Electromagnetic compatibility design in PCB circuits

Grounding design

Once an electrostatic discharge has occurred, it should be bypassed as soon as possible and not directly intrude into the internal circuitry. 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 the disturbance generated when the surrounding objects are discharged can also be introduced into the earth without affecting. Internal circuit. For a metal chassis, usually the circuit inside the chassis is grounded through an I/O cable, a power cable, etc. When an electrostatic discharge occurs on the chassis, the potential of the chassis rises, and the internal circuit is grounded, and the potential remains near the ground potential. At this time, there is a large potential difference between the chassis and the circuit. This causes a secondary arc between the chassis and the circuit. Cause damage to the circuit. The secondary arc can be avoided by increasing the distance between the circuit and the casing. 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 by one point. Prevent current from flowing through the circuit. The point at which the board is connected to the chassis should be at the cable entry. For plastic chassis, there is no problem with chassis grounding.

Cable design

A properly designed cable protection system may be the key to improving the system's non-susceptibility to ESD. As the largest "antenna" in most systems - I/O cables are particularly susceptible to large voltages or currents induced by ESD interference. On the other hand, the cable also provides a low impedance path for ESD interference if the cable shield is connected to the chassis ground. The ESD interference energy through this channel can be released from the system ground loop, thus indirectly avoiding conductive coupling. To reduce the ESD interference radiated into the cable, the line length and loop area are reduced, and common mode coupling should be suppressed and metal shielded. Shielded cables, common mode chokes, overvoltage clamp circuits, and cable bypass filters are available for input/output cables. At both ends of the cable, the cable shield must be shielded from the housing. Installing a common mode choke on the interconnect cable allows the common mode voltage caused by the electrostatic discharge to drop on the choke instead of the other end. When connecting the two chassis with shielded cables, connect the two chassis together through the shield of the cable, so that the potential difference between the two chassis is as small as possible. Here, the way the lap is made between the chassis and the cable shield is important. It is highly recommended to make a 360° overlap between the chassis at both ends of the cable and the cable shield.

Keyboard and panel

The keyboard and control panel must be designed to ensure that the discharge current flows directly to the ground without going through sensitive circuitry. For insulated keyboards, a discharge protector (such as a metal bracket) is installed between the key and the circuit to provide a discharge path for the discharge current. The discharge protector should be connected directly to the chassis or rack and not to the circuit ground. Of course, the use of a larger travel button (increasing the distance of the operator to the internal line) can directly prevent electrostatic discharge. The keyboard and control panel should be designed so that the discharge current goes directly to the ground without going through the sensitive circuitry. The use of an insulated shaft and a large knob prevents discharge to the control or potentiometer. Nowadays, more electronic product panels use film buttons and film display windows. Since the film is made of high-voltage resistant insulating material, it can effectively prevent ESD from entering the internal circuit through buttons and display windows to form interference. In addition, most of the keys of the keyboard now have a gasket made of a high-voltage resistant insulating film, which can effectively prevent ESD interference.

Circuit design

Inputs not used in the device are not allowed to be disconnected or suspended, but should be connected to the ground or power supply directly or through appropriate resistors. In general, interface circuits connected to external devices require protection circuits, including power lines, which are often overlooked by hardware design. Taking the microcomputer as an example, the links that should be considered for the protection circuit are: serial communication interface, parallel communication interface, keyboard interface, display interface, and the like.

A filter (shunt capacitor or 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, and the closer the shunt capacitor is to the input, the better. If the input impedance is low, a series of ferrites can be used to provide the best filter. These ferrites should also be as close as possible to the input.

Strengthen protective measures on internal circuits. For ports that may be subject to direct conduction ESD interference, a resistor or a parallel diode can be placed in series at the I/O interface to the positive and negative supply terminals. The input end of the MOS transistor 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 TTL tube input terminal is connected in series with a 22-100 Ω resistor, and the output terminal is connected in series with a 22-47 Ω resistor. The input end of the analog tube is connected in series from 100Ω to 100kΩ, and a parallel diode is added to divide the discharge current to the positive or negative pole of the power supply, and the output of the analog tube is connected in series with a resistance of 100Ω. Installing a capacitor on the I/O signal line shunts the ESD current induced on the interface cable to the chassis to prevent it from flowing to the circuit. But this capacitor also shunts the current on the case to the signal line. To avoid this, a ferrite bead can be placed between the bypass capacitor and the board to increase the impedance of the path to the 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. The use of a transient protection diode can also effectively protect the electrostatic discharge, but it should be noted that although the diode limits the voltage of the transient interference, the high-frequency interference component is not reduced, and the circuit should generally have The high frequency bypass capacitor in parallel with the transient protection diode suppresses high frequency interference. Gates and strobes should be used for circuit design and board layout. This type of input can only be damaged if both electrostatic discharge and gating occur simultaneously. The pulse edge trigger input mode 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 the product. This is also an important part of the ESD design part of EMC design. You can get detailed guidance from that part of the course. When electromagnetic compatibility countermeasures are applied to a finished product, it is difficult to redesign the PCB (the improvement cost is too high) and will not be described here.

software

In addition to hardware measures, software suppression schemes are also a powerful way to reduce serious errors such as system lock-up. Software ESD suppression measures fall into two common categories: refresh, check, and recovery. Refreshing involves periodically resetting to a rest state and refreshing the display and indicator status. Just do a refresh and assume that the state is correct, and you don't have to do anything else. The check/restore process is used to determine if the program is executing correctly and they are activated at certain intervals to confirm that the program is completing a function. If these features are not implemented, a recovery program is activated.

General ESD countermeasures guidelines:

(1) Adding a protection diode to a susceptible CMOS or MOS device;

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

(3) The use of electrostatic protection surface coating technology makes ESD difficult to discharge the core, which has proved to be very effective;

(4) Use shielded cables as much as possible;

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

(6) selecting a logic circuit with a low pulse frequency;

(7) Shell shield plus good grounding.