EMC Design in PCB Circuits

This page is about what is EMC and EMC design in PCB circuits for engineers.

What is EMC?

EMC--Electromagnetic compatibility refers to the ability of electronic and electrical equipment or systems to work normally according to the design requirements in the expected electromagnetic environment. It is also an important technical performance of electronic and electrical equipment or systems. On a global scale, the issue of electromagnetic compatibility has become a new discipline, and it is also a fringe science based on electromagnetic field theory, including information, electrical engineering, electronics, communications, materials, structure, and other disciplines, and it is also a practice Relatively strong disciplines require product engineers to have a wealth of practical knowledge. 

EMC Design

Grounding design

Once electrostatic discharge occurs, 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 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 disturbance generated when the surrounding objects are discharged can be led to the ground without affecting Internal circuit. For metal cases, the circuits in the case are usually grounded through I/O cables, power lines, etc. When electrostatic discharge occurs on the case, the potential of the case 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 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, the occurrence of secondary arcs 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 with chassis grounding.

Cable design

A properly designed cable protection system may be the key to improving the system's ESD non-susceptibility. 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, so conduction coupling can be avoided indirectly. To reduce the coupling of ESD interference radiation to the cable, the line length and loop area should be reduced, the common-mode coupling should be suppressed and metal shielding should be used.

For input/output cables, shielded cables, common mode chokes, overvoltage clamping circuits, and cable bypass filters can be used. 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 make the common-mode voltage drop caused by electrostatic discharge on the choke instead of on the circuit at the other end. When connecting two cabinets with shielded cables, connect the two cabinets through the shielding layer of the cable, so that the potential difference between the two cabinets can be as small as possible. Here, the overlap between the chassis and the cable shielding layer is very important. It is strongly recommended to lap 360° between the chassis at both ends of the cable and the cable shielding layer.

Keyboard and Panel

The design of the keyboard and control panel must ensure that the discharge current can flow directly to the ground without passing through sensitive circuits. For insulated keyboards, a discharge protector (such as a metal bracket) should 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, and not to the circuit ground. Of course, using a larger travel button (increasing the distance between the operator and the internal circuit) can directly prevent electrostatic discharge. The design of the keyboard and control panel should enable the discharge current to reach the ground directly without passing through sensitive circuits. The use of insulated shafts and large knobs can prevent discharge to the control keys or potentiometers.

Nowadays, more electronic product panels use thin-film buttons and thin-film display windows. Because the film is made of high-voltage-resistant insulating materials, it can effectively prevent ESD from entering the internal circuit through the buttons and display windows to cause interference. In addition, most of the keyboard keys now have pads made of high-voltage-resistant insulating films, which can effectively prevent ESD interference.

Circuit Design

The unused input terminals of the equipment are not allowed to be disconnected or suspended but should be connected to the ground or power terminal directly or through appropriate resistors. Generally speaking, the interface circuit connected with the external device needs to add a protection circuit, which also includes the power cord, which is often overlooked by the hardware design. Take the microcomputer as an example, the links that should be considered for arranging the protection circuit are serial communication interface, parallel communication interface, keyboard interface, display interface, etc.

Filters (shunt capacitors or a series of inductances or a combination of the two) must be used in the circuit to prevent EMI from coupling to the equipment. 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, and these ferrites should also be as close to the input as possible.

Strengthen protective measures on internal circuits. For ports that may suffer from direct conduction electrostatic discharge interference, you can connect a resistor or a parallel diode to the positive and negative power terminals at the I/O interface. The input end of the MOS tube is connected with a 100kΩ resistor in series, and the output end is connected with a 1kΩ resistor in series to limit the discharge current. The input end of the TTL tube is connected with a 22-100Ω resistor in series, and the output end is connected with a 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 pole of the power supply, and the output end of the analog tube is connected in series with a resistance of 100Ω. 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 and avoid flowing into the circuit. But this capacitor will also shunt the current on the chassis to the signal line. To avoid this situation, 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. The use of a transient protection diode can also effectively protect against 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 is not reduced. The circuit should generally have The high-frequency bypass capacitor connected in parallel with the transient protection diode suppresses high-frequency interference. In terms of circuit design and circuit board wiring, gate circuits and strobe pulses should be used. This input method can cause damage only when electrostatic discharge and strobe occur at the same time. The pulse edge trigger input method is very 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 ESD design in electromagnetic compatibility design. You can get detailed guidance from that part of the course. When implementing electromagnetic compatibility countermeasures for a finished product, it is difficult to redesign the PCB (improvement costs are too high), so I will not introduce it here.

Software

In addition to hardware measures, software suppression schemes are also a powerful way to reduce serious malfunctions such as system lockups. Software ESD suppression measures are divided into two commonly used categories: refresh, check, and restore. Refreshing involves periodically resetting to the dormant state and refreshing the display and indicator states. You only need to refresh once and assume that the state is correct, and you don't have to do other things. The inspection/recovery process is used to determine whether the program is executed correctly. They are activated at certain intervals to confirm whether the program is completing a certain function. If these functions are not implemented, a recovery procedure is activated.

General ESD countermeasure guidelines:

1. Add protective diodes to susceptible CMOS and MOS devices;

2. String tens of ohm resistors or ferrite beads on the susceptible transmission line (including the ground wire);

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

4. Try to use shielded cables;

5. Install the filter at the susceptible interface; isolate the sensitive interface where the filter cannot be installed;

6. Choose a logic circuit with low pulse frequency;

7. Shell shielding and good grounding.