Grounding design of PCB
Once electrostatic discharge occurs, it should be bypassed as soon as possible, and should not directly invade the internal circuit. For example, if the internal circuit is shielded by a metal chassis, the chassis 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 chassis, and at the same time, the harassment caused by the discharge of surrounding objects can also be introduced into the earth Internal circuit. For metal cases, the circuits in the case are usually grounded via I/O cables, power cords, etc.
When electrostatic discharge occurs on the case, the potential of the case rises, and the internal circuits are kept near ground due to grounding. At this time, there is a large potential difference between the chassis and the circuit. This will cause a secondary arc between the chassis and the circuit. Cause damage to the circuit. By increasing the distance between the circuit and the housing, secondary arcing can be avoided. When the distance between the circuit and the casing cannot be increased, a 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 only be connected through one point. Prevent current from flowing through the circuit. The connection point between the circuit board and the chassis should be at the cable entrance. For plastic chassis, there is no problem of chassis grounding.
Cable design of PCB boardA properly designed cable protection system may be the key to improving the non-susceptibility of the system to ESD. As the largest "antenna" in most systems-I/O cables are particularly susceptible to large voltage or current induced by ESD interference. On the other hand, the cable also provides a low-impedance channel 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, so that conductive coupling can be avoided indirectly. To reduce ESD interference radiation coupling to the cable, the wire length and loop area should be reduced, common mode coupling should be suppressed and metal shielding should be used. For input/output cables, use shielded cables, common mode chokes, overvoltage clamping 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 interconnect 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 shielding layer of the cable, so that the potential difference between the two chassis can be minimized. Here, the overlap between the chassis and the cable shield is very important. It is strongly recommended that the 360° overlap between the chassis at both ends of the cable and the cable shield.
Keyboard and panelThe 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, a discharge protector (such as a metal bracket) must be installed between the key and the circuit to provide a discharge path for the discharge current. The discharge protector should be directly connected to the chassis or rack, but 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 design of the keyboard and control panel should enable the discharge current to go directly to ground without passing through sensitive circuits. The use of insulated shafts and large knobs prevents discharge to control keys or potentiometers. At present, more electronic product panels use thin film buttons and thin film display windows. Because the film is made of high-voltage insulating material, it can effectively prevent ESD from entering the internal circuit through the keys and display windows to form interference. In addition, most of the keys of the keyboard now have pads made of high-pressure-resistant insulating film, which can effectively prevent ESD interference.
Circuit designUnused input terminals in the equipment are not allowed to be in a disconnected or floating state, but should be directly or through an appropriate resistor connected to the ground wire or power supply terminal. Generally speaking, the interface circuits connected with external devices all need to add protection circuits, which also include power lines, which is often overlooked by hardware design. Taking a microcomputer as an example, the links for arranging protection circuits 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 coupling to 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, using 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 diode can be connected in series to the positive and negative power supply terminals at the I/O interface. The input end of the MOS tube is connected in series with a 100kΩ resistor, and the output end is connected in series with a 1kΩ resistor to limit the amount of discharge current. The TTL tube input series connects 22～100Ω resistor, and the output series connects 22～47Ω resistor. 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 pole of the power supply, and the output end of the analog tube is connected with a 100Ω resistor in series. Installing a capacitor to ground on the I/O signal line can shunt 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 chassis 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.
The voltage of electrostatic discharge can be as high as several thousand volts. A transient protection diode can also effectively protect the electrostatic discharge, but it should be noted that although the voltage of the transient interference is limited by the diode, the high-frequency interference component has not been reduced. The high-frequency bypass capacitor in parallel with transient protection diodes suppresses high-frequency interference. In circuit design and circuit board wiring, gate circuits and strobes should be used. This input method can only cause damage when electrostatic discharge and gating occur simultaneously. The pulse edge trigger input method is very sensitive to transients caused by electrostatic discharge and should not be used.