EMC design in PCB circuits

Ground design

 

Once an electrostatic discharge has occurred, it should be allowed to bypass the ground as soon as possible, and not directly invade the internal circuit. For example, if the internal circuit is shielded by a metal case, the case should be well grounded, and the grounding resistance should be as small as possible, so that the discharge current can flow into the ground from the outer layer of the case, and at the same time, the disturbance caused by the discharge of surrounding objects can be introduced to the ground without affecting Internal circuit. For a metal chassis, the circuits inside the chassis are usually grounded through I / O cables, power cords, etc. When an electrostatic discharge occurs on the chassis, the potential of the chassis rises, and the internal circuit is held near ground potential due to grounding. At this time, there is a large potential difference between the chassis and the circuit. This can cause a secondary arc between the chassis and the circuit. Damage to the circuit. By increasing the distance between the circuit and the case, secondary arcing can be avoided. When the distance between the circuit and the casing cannot be increased, a layer of grounded metal baffle can be added between the casing and the circuit to block the arc. If the circuit is connected to the chassis, it should be connected through only one point. Prevent current from flowing through the circuit. The point where the circuit board connects to the chassis should be at the cable entry. For plastic cases, there is no problem with grounding the case.

 

Cable design

 

A properly designed cable protection system may be the key to improving the ESD non-susceptibility of the system. As the largest "antenna" in most systems-I / O cables are particularly vulnerable to large voltages or currents induced by ESD interference. On the other hand, cables also provide low-impedance channels for ESD interference if the cable shield is connected to the chassis ground. Through this channel, ESD interference energy can be released from the system ground loop, thereby avoiding conductive coupling indirectly. In order to reduce the coupling of ESD interference radiation to the cable, the line length and loop area should be reduced. Common-mode coupling should be suppressed and metal shields used. For input / output cables, use shielded cables, common mode chokes, overvoltage clamp circuits, and cable bypass filters. At both ends of the cable, the cable shield must be connected to the housing shield. Installing a common mode choke on the interconnecting cable can cause the common mode voltage caused by electrostatic discharge to drop on the choke instead of the circuit on the other end. When the two chassis are connected with a shielded cable, the two chassis are connected together through the shield layer of the cable, so that the potential difference between the two chassis can be as small as possible. Here, the method of bonding between the chassis and the cable shield is important. It is strongly recommended that 360 ° overlap be made between the chassis at both ends of the cable and the cable shield.

 

Keyboard and panel

 

The design of the keyboard and control panel must ensure that the discharge current can flow directly to ground without passing through sensitive circuits. For insulated keyboards, install a discharge protector (such as a metal bracket) between the keys and the circuit to provide a discharge path for the discharge current. The discharge protector should be connected directly to the chassis or rack, not to the circuit ground. Of course, using a larger travel button (increasing the distance from the operator to the internal circuit) can directly prevent electrostatic discharge. The keyboard and control panel should be designed so that the discharge current can go directly to ground without going through sensitive circuits. The use of insulated shafts and large knobs prevents discharge to control keys or potentiometers. At present, many electronic product panels use thin-film keys and thin-film display windows. Since the thin film is made of a high-voltage-resistant insulating material, it can effectively prevent ESD from entering the internal circuit through the keys and the display window to cause interference. In addition, most of the keys of today's keyboards have pads made of high-voltage-resistant insulating films, which can effectively prevent ESD interference.

 

Circuit design

 

Unused input terminals in the device must not be left unconnected or suspended, but should be connected to the ground or power terminals directly or through appropriate resistors. In general, the interface circuits connected to external devices need to be protected, including power lines, which is often overlooked by hardware design. Taking the microcomputer as an example, the links of the protection circuit should be considered: serial communication interface, parallel communication interface, keyboard interface, display interface, etc.

 

Filters (shunt capacitors or a series of inductors or a combination of both) must be used in the circuit to prevent EMI from coupling into the device. If the input is high impedance, a shunt capacitor filter is most effective because its low impedance will effectively bypass the high input impedance. The closer the shunt capacitor is to the input, the better. If the input impedance is low, a series of ferrites can provide the best filter. These ferrites should also be as close to the input as possible.

 

Strengthen protective measures on internal circuits. For ports that may be subject to direct conducted electrostatic discharge interference, a resistor or a diode can be connected in series at the I / O interface to the positive and negative power terminals. The input terminal of the MOS tube is connected in series with a 100kΩ resistor, and the output terminal is connected in series with a 1kΩ resistor to limit the amount of discharge current. The input terminal of TTL tube is connected with 22 ~ 100Ω resistor in series, and the output terminal is connected with 22 ~ 47Ω resistor in series. The input end of the analog tube is connected in series with 100Ω ~ 100kΩ, and a parallel diode is added to shunt the discharge current to the positive or negative power supply. The output end of the analog tube is connected in series with a 100Ω resistor. Installing a capacitor to the ground on the I / O signal line can divert the electrostatic discharge current induced on the interface cable to the chassis to avoid flowing to the circuit. But this capacitor will also shunt the current on the case to the signal line. To avoid this, a ferrite bead can be installed between the bypass capacitor and the circuit board to increase the impedance of the path to the circuit board. It should be noted that the withstand voltage of the capacitor must meet the requirements. Electrostatic discharge voltages can reach thousands of volts. A transient protection diode can also effectively protect the electrostatic discharge, but it needs to be noted that although the voltage of the transient interference is limited by the diode, the high-frequency interference component has not been reduced. Generally, there should be The high-frequency bypass capacitor in parallel with the transient protection diode suppresses high-frequency interference. In circuit design and circuit board layout, gate circuits and strobes should be used. This input method can only cause damage if electrostatic discharge and gating occur at the same time. The pulse edge trigger input method is sensitive to transients caused by electrostatic discharge and should not be used.

 

PCB design

 

Good PCB design can effectively reduce the impact of ESD interference on products. This is also an important part of the ESD design part of electromagnetic compatibility design. You can get detailed guidance from that part of the course. When carrying out electromagnetic compatibility countermeasures on a finished product, it is difficult to redesign the PCB (improvement cost is too high), so it will not be introduced here.

 

software

 

In addition to hardware measures, software containment schemes are also a powerful way to reduce serious outages such as system lockups. Software ESD suppression measures fall into two common categories: refresh, check, and recover. Refreshing involves periodically resetting to a rest state and refreshing the display and indicator states. You only need to refresh it once and assume that the state is correct, and nothing else needs to be done. The check / restore process is used to determine whether the program is executed correctly. They are activated at certain intervals to confirm whether the program is performing a certain function. If these functions are not implemented, a recovery procedure is activated.

 

General ESD countermeasures: 

(1) Add protection diodes to susceptible CMOS and MOS devices;

(2) Strings of tens of ohms of resistance or ferrite beads on the susceptible transmission line (including the ground);

(3) The use of electrostatic protective surface coating technology makes it difficult for ESD to discharge the movement, which has proven to be very effective;

(4) Use shielded cables as much as possible;

(5) Install a filter at the susceptible interface; isolate the sensitive interface where the filter cannot be installed;

(6) Select the logic circuit with low pulse frequency;

(7) The shield of the shell is well grounded.