Now that technology is rapidly developing, this article will give you an in-depth explanation of EMC protection circuits and their applications , and I hope to help you.
Electromagnetic Compatibility (EMC) refers to the ability of a device or system to operate in its electromagnetic environment without causing unacceptable electromagnetic interference to any device in its environment. Therefore, EMC includes two requirements: on the one hand, the electromagnetic interference generated by the equipment in the normal operation process cannot exceed a certain limit; on the other hand, the appliance has certain electromagnetic interference in the environment. The degree of immunity, ie electromagnetic sensitivity.
First, EMC protection circuit
isolation
Because the signal circuit cannot withstand the kilovolt-level voltage, this interference must be removed before the input circuit, which can be converted into a current signal and then converted into heat. The ground loop current can enter the interface and flow through the entire circuit, typically requiring galvanic isolation. Isolation is an effective method in industrial systems where the length of the connection is long or the ground loop current is large.
A 30A peak ESD pulse produces a tens of volts of resistive voltage drop across the ground, but its steep rise time (30A/ns) can produce up to several hundred volts of induced voltage on the same line, enough to cause The generation of erroneous data, such a high frequency will produce a skin effect, resulting in a significant increase in line resistance. In order to counteract this effect, large-area grounding is required to obtain low-resistance characteristics.
Fast rising pulses will generate FTB and ESD interference, capacitively coupled to the low noise region. In solving this problem, it is often wrong to add additional windings to the main Power Transformer to provide an isolated power supply. This method can only lead to further interference and affect the entire circuit.
Gas discharge tube
A butterfly capacitor filled with helium. When the voltage exceeds 100V, a plasma zone is generated to limit the maximum voltage. It can withstand large currents and has a small leakage current. The gas discharge tube can absorb high-voltage transient pulses.
Varistor
A protective device made of a metal oxide (mainly zinc). Its function is similar to that of a Zener diode, and its response speed is faster than that of a gas discharge tube, but the leakage current is relatively high, especially when the signal is close to the clamp voltage.
Transzorb diode
Used to limit fast transients of low voltage signals, the power dissipation capability is limited by their size. Similar to varistor, there is a large leakage current near the breakdown voltage.
ESD structure
A novel design that integrates bidirectional diodes into the MAX202E, MAX485E, and other RS-232/RS-485 transceiver chips. They feature low capacitance and low leakage current for ESD and FTB protection.
Choke, ferrite
It can attenuate high frequency and fast variable voltage peaks, but can not absorb extra energy. To avoid resonance, it is always used with capacitive attenuators (similar to T-type LC filters), which are often used to reject common-mode interference and act as the primary filter.
Capacitor
It is one of the important protection components. The parameters to be considered in specific applications include: equivalent series resistance (ESR), magnetic induction coefficient, rated current and rated voltage.
Series resistance
One of the important and inexpensive protection devices, the proper selection of resistance values ​​and power dissipation values ​​can replace many expensive protection devices.
Second, the application of EMC protection circuit
Thermocouple
To avoid signal distortion due to ground loop current, galvanic isolation is provided between signal acquisition and signal processing in most thermocouple applications. As shown in Figure 2, the differential signal is fed through a multiplexer to the input of the instrumentation amplifier and then sent to an A/D converter (ADC) for conversion to a digital signal, and the digital output signal of the ADC is transmitted through an optical or magnetic coupler.
Each electrode of the thermocouple is protected by a simple low-pass RC network (2kΩ & 100nF). In addition, a 1nF capacitor with a high voltage rating is required between the circuit common and the equipment cabinet ground. ESD interference is bypassed to ground to maintain DC current isolation. It also forms a capacitive divider that reduces the peak voltage of the isolated supply. Since the leakage current will cause a static signal error when flowing through the protection series resistor, it is necessary to consider the leakage current generated by the multiplexer, the buffer amplifier, and the like.
The MAX4052A multiplexer pin is pin compatible with the industry-standard device 4052. It guarantees a maximum leakage current of no more than 5nA over the extended temperature range and a typical leakage current of 2pA at 25°C. The maximum possible error is only 2μV. This error is acceptable for most thermocouples. If an instrumentation amplifier is used for signal buffering (using the MAX4524 quad op amp), the leakage current will be reduced to 100pA over the extended temperature range and typically 1pA at 25°C. In addition, the extremely low input offset voltage drift factor (only 0.3μV/°C) makes this buffer ideal for high impedance, small amplitude sources.
Angle encoder
The angle encoder can be used to determine the position of the rotor of the motor. The precision positioning system uses a two-channel, quadrature differential sinusoidal signal as the position pointer of the high-precision rotor. Such a system often needs to use RS-485/422 serial bus to set the code. Initialization parameters, sometimes these transmission lines need to transmit analog signals of several kHz or digital signals of several Mbit/s at a long distance (see Figure 3). In this case, a large resistance series resistor or a passive RC network cannot be used as the protection circuit. In the figure, the termination resistor (usually 120 ohm) is used to prevent signal reflection. To meet the asymmetry of the common-mode voltage specifications of the transmitter and receiver (EIA-422A: -7V to +12V), an asymmetric protection network is required. The MAX490E RS-422 transceiver can also be used instead of the entire protection network, which integrates ESD and FTB protection circuitry. When there is a large exchange current between two discrete ground points, a 100Ω resistor can be placed in series between the shield and ground, preferably with a bypass capacitor with a low ESR value.
If the system requires surge protection, an external protection network is used. A preferred method is to connect a current limiting resistor in series with the line termination. This is easy to implement at the receiving end, it only produces tiny signal drops. At the transmitter, verify that the series resistance of approximately 10 Ω is acceptable because the differential output impedance of the MAX490E is approximately 40Ω. In practical circuits, PTC fuses are usually connected in series on the data line.
Standard signal interface
A ±10V interface is often used to set the target position in motor control applications. The application environment is very noisy. Once the wiring fails, the 24V industrial power supply will be damaged. The signal line protectors MAX4506 and MAX4507 have an on-resistance of 60Ω and a maximum leakage current of 20nA over the full temperature range, providing an excellent interface protection. When a large amplitude signal passes through the chip, the system passes unaffected. The IC. If the interference causes the protection signal to exceed the supply voltage (positive or negative), the line protector will exhibit a high impedance to this fault signal. It can withstand a fault voltage of 36V (±40V when power is off).
In summary, this article has explained the EMC protection circuit and its application . I believe that everyone's understanding of EMC protection circuit and its application is deeper and deeper. I hope this article can have a relatively large reference value for readers.
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