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How to pass the electromagnetic compatibility tests
Release time:2014-12-04   Click times:3499

Grounding Design: Once an electrostatic discharge occurs, people should be allowed to pass as soon as possible, do not directly invade the internal circuitry. For example, the internal circuit such as a metal shielding enclosures, the enclosure should be well grounded, grounding resistance should be as small as possible, so that the discharge current can flow from the outer enclosure of the earth, but also can be formed when the harassment will discharge into the earth around the object will not affect internal circuit. Metal chassis, circuit generally within the chassis through I / O cable, power lines, and grounding, when electrostatic discharge occurs on the chassis, the chassis of the potential increase in the internal circuit due to ground potential was kept near ground potential. At this time, there is a considerable potential difference between the chassis and the circuit. This can cause arcing between the chassis and the secondary circuit. The circuit damage. By increasing the distance between the circuit and the housing to avoid the occurrence of the secondary arc. When the distance between the circuit and the housing can not be increased, it can add a layer between the housing and the circuit grounded metal shield, blocking arc. If the circuit together with the chassis, only a little connection should be adopted. Preventing current from flowing through the circuit. Board and chassis connection point should be at the entrance of the cable. Plastic chassis, the chassis ground is not an issue.

Cable Design:

Cable protection systems a properly designed system may be a non-ESD susceptibility key to improving. As most systems biggest "antenna" - I / O cables are especially susceptible to ESD interference induced a large voltage or current. On the other hand, the cable also provides a low impedance path ESD interference, if the cable shield with the chassis ground connection of words. ESD interference energy can be released from the system ground loops through the channel, thus indirectly prevent conduction coupling. To reduce ESD interference radiation is coupled to the cable, cable length and loop area to reduce should suppress common mode coupling and the metal shield. For input / output cable can use shielded cable, common mode choke, overvoltage clamp and cable bypass filter measures. At both ends of the cable, shielded cable shield must be connected to the housing. Installing a common mode choke on the interconnect cable can make the electrostatic discharge caused by the common mode voltage drop across the choke, instead of on the other side of the circuit. With shielded cable connection, via cable shield connection between the two chassis two chassis together, this could make the difference in potential between the two chassis as small as possible. Here, an overlap between the chassis and cable shielding is important. It is strongly recommended between the chassis and the cable shield at both ends of the cable 360 ​​° lap.

Keyboard and panel:

Keyboard and control panel must be designed to ensure that the discharge current flows to the ground directly, not through the sensitive circuit. For the insulating keyboard, between the key and the circuit you want to install a discharge protection device (such as a metal bracket), provides a discharge path for the discharge current. Discharge protection device to be connected directly to the chassis or rack, and not connected to circuit ground. Of course, with a larger trip button (increase operator from the internal wiring to) directly against electrostatic discharge. Keyboard and control panel should be designed to allow the discharge current sensing circuit does not pass through directly to the ground. Insulated shaft and prevents the large knob to control key or potentiometer discharge. Now, more electronic panel with membrane keypad and display window film, since the film by the high pressure of the insulating material, which can effectively prevent ESD interfere by buttons and display window into the internal circuitry. In addition, most of the internal key pad of the keyboard now have pressure-resistant insulating thin film, which can effectively prevent ESD interference.

Circuit Design: the device input is not allowed without the connection or suspension, but should be communicated directly or through an appropriate resistor to ground or power terminals of. In general, the interface circuit with an external device connected to the need to increase protection circuitry, including power lines, it is often overlooked by hardware design. In terms of the microcomputer as an example, should be considered part of the protective circuit arrangements are: serial communication interface, a parallel communication interface, keyboard interface, display interface.

Filter (series inductance or shunt capacitance or a combination of both) to be used in the circuit is coupled to the device to prevent EMI. If the input is high impedance and a shunt capacitor filter is the most effective, because it will effectively bypass the low impedance high input impedance, input shunt capacitance closer the better. If the input impedance is low, using a series of ferrite can provide the best filters, ferrites these should also be as close as possible input.


Strengthen protective measures on the internal circuitry. Port for direct conduction may suffer electrostatic discharge interference can resistor in series or in parallel diode to the positive and negative power supply terminal in the I / O interface. Input MOS tube 100kΩ series resistor, the output of 1kΩ series resistor to limit the discharge current amount. TTL input end of the tube 22 ~ 100Ω resistor in series, the output of 22 ~ 47Ω series resistor. Analog tube series with the input 100Ω ~ 100kΩ, and add-parallel diode shunt discharge current to the power supply positive or negative, the analog output of the tube series resistance of 100Ω. In the I / O signal line to install a capacitance to ground can be induced on the interface cable electrostatic discharge current shunt to the chassis to prevent flow to the circuit. But this will also capacitive shunt current cabinet to the signal line. To prevent this from happening, you can install a ferrite bead between the bypass capacitor and the circuit board, the board increased the flow impedance path. It should be noted that the capacitor voltage must meet the requirements. Electrostatic discharge voltage can be as high as several thousand volts. With a transient protection diode for electrostatic discharge can also play an effective protection, but need to pay attention, although the diode voltage transient disturbances locked, but did not reduce high-frequency interference component of the circuit and it should normally transient protection diode in parallel with the high-frequency bypass capacitors suppress high frequency interference. In the circuit design and board layout aspect should be used gates and strobes. This input mode only when the electrostatic discharge and strobe simultaneously to cause damage. The transient pulse edge trigger input due to electrostatic discharge is very sensitive, should not be used.

PCB design: Good PCB design can effectively reduce the impact on the product caused by ESD interference, which is one of EMC design ESD design part of the important content, you can get from that part of the course of the detailed guidelines. When a finished product EMC countermeasure, very difficult to re-design the PCB (improved cost is too high), here no longer be introduced.

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