Knowledge about surge current and surge suppressor

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Knowledge about surge current and surge suppressor [Copy link]

Inrush current refers to the peak current that flows into the power supply device when the power is turned on. Since the input filter capacitor charges quickly, the peak current is much larger than the steady-state input current. The power supply should limit the surge level that the AC switch, rectifier bridge, fuse, and EMI filter components can withstand. With repeated switching loops, the AC input voltage should not damage the power supply or cause the fuse to blow. Inrush current also refers to the non-repetitive maximum forward overload current that causes the junction temperature to exceed the rated junction temperature due to abnormal circuit conditions.

Classification

1. Discharge gap (also called protection gap):

It is generally composed of two metal rods exposed to the air with a certain gap between them. One of the metal rods is connected to the power phase line L1 or the neutral line (N) of the equipment to be protected, and the other metal rod is connected to the ground wire (PE). When the instantaneous overvoltage strikes, the gap is broken down, and part of the overvoltage charge is introduced into the earth, avoiding the voltage increase on the protected equipment. The distance between the two metal rods of this discharge gap can be adjusted as needed, and the structure is relatively simple. 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. It extinguishes the arc by the action of the electric force F of the circuit and the rising effect of the hot air flow.

2. Gas discharge tube:

It is composed of a pair of cold cathode plates separated from each other and encapsulated in a glass tube or ceramic tube filled with a certain amount of inert gas (Ar). In order to increase the probability of triggering 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 tripod type.

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

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

Use under AC conditions: U dc ≥ 1.44Un (Un is the effective value of the AC voltage when the line is working normally)

3. Varistor:

It is a metal oxide semiconductor nonlinear resistor with ZnO as the main component. When the voltage applied to its two ends reaches a certain value, the resistor is very sensitive to voltage. Its working principle is equivalent to the series and parallel connection of multiple semiconductor PNs. The characteristics of varistor are good nonlinear characteristics (nonlinear coefficient α in I=CUα), large current capacity (~2KA/cm2), small normal leakage current (10-7~10-6A), low residual pressure (depending on the working voltage and current capacity of the varistor), fast response time to transient overvoltage (~10-8s), and no continuous current.

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

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

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

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

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

4. Suppression diode:

The suppressor diode has a clamping voltage limiting function. It works in the reverse breakdown region. Because it has the advantages of low clamping voltage and fast action response, it is particularly suitable for use as the last few levels of protection components in a multi-level protection circuit. The volt-ampere characteristic of the suppressor diode in the breakdown region can be expressed by the following formula: I=CUα, where α is the nonlinear coefficient. For Zener diodes, α=7~9, and for avalanche diodes, α=5~7.

The technical parameters of the suppressor diode are mainly

(1) Rated breakdown voltage, which refers to the breakdown voltage at a specified reverse breakdown current (usually LMA). The rated breakdown voltage of a Zener diode is generally in the range of 2.9V to 4.7V, while the rated breakdown voltage of an avalanche diode is usually in the range of 5.6V to 200V.

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

(3) Pulse power: It refers to the product of the maximum clamping voltage at both ends of the tube and the current in the tube under a specified current waveform (such as 10/1000μs).

(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, under which the tube should not break down. This reverse displacement voltage should be significantly higher than the highest operating voltage peak of the protected electronic system, that is, 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 tube 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 ferrite as the core. It consists of two coils of the same size and number of turns symmetrically wound on the same ferrite ring core to form a four-terminal device. It has a large inductance to suppress the common-mode signal, but has a very small leakage inductance to have almost no effect on the differential-mode signal. Choke coils can effectively suppress common-mode interference signals (such as lightning interference) in balanced lines, while having no effect on the differential-mode signals transmitted normally on the line.

The choke coil should meet the following requirements when it is manufactured:

1) The wires wound on the coil core must be insulated from each other to ensure that there is no breakdown or short circuit between the turns of the coil under the action of transient overvoltage.

2) When a large instantaneous current flows through the coil, the magnetic core should not be saturated.

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

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

6. 1/4 wavelength short circuit

The 1/4 wavelength short-circuit device is a microwave signal surge protector made based on the spectrum analysis of lightning waves and the standing wave theory of antenna feed lines. The length of the metal short-circuit rod in this protector is determined by the size of 1/4 wavelength of the working signal frequency (such as 900MHZ or 1800MHZ). For the working signal frequency, the impedance of this parallel short-circuit rod is infinite, which is equivalent to an open circuit and does not affect the transmission of the signal. However, for lightning waves, since the lightning energy is mainly distributed below n+KHZ, the impedance of this short-circuit rod is very small, which is equivalent to a short circuit, and the lightning energy level is discharged into the ground.

Since the diameter of the 1/4 wavelength short-circuit rod is generally a few millimeters, it has good impact current resistance performance, which can reach more than 30KA (8/20μs), and the residual voltage is very small. This residual voltage is mainly caused by the self-inductance of the short-circuit rod. Its disadvantage is that the power frequency band is narrow, with a bandwidth of about 2% to 20%. Another disadvantage is that it cannot add DC bias to the antenna feed facility, which limits some applications.