PCB Technical - Anti interference design of PCB circuit

PCBcircuit anti-interference in the electronic system design, in order to avoid detours and save time, we should fully consider and meet the anti-interference requirements, and avoid taking anti-interference remedial measures after the design is completed. There are three basic elements of interference:

(1) Interference source refers to the element, equipment or signal generating interference. It is described in mathematical language as follows: Du / DT, where di / DT is large, it is the interference source. For example, lightning, relay, thyristor, motor and high-frequency clock may become interference sources.

(2) Propagation path refers to the path or medium through which interference propagates from interference source to sensitive device. Typical interference propagation paths are conduction through wires and radiation in space.

(3) A sensitive device is one that is easily disturbed. Such as: A / D, D / a converter, single chip microcomputer, digital IC, weak signal amplifier, etc.

The basic principle of anti-interference design is to suppress the interference source, cut off the interference propagation path and improve the anti-interference performance of sensitive devices. (similar to the prevention of infectious diseases)

1 suppression of interference sources

Suppressing the interference source is to reduce the Du / dt and di / dt of the interference source as much as possible. This is the highest priority and most important principle in anti-interference design, which often achieves twice the result with half the effort. Reducing Du / dt of interference source is mainly realized by paralleling capacitors at both ends of interference source. Reducing di / dt of interference source is realized by connecting inductance or resistance in series and adding freewheeling diode in interference source circuit.

Common measures to suppress interference sources are as follows:

(1) A freewheeling diode is added to the relay coil to eliminate the back EMF interference generated when the coil is disconnected. Only adding freewheeling diode will delay the disconnection time of the relay. After adding zener diode, the relay can act more times in unit time.

(2) Connect spark suppression circuit (generally RC series circuit, resistance is generally several K to tens of K, capacitance is 0.01uF) at both ends of relay contact in parallel to reduce the impact of electric spark.

(3) Add filter circuit to the motor, and pay attention to the lead of capacitance and inductance as short as possible.

(4) Each IC on the circuit board shall be connected with a 0.01 in parallel μ F～0.1 μ F high frequency capacitor to reduce the influence of IC on power supply. Pay attention to the wiring of high-frequency capacitor. The wiring should be close to the power end and as thick and short as possible. Otherwise, it will increase the equivalent series resistance of the capacitor and affect the filtering effect.

(5) Avoid 90 degree broken lines during wiring to reduce high-frequency noise emission.

(6) Both ends of the thyristor are connected with RC suppression circuit in parallel to reduce the noise generated by the thyristor (when the noise is serious, the thyristor may be broken down).

According to the propagation path of interference, it can be divided into conduction interference and radiation interference.

The so-called conducted interference refers to the interference transmitted to sensitive devices through wires. The frequency band of high-frequency interference noise is different from that of useful signal. The propagation of high-frequency interference noise can be cut off by adding a filter on the conductor, and sometimes an isolation optocoupler can be added. Power supply noise is the most harmful, so special attention should be paid to treatment. The so-called radiation interference refers to the interference transmitted to sensitive devices through space radiation. The general solution is to increase the distance between the interference source and the sensitive device, isolate them with ground wire and add a mask on the sensitive device.

The common measures to cut off the interference propagation path are as follows:

(1) Fully consider the influence of power supply on MCU. If the power supply is well done, more than half of the anti-interference of the whole circuit is solved. Many single chip computers are very sensitive to power supply noise. Filter circuit or voltage regulator should be added to the single chip microcomputer power supply to reduce the interference of power supply noise to single chip microcomputer. For example, a π shaped filter circuit can be composed of magnetic beads and capacitors. Of course, when the conditions are not high, a 100 Ω resistor can also be used to replace the magnetic beads.

(2) If the I / O port of single chip microcomputer is used to control noise devices such as motor, isolation shall be added between the I / O port and noise source (π shaped filter circuit shall be added). For noise control devices such as motor, isolation shall be added between I / O port and noise source (π shaped filter circuit shall be added).

(3) Pay attention to crystal oscillator wiring. The crystal oscillator and the single chip microcomputer pin shall be as close as possible, the clock area shall be isolated with ground wire, and the crystal oscillator shell shall be grounded and fixed. This measure can solve many difficult problems.

(4) The circuit board shall be divided reasonably, such as strong and weak signals, digital and analog signals. Keep the interference source (such as motor, relay) away from the sensitive element (such as single chip microcomputer) as far as possible.

(5) Use ground wire to isolate the digital area from the analog area, separate the digital ground from the analog ground, and finally connect to the power ground at one point. The wiring of a / D and D / a chips is also based on this principle. The manufacturer has considered this requirement when allocating the pin arrangement of a / D and D / a chips.

(6) The ground wire of single chip microcomputer and high-power devices should be grounded separately to reduce mutual interference. High power devices shall be placed on the edge of the circuit board as far as possible.

(7) The anti-interference performance of the circuit can be significantly improved by using anti-interference components such as magnetic beads, magnetic rings, power filter and shielding cover in key places such as MCU I / O port, power line and circuit board connecting line.

3 improve the anti-interference performance of sensitive devices

Improving the anti-interference performance of sensitive devices refers to the method of minimizing the pickup of interference noise and recovering from abnormal state as soon as possible.

Common measures to improve the anti-interference performance of sensitive devices are as follows:

(1) During wiring, the area of loop shall be reduced as much as possible to reduce induced noise.

(2) When wiring, the power line and ground wire shall be as thick as possible. In addition to reducing the voltage drop, it is more important to reduce the coupling noise.

(3) For the idle I / O port of single chip microcomputer, do not hang in the air, but ground or power supply. The idle terminals of other ICs are grounded or connected to the power supply without changing the system logic.

(4) Use power monitoring and watchdog circuit for single chip microcomputer, such as imp809, imp706 and imp813