EMC protection circuit and its application

Electromagnetic compatibility ( EMC ) refers to the ability of a device or system to operate in accordance with requirements in its electromagnetic environment and not to cause intolerable electromagnetic interference to any device in its environment. Therefore, EMC includes two requirements: on the one hand, the electromagnetic interference generated by the device to the environment during normal operation cannot exceed a certain limit; on the other hand, it means that the device has a certain degree of immunity to the electromagnetic interference in the environment, that is, electromagnetic sensitivity.

1. EMC protection circuit

isolation

Because signal circuits cannot withstand kilovolt voltages, this interference must be eliminated before it enters the circuit, where it can be converted into a current signal and then into heat for consumption. Ground loop current can enter the interface and flow through the entire circuit, generally requiring current isolation. Isolation is an effective method in industrial systems with long connection lines or large ground loop currents.

A peak ESD pulse of 30A will produce a resistance drop of tens of millivolts on the ground line, but its steep rise time (30A/ns) can produce an induced voltage of up to hundreds of volts on the same line, which is enough to cause erroneous data. Such a high frequency will produce a skin effect, which will significantly increase the line resistance. In order to offset this effect, a large area grounding is required to obtain low resistance characteristics.

Fast rising pulses will generate FTB and ESD interference, which will be coupled to the low noise area through capacitance. When solving this problem, people often mistakenly add additional 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.

Gas Discharge Tube

A butterfly capacitor filled with neon gas. When the voltage exceeds 100V, a plasma 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 protection device made of metal oxides, primarily zinc. It functions similarly to a Zener diode, responding faster than a gas discharge tube, but with higher leakage current, especially when the signal is close to the clamping voltage.

Transzorb Diode

Used to limit fast transients of low voltage signals, its power dissipation capacity is limited by its size. Similar to varistors, it has a large leakage current when approaching the breakdown voltage.

ESD Structure

A novel design that integrates bidirectional diodes into MAX202E, MAX485E, and other RS-232/RS-485 transceiver chips. They have low capacitance and low leakage current characteristics, suitable for ESD and FTB protection.

Chokes, Ferrites

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

Capacitors

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

Series resistance

It is one of the important and cheap protection devices. By properly selecting the resistance value and power dissipation value, it can replace many expensive protection devices.

2. Application of EMC protection circuit

Thermocouple

To avoid signal distortion due to the influence of ground loop current, most thermocouple applications provide galvanic isolation between signal acquisition and signal processing. As shown in Figure 2, the differential signal is fed to the input 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 by a simple low-pass RC network (2kΩ & 100nF). In addition, a 1nF capacitor with a high voltage rating is connected between the circuit common and the equipment cabinet ground. This capacitor bypasses the ESD interference to the ground and maintains the isolation of the DC current. It also forms a capacitive voltage divider to reduce the peak voltage of the isolated power supply. Since leakage current will cause static signal errors when flowing through this protective series resistor, leakage current generated by multiplexers, buffer amplifiers, etc. needs to be considered.

The MAX4052A multiplexer is pin-compatible with the industry-standard 4052 device, and guarantees a maximum leakage current of no more than 5nA over the extended temperature range, with a typical leakage current of 2pA at 25°C, and a maximum error of only 2μV. This error is acceptable for most thermocouples. If an instrumentation amplifier is used as a 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 input offset voltage temperature drift coefficient (only 0.3μV/°C) makes this buffer very suitable for high-impedance, small-amplitude signal sources.

Angle encoder

Angle encoders can be used to determine 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 encoder initialization parameters. Sometimes these transmission lines need to transmit analog signals of several kHz or digital signals of several Mbit/s over long distances (see Figure 3). In this case, large-value series resistors or passive RC networks cannot be used as protection circuits. In the figure, the terminal resistor (usually 120ohm) is used to prevent signal reflection. 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 MAX490E RS-422 transceiver can also be used to replace the entire protection network, which integrates ESD and FTB protection circuits. When there is a large exchange current between two discrete ground points, a 100Ω resistor can be connected in series between the shield layer 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. A desirable method is to connect a current-limiting resistor in series at the line terminal. This is easy to implement at the receiving end, and it will only produce a small signal drop. At the transmitting end, it is necessary to confirm whether a series resistor of about 10Ω is acceptable, because the differential output impedance of the MAX490E is about 40Ω. In practical circuits, a PTC fuse is generally 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. Its application environment is very noisy, and a wiring fault will cause damage to the 24V industrial power supply. 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 larger signal passes through the chip, the system passes through the IC unaffected. If interference causes the protected end signal to exceed the power supply voltage (positive or negative), the line protector will present a high impedance to this fault signal. It can withstand a fault voltage of 36V (up to ±40V when powered off).