As we have seen before, the EFT waveform specified in the IEC/EN standard document is intended to simulate the transients created by the interrupting inductive loads on the power mains, switching relays etc. Though primarily intended for injection on the product’s AC power cord, the EFT waveform can also be applied onto signal and control lines to simulate the coupling of the EFT onto these lines.
The EFT waveform specified in IEC 61000-4-4 has a rise time between 3.5 ns and 6.5 ns resulting, in a noise bandwidth (1/πtr) of approximately 90 MHz. Again, the EFT noise with frequencies upto (0.5/tr), i.e. 140MHz could also cause trouble. Hence let us consider the EFT noise with effective bandwidth upto 140 MHz could enter an electronic device or system through the cables connected to the ports. We need to design the filter circuit accordingly.
Mainly two types of ports are distinguished by the standard EN/IEC 61000-4-4: power ports and signal ports. A signal port could be an IO port, sensor input port or any communication port such as Ethernet, USB, serial etc. In general the common tactics to deal with EFT noise and similar are:
1. Avert:
Implement a circuit which can resist the noise at the port entry so that the noise to the extent enough to upset the device, doesn't enter through the ports. Using a common-mode choke at the power entry or on communication channels carrying differential signals at the port is one good example.
2. Divert:
Implement a circuit which can bypass the noise to a good ground connection (such as chassis ground). Implementing port filter or using EFT protection devices such as TVS diodes, capacitors are good examples of this method. TVS devices are used to divert the transient current, clamp the voltage and keep the internal circuitry safe.
Most of the time, a combination of both the above is desirable to be implemented in the design. As shown below, the TVS diode D1, capacitors CL1, CH1 and the Common Mode Choke forms the circuit which filters the noise at the port entry and prevents the noise (Averts) to reach the device. After the Common Mode Choke on the device-side, there are two decoupling capacitors CL2 and CH2 shown which divert the noise to the ground and prevents it further from entering to the device. Please note that the circuit below can handle common-mode noise only. Usually EFT noise is Common Mode in nature, but there should be some provision kept to handle Differential Mode noise also (not shown below).
The red arrow mark in the picture below shows that the noise gets weaker at each stage of the filter/protection circuit. The strength of the noise entering the device gets weaker enough not to upset its functionality. Please note that the ground path needs to be very good to provide a strong & solid return path for the noise.
I plan to mention some of the board layout level techniques after we discuss all other EMC tests. In the next part, we will venture into high energy voltage surge.
The EFT waveform specified in IEC 61000-4-4 has a rise time between 3.5 ns and 6.5 ns resulting, in a noise bandwidth (1/πtr) of approximately 90 MHz. Again, the EFT noise with frequencies upto (0.5/tr), i.e. 140MHz could also cause trouble. Hence let us consider the EFT noise with effective bandwidth upto 140 MHz could enter an electronic device or system through the cables connected to the ports. We need to design the filter circuit accordingly.
Mainly two types of ports are distinguished by the standard EN/IEC 61000-4-4: power ports and signal ports. A signal port could be an IO port, sensor input port or any communication port such as Ethernet, USB, serial etc. In general the common tactics to deal with EFT noise and similar are:
1. Avert:
Implement a circuit which can resist the noise at the port entry so that the noise to the extent enough to upset the device, doesn't enter through the ports. Using a common-mode choke at the power entry or on communication channels carrying differential signals at the port is one good example.
2. Divert:
Implement a circuit which can bypass the noise to a good ground connection (such as chassis ground). Implementing port filter or using EFT protection devices such as TVS diodes, capacitors are good examples of this method. TVS devices are used to divert the transient current, clamp the voltage and keep the internal circuitry safe.
Most of the time, a combination of both the above is desirable to be implemented in the design. As shown below, the TVS diode D1, capacitors CL1, CH1 and the Common Mode Choke forms the circuit which filters the noise at the port entry and prevents the noise (Averts) to reach the device. After the Common Mode Choke on the device-side, there are two decoupling capacitors CL2 and CH2 shown which divert the noise to the ground and prevents it further from entering to the device. Please note that the circuit below can handle common-mode noise only. Usually EFT noise is Common Mode in nature, but there should be some provision kept to handle Differential Mode noise also (not shown below).
The red arrow mark in the picture below shows that the noise gets weaker at each stage of the filter/protection circuit. The strength of the noise entering the device gets weaker enough not to upset its functionality. Please note that the ground path needs to be very good to provide a strong & solid return path for the noise.
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