Generation and protection of lightning surge of switching power supply
Analysis of the flow loop of a lightning surge generated by a lightning surge in a switching power supply (common mode signal and differential mode signal) A design of a switching power supply circuit for preventing lightning surges.
The reliability of the lightning surge circuit is artificially generated and the lightning resistance surge circuit is tested.
Lightning strikes caused by lightning surges are a kind of discharge phenomenon caused by the close proximity of charged clouds or charged clouds and the ground. This discharge process produces intense lightning and loud sounds with a lot of energy transfer. There are three main types of lightning strikes: direct lightning, conducted lightning and inductive lightning. With the understanding and in-depth study of the formation of lightning bodies, people have already had better protection measures against the catastrophic damage of lightning strikes and conducted lightning, but indirect lightning (such as lightning strikes within clouds, between clouds, or adjacent objects) The lightning strike can still induce surge voltage and current on the outdoor overhead line. In addition, when the large power switch is switched. Large surge voltages and currents are also induced on the power supply line: surges in the electromagnetic compatibility field are typically derived from this lightning strike transient and switching transients.
2 electronic product surge (lightning strike) damage mechanism
2.1 Surge (lightning strike) access to electronic equipment
The way of lightning strikes electronic devices can be divided into two situations: 1) high-energy lightning shockwaves through outdoor transmission lines, connection lines between devices, and power line intrusion devices. The electronic equipment connected in the middle of the line or the terminal is damaged; 2) lightning strikes the ground or the grounding conductor, causing a local instantaneous ground potential rise. Spreads nearby electronic equipment, causing impact on the equipment and damaging its insulation to the ground.
2.2 Surge damage mechanism of electronic equipment
Generally, the surge pulse has a long rise time, a wide pulse width, does not contain a high frequency component, and enters the device through conduction. The effect of longitudinal (common mode) impact on the components of the balance circuit of the equipment is: damage to the components connected to the ground and the ground or its insulating medium: breakdown of the transformer between the line and the equipment, the inter-layer, or Line to ground insulation, etc. Lateral (differential mode) shocks can also be transmitted in the circuit. A semiconductor device that damages the internal circuit's capacitive inductance and poor impact resistance. The degree to which the components in the device are damaged by surge. Depending on the insulation level of the component and the strength of the impact: for insulation with self-recovery, the breakdown is only temporary. Once the impact disappears, the insulation is quickly restored. Some non-self-recovering insulating media, if only a small current flows after breakdown. Often does not immediately interrupt the operation of the device, but over time. The wetted insulation of the component will gradually decrease, the circuit characteristics will deteriorate, and finally the circuit will be interrupted. Some components, such as the collector and emitter of the transistor or the emitter and base, if reverse breakdown occurs. Permanent damage to components that are susceptible to energy damage. The degree of damage depends mainly on the current flowing through it and its duration.
3 Comprehensive protection against surges
3.1 lightning protection of buildings
According to the scope of protection, the protective measures of electronic products can be divided into two categories, external protection and internal protection. External protection refers to the safety protection of the building body on which electronic products are installed. Lightning rods, shunts, shielded nets, balanced potentials, and grounding can be used. People pay more attention to these protective measures, and the application is relatively common, relatively perfect. Internal protection refers to the protection of over-voltage (lightning or over-voltage inside the power system) of electronic products inside the building. The measures include: equipotential bonding, shielding, protection and isolation, reasonable wiring and use of overvoltage protectors.
3.2 Surge suppression method for electronic products
The surge (lightning strike) protection measures described above can in principle greatly reduce the possibility of electronic products suffering from surge (lightning strike) damage, providing a relatively safe environment for electronic products. However, it is not enough to ensure that electronic products are protected from surges by these measures. Only by simultaneously improving the resistance of electronic products to lightning surges can a complete integrated surge protection system be formed. The surge impact is mainly transmitted to the inside of the electronic product through the AC/DC power supply and the signal/control line connected to the outside, which poses a hazard to the product. To effectively prevent the impact of surges on the product, surge suppression devices must be installed at the AC and DC power ports and signal/control ports of the product to absorb the surge and prevent them from entering the product and posing a hazard to the circuit.
The biggest characteristic of lightning surge is that the energy is particularly large, so the scheme of filtering and absorbing by ordinary filter and ferrite core is basically invalid, and gas discharge tube, varistor, silicon transient absorption diode and semiconductor discharge tube must be used. Special surge suppression devices are required. The basic use of surge suppression devices is to directly connect the surge absorbing device in parallel with the protected device to absorb or transfer energy to surge voltages that exceed the device's ability to withstand. A common feature of surge suppression devices is that their impedance is different when there is a surge voltage and when there is no surge voltage. At normal voltages, its impedance is high and has no effect on the operation of the circuit. When a high surge voltage is applied to it, its impedance becomes very low, bypassing the surge energy. This type of device is used in parallel between the line and the reference ground and is quickly turned on when a surge voltage occurs to limit the voltage amplitude to a certain value.
Varistors, transient suppression diodes, and gas discharge tubes have different volt-ampere characteristics, so the changes that occur when surges pass through them are different. As shown in Figure 1, the changes in surges through these three devices were compared.
4 Common Surge Suppression Device Features and Applications
4.1 Metal oxide varistor (MOV)
The varistor is made of a metal oxide (mainly zinc oxide) material and is a clamp-type device. Its characteristics are very similar to those of two back-to-back Zener tubes, with nanosecond response speed. The varistor's ability to absorb transient signals is proportional to its volume: its thickness is proportional to the voltage; the area is proportional to the current. Varistors are currently the most widely used surge suppression devices in electronics. When the voltage across the varistor exceeds a certain range, the resistance of the resistor is greatly reduced, thereby discharging the surge energy. Under the influence of the surge voltage, the voltage on the varistor after conduction (generally referred to as the clamp voltage) is equal to the current flowing through the varistor multiplied by the resistance of the varistor, so the peak value of the surge current The clamp voltage is at its highest. 4.1.1 Characteristics of varistor:
a) Advantages: Wide voltage range, from a few volts to several thousand volts; absorption surge current can range from tens to thousands of amps, fast response, no polarity, no freewheeling, high peak current withstand capability, price low. b) Disadvantages: The clamp voltage is higher, generally can reach 2 to 3 times of the working voltage; moreover, as the number of surge impacts increases, the leakage current increases; in addition, the response time is longer and the parasitic capacitance is larger. c) Applicable occasions: DC power line, low frequency signal line, or in series with the gas discharge tube for use on the AC power line.
4.1.2 Selection of varistor a) From the perspective of suppressing transient interference, the varistor voltage should be reduced as close as possible to the operating voltage of the protected circuit; from the perspective of improving the life of the component, it is necessary to open the gap between the two. . The general compromise selection scheme is: for the AC working circuit, the varistor voltage value is 2.2 times the working voltage; for the DC working circuit, the varistor voltage value is 1.5 times the working voltage. b) Selection of flow rate: In practical applications, the maximum surge current absorbed by the varistor should be less than its maximum flow rate. For the same application, when the maximum flow rate is doubled, the life of the varistor is also doubled simultaneously.
4.2 Silicon Transient Voltage Absorption Diode (TVS)
TVS is a voltage clamp type of operation, sub-nanosecond response speed. TVS is available in a variety of packages to meet the needs of different occasions. When the voltage on the TVS exceeds a certain range, the device turns on quickly. The surge energy is released by reverse overpressure avalanche breakdown of the PN junction. Since these devices have low impedance after turn-on, their clamping voltage is very flat and very close to the operating voltage.
4.2.1 Characteristics of Silicon Transient Voltage Absorption Diodes a) Advantages: short response time, low leakage current, and small breakdown voltage. The clamping voltage is low (relative to the operating voltage), the operation accuracy is high, there is no following current (freewheeling), the volume is small, the performance is not degraded after each transient voltage is applied, and the reliability is high. b) Disadvantage: Since all power is dissipated on the PN junction of the diode, it is subjected to a small amount of power and allows a small current to flow. A typical TVS device has a large parasitic capacitance, such as used on a high speed data line. Special low-capacitance parts are used, but the low-capacitance parts tend to be less rated. c) Applicable occasions: occasions where the surge energy is small. If the surge energy is large. To be used with other high-power surge suppression devices, use it as a post-level protection.
4.2.2 Selection of Silicon Transient Voltage Absorption Diodes
a) The maximum clamp voltage VCMAX should be no greater than the maximum allowable safe voltage of the current. b) The maximum reverse operating voltage VRWM should not be lower than the maximum operating voltage of the circuit. Generally slightly higher than the operating voltage of the circuit. c) The maximum pulse power rated by the TVS must be greater than the maximum transient surge power present in the circuit. d) For protection of small current loads, a suitable current limiting resistor can be connected in series with the diode. Therefore, a TVS with a small peak absorption power can be selected to perform this function. 4.3 Gas discharge tube The gas discharge tube is made of ceramic sealed package. The inside is composed of two or several metal electrodes with gaps, which are filled with inert gas (argon or helium). When the voltage applied to the ends of the two electrodes reaches a breakdown of the gas in the gas discharge tube, the gas discharge tube begins to discharge, and the device becomes short-circuited so that the voltage across the electrodes does not exceed the breakdown voltage. Once the gas discharge tube is turned on, the voltage across it will be low. The gas discharge tube has two poles and three poles, which can be used for protection between the line and the line.
4.3 gas discharge tube
The gas discharge tube is made of a ceramic hermetic package, and the inside is composed of two or a plurality of metal electrodes with a gap, and is filled with an inert gas (argon or helium). When the voltage applied to the ends of the two electrodes reaches a breakdown of the gas in the gas discharge tube, the gas discharge tube begins to discharge, and the device becomes short-circuited so that the voltage across the electrodes does not exceed the breakdown voltage. Once the gas discharge tube is turned on, the voltage across it will be low. The gas discharge tube has two poles and three poles, which can be used for protection between the line and the line. 4.3.1 Characteristics of gas discharge tubes
a) Advantages: high current withstand, high insulation resistance, low leakage current and low parasitic capacitance. b) Disadvantages: high ignition voltage, high residual voltage, slow reaction time (≥100 ns), low operating voltage accuracy, chronic air leakage, photosensitive effect, and large dispersion. There is a follower current (freewheeling). If the time of following the current is long, the discharge tube contacts will burn out quickly, thereby shortening the life of the discharge tube. c) Applicable occasions: The signal line or working voltage is lower than the DC power line that conducts the sustain voltage (generally less than 10 V); it is combined with the varistor for use on the AC power line. It has a strong impact current absorption capability. However, it has a high arcing voltage, so it is more suitable for the first level of rough protection.
4.3.2 Selection of gas discharge tube
The nominal voltage of the gas discharge tube in the DC circuit is selected to be 1.8 times the operating voltage: 2.5 times the effective value of the operating voltage is selected in the AC circuit. The nominal current capacity of the gas discharge tube should be greater than the maximum surge impact capacity of the protected circuit. Due to the following current (freewheeling), the gas discharge tube is generally not usable in the DC circuit unless the DC operating voltage is lower than the breakdown sustaining voltage of the gas discharge tube.
4.4 Other Surge Absorption Devices 4.4.1 Solid-state discharge tubes are a new type of transient voltage absorbing device. They are the same energy transfer protection devices as gas discharge tubes, but the performance is better. For example, the on-state voltage drop is only about 3 V, close to short circuit; nanosecond response speed; stable operating voltage; long service life; capable of absorbing positive/negative transient voltages in both directions.
The solid discharge tube has a certain junction capacitance; the trigger voltage is slightly higher than the DC breakdown voltage in the pulse state (for example, a 200 V tube has a pulse trigger voltage of 350 V), which is much better than a gas discharge tube. The failure mode of a solid discharge tube is a short circuit. The point is that it will not expand the fault. It also makes it easy for duty personnel to find faults and handle faults in time.
4.4.2 Thyristor type protection device
There are two types of thyristor protection devices: a) Control gate type bidirectional three-terminal devices, such as SCR, TRLAO, etc. Because most power supply circuits have voltage overload protection at the output, the control gate with a level-triggered SCR shorts the output and interrupts the supply. The response time is about 100 IXs, which can cause damage to voltage-sensitive devices. It has the advantage of high current consumption. The disadvantage is that the ignition voltage is easy to change and the response time is slow. b) Control the current-maintaining bidirectional terminal device. It consists of five layers of PNPNP, and its structure is a composite device composed of reverse parallel on a single chip. The device also has the advantages of fast response rate, no multi-level protection circuit, large current consumption, small electrostatic capacity and high reliability, and is especially suitable for protecting lightning surges.
4.5 Gas discharge tube and varistor combination application
Gas discharge tubes and varistors are not suitable for use on AC power lines alone. A practical solution is to use a gas discharge tube in series with a varistor. If a capacitor is connected in parallel with the varistor, the voltage can be applied to the gas discharge tube more quickly when the surge voltage comes. Shorten the on time. The combination of such a gas discharge tube and a varistor can avoid the above disadvantages. Another benefit is that the limit voltage can be reduced. A varistor with a lower turn-on voltage can be used, thereby reducing the limiter voltage value. The suppression of the surge voltage by this connection method is shown in Fig. 2. The combined protection scheme can take advantage of the different characteristics of different protection devices to achieve the best protection
5 Electronic product surge protection design
The surge resistance of the product is tested by the surge (impact) immunity test. This test item is suitable for use when electrical and electronic equipment is operating under specified operating conditions. A reaction to a surge (shock) voltage at a certain level of damage caused by a switch or lightning. This test item is applicable to the AC power supply 1:3 test of electrical and electronic equipment powered by a public power supply network. It is also suitable for testing power, control, and signal ports with outdoor wires and cables. There are two modes of application: common mode and differential mode. Therefore, in the product design, it is necessary to take corresponding suppression measures against the common/differential mode surge of these ports.
5.1 Surge suppression of the power port
An ideal AC power surge suppression scheme is shown in Figure 3. It takes full advantage of the respective advantages of different absorption devices.
The ideal working condition is: when a surge arrives. TVS starts first. The instantaneous overvoltage will be accurately controlled to a certain level; if the surge current is large. Then the varistor is then activated and a certain surge current is discharged; the voltage at both ends is increased until the discharge of the gas discharge tube of the preceding stage is pushed. Discharge large currents to the ground. The circuit combines the advantages of fast action, low voltage limit and high discharge capability. The middle filter inductor acts as a high frequency filter (absorbing the leading edge high frequency energy of the surge pulse) and interstage isolation.
For 220 V/50 Hz AC power systems, the third-stage TVS can take 380 V rated voltage products. The second stage varistor is available in a 470 V rated voltage product. The first stage gas discharge tube is selected for 550 V rated voltage products. The first stage varistor is available in a 400 V rated voltage product. In order to reduce the reflection time of the front gas discharge tube, a high frequency capacitor of 1 000 pF to 10 000 pF can be connected in parallel to the front varistor.
The current capacity of the first protection circuit should be greater than the maximum current capacity that the circuit can withstand. The surge current capacity of the second- and third-stage protection circuits can be gradually reduced. For products where the surge voltage does not require too high a test level, the first stage gas discharge tube and varistor series circuit and the corresponding interstage isolation inductance can be omitted. For products that are not sensitive to the residual voltage of the protector, the TVS protection circuit of the third stage and the corresponding isolation inductor between the stages can be omitted. Due to the limited current absorption capability of the TVS, it is generally not used separately on the AC power port. This clipping does not affect the selection of the rated voltage of the protection device as exemplified above, but the current capacity of the protection circuit should be varied accordingly. For the DC power port, there is always one pole grounded. We can take the combined protection circuit as shown in Figure 4. This circuit is only properly tailored to Figure 3 and works the same way.
There is one point to note in this protection circuit: if the inrush current to be tolerated by the device under test is not very large, it is recommended not to use the first stage gas discharge tube; if the DC circuit operating voltage is greater than 10 V, the first stage gas discharge tube unavailable. At this time, the surge level requirement of the device can be satisfied by increasing the current capacity of the second-stage varistor. For products that are not sensitive to the residual voltage of the protector, the TVS protection circuit of the third stage can be omitted. In this circuit, the rated voltage of the gas discharge tube should be greater than or equal to 1.8 times the working voltage, and the rated voltage of the varistor should be greater than or equal to 1.5 times the operating voltage. The current capacity of the foremost protection component should be greater than the maximum inrush current. The current capacity of the latter protection circuit can be decremented step by step. 5.2 Surge suppression of communication port The technical requirements of the surge suppression circuit of the communication interface are high, because in addition to meeting the surge protection requirements, the transmission index must be guaranteed to meet the requirements. In addition, the equipment connected to the communication line has a low withstand voltage and strict requirements for surge residual voltage, so it is difficult to select a protective device. The ideal surge suppression circuit should be small in capacitance, low in residual voltage, large in current, and fast in response.
As shown in FIG. 5, the communication-connected El combination protection circuit is actually a modification of FIG. 4, but the high-frequency filter inductor of FIG. 4 is replaced with a PTC-type self-recovery fuse in order to meet the requirements of high-speed signal transmission of the communication interface. The normal working impedance of the PTC is approximately zero, and has no adverse effect on the communication line. When the surge arrives, the TVS and the varistor are turned on, and the large inrush current passes through the PTC, and the PTC becomes a high-resistance state after being heated. The large surge voltage is divided to protect the subsequent surge suppression components and communication circuits; when the surge disappears, the PTC temperature drops, returning to the normal low resistance state, and the communication circuit is restored to the normal state. If the communication circuit requires a wide interface impedance, a low impedance resistor can be used instead of the PTC to reduce the line cost. This circuit is suitable for single-channel communication interfaces for unbalanced transmission. For balanced communication interfaces, the T2 channel should also be symmetrical as Tl channel plus PTC. If it is a multi-channel communication interface, the protection circuit of each channel is the same. For the balanced transmission communication interface, when the device is a metal casing, it is also necessary to consider the surge impact between the device and the casing ground. The protection circuit of each communication line to the ground is the same as the circuit of FIG. 5, and only T2 is replaced by The outer casing can be used. The rated voltage of each protection component should be compatible with the peak value of the normal operating voltage of the communication interface, and the current should be compatible with the maximum surge current.
The protection circuit needs to be noted that if the communication interface circuit contains a DC signal with an absolute value exceeding 10 V (such as a telephone network containing 48 V DC), the gas discharge tube is not available; the varistor has a large capacitance and is only suitable for audio communication. Signal transmission. For the high-frequency interface protection circuit without DC, the second-stage varistor can be eliminated. This protection circuit can reach a frequency of several tens of MHz. (If the communication circuit contains DC, the arc-extinguishing voltage should be higher than the working DC. The gas discharge tube; or the protection circuit consists only of PTC and TVS, at which time the surge protection capability is low). Higher frequency protection is mainly based on the discharge tube, otherwise it is difficult to meet the transmission requirements.
5.3 Surge suppression of the antenna port
Antenna ports are a type of interface that is very susceptible to surge damage. The external antenna port of the wireless communication device generally needs to be connected with the antenna at the outdoor high to realize the transmission and reception of the wireless signal. The antenna ports of the AV products are also connected to outdoor antennas or CATV systems, which are connected to outdoor leads. Although the antennas in the outdoor high places should generally be protected by lightning rods, there is also a pre-stage (lightning strike) surge protector protection after entering the room. However, on the one hand, lightning rods and protectors are not necessarily protected (these protective measures are also difficult to be discovered by product users. Generally, surges are found to be ineffective before the product is destroyed); on the other hand, these outdoor antennas are very It may be installed by the user (such as the outdoor TV antenna in rural areas), and the protection measures are missing. In addition, the antennas of the products are long-term connections. Unless the products are moved, they are usually disconnected after being connected. These characteristics determine that the product antenna port is vulnerable to surges, unfortunately. The circuits connected to the product antenna port are low voltage electronic circuits that are very sensitive to surges, so surge protection for the antenna ports is necessary.
The RF coaxial antenna port combination protection circuit is shown in Figure 6. The pre-stage protection circuit of the circuit is composed of a gas discharge tube, and the post-stage protection circuit is composed of a TVS and a high-frequency choke inductor L. The purpose of adding the inductor L is to prevent the high frequency signal on the antenna from being short-circuited to ground by the TVS inter-electrode capacitance. To reduce the high frequency attenuation of the protection circuit. The interstage isolation resistor is removed. This protection circuit has an operating frequency limit of up to 2 GHz. If the antenna port contains DC (such as powering the preamplifier antenna), a gas discharge tube with an arc extinguishing voltage higher than the operating DC should be used. There are also protection circuits that use high-pass filters. Because the energy spectrum of the surge is concentrated between tens of hertz and one megahertz, the energy is mainly concentrated below tens of kilohertz, and the high frequency operating frequency is relatively low with respect to the antenna port, and the surge can be passed through the high-pass filter. Separate and absorb in the working signal. For the point frequency communication antenna, a quarter-wavelength short-circuit line can also be used to form a band-pass filter, and the lightning protection effect is better. However, both methods will short-circuit the DC transmitted on the antenna, and its application range is limited.
5.4 Surge suppression of other signals/control ports
For other signal cavity ports, if the port wiring comes from outside or the line length exceeds a certain length. Then the corresponding port is in danger of being damaged by the induced surge. It is also necessary to take corresponding surge suppression measures. If the working signal is DC level, the surge suppression mode can be designed by referring to the surge suppression mode of the DC power port; if the working signal is a medium and low frequency signal, the surge suppression mode can be referred to the surge suppression mode of the communication port. Design; if the working signal is a high frequency signal, the surge suppression mode can be designed by referring to the surge suppression mode of the antenna terminal.
However, it should be noted that if the port is isolated by a transformer or an optocoupler, in order to prevent the transformer or optocoupler from being broken by the surge, in addition to the surge suppression between the interface wires, the interface wire is also between the ground terminals of the product. There should be a corresponding surge suppression circuit. In order to ensure the electrical isolation of the internal and external circuits, only the gas discharge tube can be used for surge suppression. In order to ensure the gas discharge tube after the breakdown of the surge can normally extinguish the arc. There should be no DC potential difference greater than 10 V at both ends of the transformer or optocoupler isolation.
5.5 Ground line rebound suppression
When a parallel type surge suppressor works. It bypasses the surge energy to the ground. Because the ground wire has a certain impedance. Therefore, when current flows through the ground, there is a voltage on the ground. This phenomenon is generally referred to as ground bounce. When the ground of the surge suppressor is not at the same point as the ground of the device, the line of the device is not actually protected. Higher surge voltages are still added between the device's power line and ground. The solution is to connect a surge suppressor in parallel between the line (ground) and the enclosure (ground) of the device, or to select the two locations at the same point. Protected devices are connected to other devices. Due to ground bounce, another device is subject to common mode voltage. This common mode voltage appears on all cables connecting device 1 (protected device) to device 2 (unprotected device). The solution is to install a surge suppressor at the end of the device 2 that interconnects the cables.
6 Conclusion
With the increasing integration and widespread use of semiconductor devices, electronic products are becoming more and more fragile and less resistant to surges. In order to ensure the safety of electronic products, it is necessary to understand the ways in which surges invade products and the mechanism of damage, and find corresponding countermeasures to improve the product's surge resistance. This paper discusses some aspects of surge damage mechanism, surge suppression countermeasures and product anti-surge design. And introduced various aspects of the surge countermeasure device. To facilitate product designer reference and selection.
References: [1] 1 GB 3483-83, lightning test test guidelines for electronic equipment [S] [2] Yang Jishen, product development and certification of electromagnetic compatibility technology [M], Beijing: Electronic Industry Press. 2004. [3] Qian Zhenyu, Electromagnetic Compatibility Testing and Countermeasures in 3C Certification [M], Beijing: Publishing House of Electronics Industry. 2004. [4] Chen poor, electromagnetic compatibility engineering design manual [M], Beijing: National Defense Industry Press. Author of the author: Author: China Saibao Laboratory E-mail:
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