Electromagnetic Compatibility (EMC) Protection Measures

    Abstract: An ideal electronic product should be able to withstand any external electromagnetic interference. Adopting linear filtering, reasonable circuit board layout, correct power supply design and shielding measures can effectively reduce active interference. Electromagnetic interference mainly comes from the power supply or is transmitted through air radiation from capacitive, magnetic, and electromagnetic components. This article mainly discusses how to protect equipment from damage due to the impact of harmful external voltage or current.

    Relevant departments have long recognized the importance of EMC protection and formulated corresponding equipment compatibility standards. For example, IEC1000-4 standard test methods, which include: IEC1000-4-2 electrostatic discharge (ESD) test method, IEC1000-4-4 fast transient (FTB) test method and IEC1000-4-5 high energy transient response ( Surge) test methods, all test methods are based on actual models of electrical noise.

1. EMC protection circuit

    isolation

    Because the signal circuit cannot withstand kilovolt voltage, this interference must be eliminated before the input circuit, which can be converted into a current signal and then converted into heat for consumption. Ground loop current can enter the interface and flow through the entire circuit, generally requiring galvanic isolation. Isolation is an effective method in industrial systems with long connecting wires or large ground loop currents.

    An ESD pulse with a peak value of 30A will produce a resistive voltage drop of tens of millivolts on the ground line, but its steep rise time (30A/ns) can produce up to 1nH/cm on the same line (assuming a line inductance of 1nH/cm) An induced voltage of several hundred volts is enough to cause erroneous data. Such a high frequency will produce a skin effect, significantly increasing line resistance. To counteract this effect, a large area ground is required to obtain a low resistance characteristic.

    Fast rising pulses will generate FTB and ESD interference that will couple capacitively into the low noise area. When solving this problem, people often mistakenly add extra windings to the main power transformer to provide an isolated power supply. This method can only cause the interference to spread further and affect the entire circuit.

    A low-cost solution is to use a MAX253 driver to form a forward voltage converter to provide the isolation voltage. The MAX253 has a small package that provides an effective noise barrier. The microtransformer requires a coupling capacitance of less than 10pF, a power of 1W, and an isolation voltage of 1kV (see Figure 1).

    Gas discharge tube

    A butterfly capacitor filled with neon gas. When the voltage exceeds 100V, a plasma zone is generated that can limit the maximum voltage. It can withstand larger currents and has smaller leakage currents. The gas discharge tube can absorb high-voltage transient pulses. However, since it takes a certain amount of time to generate a plasma zone, it cannot absorb fast transient pulses and cannot be used as a primary means of protection.

    Varistor

    A protective device made of metal oxides (mainly zinc). Its function is similar to a Zener diode, and its response speed is faster than that of a gas discharge tube, but its leakage current is relatively high, especially when the signal is close to the clamping voltage.

    Transzorb diodes

    are used to limit fast transients in low-voltage signals, and their power dissipation capabilities are limited by their size. Similar to the varistor, there is a large leakage current when it is close to the breakdown voltage. The node capacitance is large and is decoupled with a diode bridge in fast transient systems.

    ESD structure is

    a novel design scheme that integrates bidirectional diodes in MAX202E, MAX485E and other RS-232/RS-485 transceiver chips (newly launched analog switches, such as MAX4558, also integrate similar functions). They feature low capacitance and low leakage current, making them suitable for ESD and FTB protection.

    Chokes and ferrites

    can attenuate high-frequency and rapidly changing voltage peaks, but cannot absorb additional energy. To avoid resonance, it is always used together with a capacitive attenuator (similar to a T-structure LC filter). These devices are often used to suppress common-mode interference and serve as the main filtering device.

    Capacitor

    is one of the important protection components. Parameters that need to be considered in specific applications include: equivalent series resistance (ESR), magnetic induction coefficient, rated current and rated voltage.

    Series resistor

    is one of the important and cheap protection devices. Properly selected resistance value and power dissipation value can replace many expensive protection devices.

2. Application of EMC Protection Circuit

    Thermocouple

    In order to avoid signal distortion due to the influence of 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 to the input end of the instrumentation amplifier through a multiplexer, and then sent to the A/D converter (ADC) to be converted into a digital signal. The digital output signal of the ADC is transmitted through an optical or magnetic coupler.

    Each electrode of the thermocouple is protected with 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. This capacitor will ESD interference is bypassed to ground to maintain isolation from DC current. It also forms a capacitive voltage divider to reduce the peak voltage of the isolated power supply. To further limit the peak voltage, a varistor can be connected in parallel with the capacitor. The 2kΩ resistor in the picture must be able to withstand high voltage (8kV ESD) and be able to dissipate considerable power during FTB and surge testing. However, since leakage current will cause static signal errors when flowing through the protection series resistor, the leakage current generated by multiplexers, buffer amplifiers, etc. needs to be considered.

    The MAX4052A multiplexer is pin-compatible with the industry standard device 4052 and ensures that the maximum leakage current does not exceed 5nA within the extended temperature range. The typical leakage current is 2pA at 25°C, and the maximum possible error is only 2μV. This error is acceptable for most thermocouples. If an instrumentation amplifier is used as signal buffer (using the MAX4524 quad op amp), the leakage current will be reduced to 100pA over the extended temperature range, with a typical value of 1pA at 25°C. In addition, the extremely low temperature drift coefficient of the input offset voltage (only 0.3μV/℃) makes this buffer very suitable for high-impedance, small-amplitude signal sources.

    Another alternative is to use the MAX1402 monolithic signal collector. The chip includes a Σ-Δ A/D converter, a buffer amplifier, a multiplexer, and a current source for sensor excitation. It has very low input leakage. current, greatly simplifying system design.

    Angle encoder

    Angle encoder can be used to measure the position of the motor rotor. Precision positioning systems use dual-channel, orthogonal differential sinusoidal signals as high-precision rotor position pointers. Such systems often require the use of RS-485/422 serial bus. To set the encoder initialization parameters, sometimes these transmission lines need to transmit analog signals of several kHz or digital signals with a rate of several Mbit/s over long distances (see Figure 3). In this case, it is impossible to use a large value series resistor or a passive resistor-capacitor network as a protection circuit. In the figure, the terminal resistor (usually 120ohm) is used to prevent signal reflection. The system first needs to provide ESD and FTB interference protection. In a traditional data transceiver (Figure 4), the differential transmitter output voltage is limited by a Transzorb diode, and the receiver also has the same protection. In order to meet the asymmetry of the common-mode voltage indicators of the transmitter and receiver (EIA-422A: -7V to +12V), an asymmetric protection network is required. The transmitter and receiver inputs have the same common mode range and can use the same Transzorb diode. The entire protection network can also be replaced with the MAX490E RS-422 transceiver, which integrates ESD and FTB protection circuitry. In practical applications, the ground of the transceiver must be connected to the chassis/earth as short a distance as possible. If a shielded wire is used, the shielding layer also needs to be connected to this point as short a distance as possible. When there is a large exchange current between two separate ground points, a 100Ω resistor can be connected in series between the shield and the ground, preferably with a bypass capacitor with a low ESR value.

    If the system requires surge protection, an external protection network is used. One preferable method is to connect current-limiting resistors in series with the line terminals. This is easy to implement on the receiving end and produces only a tiny signal drop. On the transmitting end, you need to confirm whether a series resistance of approximately 10Ω is acceptable, because the differential output impedance of the MAX490E is approximately 40Ω. In practical circuits, PTC fuses are generally connected in series on the data lines (Figure 4).

    The choice of standard signal interface

    signal transmission mode depends on the signal source and signal transmission distance of the system. For broadband and long-distance transmission, it is often necessary to convert the signal to a high level: 0V to 10V, -10V to +10V, 4mA to 20mA, Or use differential analog signals, and some systems transmit digital signals through wired, optical fiber, wireless, etc. In any case, the sensor needs to be installed as close to the signal source as possible to reduce the impact of noise.

    A ±10V interface is often used to set target positions 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 larger amplitude signals pass through the chip, the system passes unaffected. the IC. If interference causes the protected terminal signal to exceed the supply voltage (positive terminal or negative terminal), the line protector will present a high impedance to the fault signal. It can withstand fault voltages up to 36V (up to ±40V during power outage).