A brief introduction to EMC common components (2)
Electromagnetic compatibility components are the key to solve the problems of electromagnetic interference emission and electromagnetic sensitivity, and the correct selection and use of these components is the premise of doing a good job in electromagnetic compatibility design. Since each electronic component has its own characteristics, it should be carefully considered in the design. Some common electronic component and circuit design techniques used to reduce or suppress EMC are discussed next.
1. Transient suppression diode (TVS)
Transient suppression diode (TVS tube) is a special diode made of semiconductor silicon material. It is used in parallel with the circuit. When the circuit is normal, TVS is in the off state and presents high impedance. Transform from high impedance to low impedance to absorb the surge power, and make the surge voltage pass through itself to the ground, thus protecting the circuit from damage. It is characterized by short action time, high voltage amplitude, and large transient energy. The transient voltage superimposed on the working voltage of the circuit will cause the circuit to be damaged due to "overvoltage".
(1) Main characteristic parameters of TVS diodes
①Maximum reverse leakage current ID and rated reverse turn-off voltage VWM.
VWM is the maximum continuous working DC or pulse voltage of TVS. When this reverse voltage is added between the two poles of TVS, it is in the reverse off state, and the current flowing through it should be less than or equal to its maximum reverse leakage current ID.
② minimum breakdown voltage VBR.
VBR is the minimum avalanche voltage of TVS. At 25C, before this voltage, TVS is non-conductive. When the TVS flows through the specified test current, the voltage added between the two poles of the TVS is its minimum breakdown voltage VBR. According to the degree of dispersion between VBR of TVS and the standard value, TVS can be divided into two types: ±5%VBR and ±10%VBR. For ±5%VBR, VWM=0.85VBR; for ±10%VBR, VWM=0.81VBR.
③ Maximum clamp voltage Ve and maximum peak pulse current Ipp.
When the pulse peak current Ipp with a duration of 20μs flows through TVS, the maximum peak voltage that appears between its two poles is Vc. It is a combination of the voltage rise across the series resistor and due to the temperature coefficient. Vc and Ipp reflect the surge suppression capability of TVS devices. The ratio of Vc to VBR is called the clamping factor, generally in the range of 1.2 to 1.4.
④ Capacitance C.
The capacitance C is determined by the TVS avalanche junction cross section and measured at a specific frequency of 1MHz. The size of C is proportional to the current bearing capacity of TVS, if C is too large, the signal will be attenuated. Therefore, C is an important parameter for selecting TVS for the data interface circuit.
⑤ Maximum peak pulse power consumption PM.
PM is the maximum peak pulse power dissipation that the TVS can withstand. For the specified test pulse waveform and PM values of various TVS, please refer to the relevant product manuals. Under a given maximum clamping voltage, the greater the power consumption PM, the greater its surge current tolerance; under a given power consumption PM, the clamping voltage V. The lower it is, the greater its ability to withstand surge current.
In addition, the peak pulse power consumption is also related to the pulse shape, duration and ambient temperature. Moreover, the transient pulse that TVS can bear is non-repetitive. The pulse repetition frequency (ratio of duration to intermittent time) specified by the device is 0.01%. If repetitive pulses appear in the circuit, the "accumulation" of pulse power should be considered. It is possible to damage the TVS.
⑥ Clamping time TC.
TC is the time for the voltage across the TVS to go from zero to the minimum breakdown voltage VBR. For unipolar TVS, it is generally 1X10-12 seconds; for bipolar TVS, it is generally 1X10-11 seconds.
(2) Transient suppression diode selection and precautions
① Determine the maximum DC or continuous working voltage of the protected circuit, the rated standard voltage of the circuit and the "high end" tolerance.
②TVS rated reverse turn-off VWM should be greater than or equal to the maximum working voltage of the protected circuit. If the selected VWM is too low, the device may enter an avalanche or affect the normal operation of the circuit due to too much reverse leakage current. Connect in series to divide the voltage, and connect in parallel to divide the current.
③The maximum clamping voltage V of TVS. It should be less than the damage voltage of the protected circuit.
④In the specified pulse duration, the maximum peak pulse power consumption PM of TVS must be greater than the peak pulse power that may appear in the protected circuit. After determining the maximum clamping voltage, its peak pulse current should be greater than the transient surge current.
⑤ For the protection of the data interface circuit, attention must also be paid to selecting a TVS device with a suitable capacitance C.
⑥ Select the polarity and packaging structure of TVS according to the application. It is more reasonable to choose bipolar TVS for AC circuit; it is more favorable to choose TVS array for multi-line protection.
⑦ Temperature considerations. The transient voltage suppressor can work between -55℃~+150C. If the TVS needs to work at a changing temperature, because its reverse leakage current ID increases with the increase of temperature; the power consumption decreases with the increase of TVS junction temperature, from +25°C to +175°C, approximately linearly down by 50% , The breakdown voltage VBR increases by a certain coefficient with the increase of temperature. Therefore, it is necessary to consult relevant product information and consider the influence of temperature changes on its characteristics.
(3) Typical application
TVS is mainly used in 485 interface, 232 interface, USB interface, VGA interface, etc., which require anti-static and hot-swappable ports.
2. Gas discharge tube
A gas discharge tube (GDT) is a device consisting of one or more discharge gaps sealed in the medium of the gas discharge tube (in air not under atmospheric pressure), used to protect equipment or people from high voltage hazards.
(1) Main characteristic parameters of gas discharge tube
●DC breakdown voltage (100V/s).
●Impact breakdown voltage (1000V/μus).
●Insulation resistance.
● inter-electrode capacitance.
(2) Selection and Precautions of Gas Discharge Tube
①Under fast pulse impact, it takes a certain amount of time (generally 0.2~0.3μs) for the gas ionization of ceramic gas discharge tube, so a sharp pulse with a higher amplitude will leak to the back . To suppress this spike, a two-level protection circuit is generally considered, with a gas discharge tube as the first level, a TVS diode or a semiconductor discharge tube as the second level, and a resistance, inductance or self-recovery fuse between the two levels. .
② Selection of DC breakdown voltage Vsdc: The minimum value of DC breakdown voltage Vsdc should be greater than 1.2 times the highest possible peak power supply voltage or the highest signal voltage.
③ Selection of impulse discharge current: It should be selected according to the maximum surge current that may appear on the line or the maximum surge current that needs to be protected. The discharge tube impulse discharge current should be calculated according to the nominal impulse discharge current (or half of the single impulse discharge current).
④Follow current problem: In order to make the discharge tube normally extinguish the arc after the impact breakdown, in places where freewheeling may occur (such as in active circuits), a varistor or self-recovery fuse can be connected in series with the discharge tube to limit the arc. Freewheeling, making it less than the sustaining current of the discharge tube.
Pay attention to the following items:
Ceramic gas discharge tubes cannot be used directly on the power supply for differential mode protection.
The breakdown voltage should be greater than the maximum signal frequency voltage on the line.
The withstand current cannot be less than the maximum abnormal current that may occur on the line.
The pulse breakdown voltage must be less than the protected line voltage.
(3) Typical applications
Gas discharge tubes are mainly used in AC power supply, DC power supply interface, 485 circuit, video interface, XDSL, Ethernet interface and other interfaces that require lightning protection.
3. Semiconductor discharge tube
Semiconductor discharge tube (TSS), also known as surge suppression thyristor, is a PNPN junction four-layer structure device made by semiconductor technology. Its volt-ampere characteristics are similar to thyristors and have typical switching characteristics. TSS is generally connected in parallel in the circuit. Under normal working conditions, TSS is in the cut-off state. When abnormal overvoltage occurs in the circuit due to induction lightning, operating overvoltage, etc., TSS quickly conducts the discharge current to protect the back-end equipment from abnormal overvoltage. After the abnormal overvoltage disappears
, the TSS returns to the cut-off state.
(1) The main characteristic parameters of the semiconductor discharge tube
VprRM reverse cut-off voltage (off-state repetitive peak voltage): also known as off-state repetitive peak voltage, the rated highest (peak) instantaneous voltage applied at the off-state moment including all DC and repetitive voltage components Voltage. IprM reverse maximum leakage current (off-state repetitive peak current): also known as off-state repetitive peak current, refers to the
maximum (peak) off-state current generated by applying off-state repetitive peak voltage VpRM. Iu: The minimum current to maintain the on-state of the thyristor.
(2) Selection and Precautions
The following points should be paid attention to when selecting a semiconductor discharge tube:
①The maximum instantaneous peak current Ipp must be greater than the specified value of the communication equipment standard. For example, the Ipp of FCC Part68A type should be greater than 100A; the Ipp of Bellcore 1089 should be greater than 25A.
② The breakover voltage VBO must be less than the maximum instantaneous peak voltage allowed by the protected circuit.
③When the semiconductor discharge tube is in the conduction state (conduction), the power loss P should be less than its rated power Pcm, Pcm=KVTXIpp, where K is determined by the waveform of the short-circuit current. For exponential wave, square wave, sine wave and triangular wave, K values are 1.00, 1.4, 2.2, 2.8 respectively
④ The reverse breakdown voltage VR must be greater than the maximum working voltage of the protected circuit. For example, in POTS applications, the sum of the peak voltage (150X1.41=212.2V) of the maximum ringing voltage (150V) and the peak value of the DC bias voltage (56.6V) is 268.8V, so a device with a VBR greater than 268.8V should be selected. Another example is that in ISDN applications, the sum of the maximum DC voltage (150V) and the maximum signal voltage (3V) is 153V, so a device with a VBR greater than 153V should be selected.
⑤ To make the semiconductor discharge tube self-reset after a large surge current, the holding current IH of the device must be greater than the current value that the system can provide. That is IH (system voltage/source impedance).
(3) Typical applications
Semiconductor discharge tubes are mainly used in interfaces that require lightning protection such as 485 circuits, video interfaces, XDSL, and telephone interfaces.