01

Selection of PPTC self-recovery fuse

02

Selection of transient suppression diode TVS

TVS, as a member of the diode family, has many models, so how should we choose the TVS model that suits us? In fact, it is very simple: 1. Determine the DC voltage or continuous working voltage of the circuit to be protected. If it is AC, the maximum value should be calculated, that is, the effective value *1.414. 2. The reverse displacement voltage of TVS tube is the working voltage (VRWM)-- choose VRWM of TVs equal to or greater than the operating voltage specified in step 1 above. This ensures that the current absorbed by TVS can be neglected under normal working conditions. If the voltage specified in step 1 is higher than VRWM of TVS, TVS will absorb a large amount of leakage current and be in an avalanche breakdown state, thus affecting the operation of the circuit. 3. The maximum clamping voltage (VC) of the selected TVS should be lower than the maximum withstand voltage allowed by the protected circuit. 4. Maximum peak pulse power: Determine the interference pulse condition of the circuit, and determine the peak pulse power of TVS that can effectively suppress the interference according to the waveform and pulse duration of the interference pulse. 5. After the maximum clamping voltage of TVS is determined, its peak pulse current should be greater than the transient surge current. 6. Unipolar or Bipolar-It is often misunderstood that bidirectional TVS is used to suppress reverse surge pulses, but this is not the case. Bidirectional TVS is used for alternating current or from positive and negative bidirectional pulses. TVS is sometimes used to reduce capacitance. If the circuit only has a positive level signal, then one-way TVS is sufficient. TVS operation mode is as follows: TVS is in reverse avalanche breakdown state during forward surge; In reverse surge, TVS conducts like a forward-biased diode and absorbs surge energy. This is not the case in low capacitance circuits. Bidirectional TVS should be selected to protect the low capacitance devices in the circuit from reverse surge. 7. If you know the accurate surge current IPP, then you can use VC to determine its power. If the power range cannot be determined, generally speaking, it is better to choose a larger power. 8. One-way TVS diode is generally used for DC protection, two-way TVS diode is generally used for AC protection, TVS array device is used for multi-channel protection, and special protection module for TVS is used for high-power protection. In special cases, such as RS-485 and RS-232 protection, bidirectional TVS diode or TVS array can be selected. 9.TVS diode can work between -55℃ and +150℃. If TVS needs to work at a variable temperature, its reverse leakage current ID increases with the increase of temperature; The power consumption decreases with the increase of junction temperature of TVS, so the influence of temperature change on its characteristics should be considered when selecting TVS. 10.TVS diodes can be applied in series/parallel, with serial connection for voltage division and parallel connection for current division. However, considering the discreteness of TVS, the number of serial/parallel should be reduced as much as possible.

03

GDT selection of ceramic gas discharge tube

Ceramic discharge tube is sealed with ceramic. The inside of ceramic discharge tube is made up of two or more metal electrodes with gaps, which are filled with inert gases such as argon and neon. When the voltage applied to the two electrode terminals reaches the level of gas breakdown in the gas discharge tube, the gas discharge tube begins to discharge, changing from high impedance to low impedance, so that the surge voltage is quickly short-circuited to near zero voltage, and the overcurrent is released into the ground, thus protecting the subsequent circuit. Since ceramic gas discharge tube has such a function, how should we choose the type of ceramic gas discharge tube?

Under the rapid pulse impact, it takes a certain time (generally 0.2 ~ 0.3 μ s, and the fastest one is about 0.1μs) for gas ionization in ceramic gas discharge tube, so a sharp pulse with high amplitude will leak to the back.

To suppress this spike, there are several ways:
A, connecting a capacitor or a varistor in parallel with the discharge tube;

B, connecting an inductor in series behind the discharge tube or leaving a transmission line with an appropriate length to attenuate the sharp pulse to a lower level;

C, Adopt two-stage protection circuit, with discharge tube as the first stage and TVS tube or semiconductor discharge tube as the second stage, and the two stages are isolated by resistance, inductance or self-recovery fuse.

Selection of DC breakdown voltage Vsdc: The minimum value of DC breakdown voltage Vsdc should be more than 1.2 times of the highest possible peak voltage of power supply or the highest signal voltage.Selection of impulse discharge current: according to the maximum surge current that may occur on the line or the maximum surge current that needs protection. The impulse discharge current of the discharge tube should be calculated according to the nominal impulse discharge current (or half of the single impulse discharge current).

Ceramic gas discharge tubes are generally not used in parallel because of large breakdown voltage error.Free-wheeling problem: In order to make the discharge tube normally extinguish the arc after impact breakdown, in places where free-wheeling may occur (such as active circuits), series varistor or self-recovery fuse can be connected to the discharge tube to limit the free-wheeling, so that it is smaller than the holding current of the discharge tube.

In addition, it should be noted that the discharge tube with appropriate volume should be selected according to the product size.

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