one tenth without affecting the normal operation of the instrument. In addition, their resistance is very small during normal operation, so the power dissipated is also very small. This type of component has been widely used in various types of switching power supplies.

(5) Transient Voltage Suppressor (TVS)

Transient voltage suppressor (TVS) is a special silicon diode avalanche device, so it is also called closed-type voltage suppression diode. Its working principle is similar to that of Zener diode. Its characteristics and symbols are the same as those of Zener diode, but the difference from general Zener diode is that TVS device has a large PN junction, which has the ability to withstand large instantaneous current. In addition, its reverse characteristic is a typical avalanche type, with low dynamic impedance and low clamping voltage during avalanche. As long as TVS is connected to the circuit to be protected, when transient voltage occurs, TVS will respond quickly (breakdown) to dissipate large surge current, and the circuit is kept at low voltage, so that the circuit can be protected.

(6) CSSPD

CSSPD is a control-maintain current type silicon surge protective device, which was successfully developed by Shindengen Industries in Japan in 1988. The device is a bidirectional two-terminal device, consisting of five layers of pnpnp, and is a composite device composed of reverse parallel connection on a single chip. The advantages of CSSPD are fast response speed, no need for multi-level protection circuits, large current resistance, small electrostatic capacitance and high reliability, and it is particularly suitable for lightning surge protection. Shindengen Industries has launched a variety of leaded and leadless CSSPDs, such as DV3-5, KP4, KP15N series, SV1-2 and VR-60 series. The company also launched a leadless bridge rectifier (four-terminal), model KW4R, which contains a CSSPD (KP4R20) and four rectifier diodes (S1WB60), which is particularly suitable for the protection of communication circuits and capacitive power supplies.

References :

1. Wang Shuiping, Fu Minjiang. Switching Power Supply. Xidian University Press, 1997

2. Chen Keming. Selection of electronic components for complete machines. World Electronic Components, 1998

3. Li Chengzhang, Wang Shufang. Principle of New UPS Uninterruptible Power Supply. Electronic Industry Publishing House, 1995

4. Yu Chengjie. Electromagnetic pulse protection characteristics of zinc oxide surge absorber. Piezoelectricity and Acousto-Optics, 1996.2