PCB Technical - EMI interference: conduction is the culprit

Radiated EMI interference can come from a non-directional emission source and an unintentionally formed antenna. Conducted EMI interference can also come from a source of radiated EMI interference, or caused by some circuit board components. Once your circuit board receives conductive interference, it resides in the PCB trace of the application circuit. Some common sources of radiated EMI interference include the components discussed in previous articles, as well as on-board switching power supplies, connecting wires and switches, or clock networks.

Figure 1 Coupling medium of conductive EMI signal

Conducted EMI interference is the result of the normal operation of the switching circuit and the combined effect of parasitic capacitance and inductance. Figure 1 shows some of the EMI interference sources that will enter your PCB traces. Vemi1 is derived from a switching network, such as clock signals or digital signal traces. The coupling methods of these interference sources are through the parasitic capacitance between the traces. These signals bring current spikes into adjacent PCB traces. Similarly, Vemi2 originates from the switch network, or from an antenna on the PCB. The coupling methods of these interference sources are through the parasitic inductance between the traces. This signal brings voltage disturbances into adjacent PCB traces. Every three EMI sources come from adjacent wires in the cable. Signals propagating along these wires can produce crosstalk effects.

The switching power supply generates Vemi4. The interference generated by the switching power supply resides on the power trace and appears in the form of a Vemi4 signal.

During normal operation, switched mode power supply (SMPS) circuits provide opportunities for the formation of conductive EMI. The "on" and "off" switching operations in these power supplies will generate strong discontinuous currents. These discontinuous currents exist at the input of the buck converter, the output of the boost converter, and the input and output of the flyback and buck-boost topologies. The discontinuous current caused by the switching action will produce voltage ripples, which propagate to other parts of the system through the PCB traces. The input and/or output voltage ripple caused by SMPS can endanger the operation of the load circuit. Figure 2 shows an example of the frequency composition of a DC/DC step-down SMPS input operating at 2 MHz. The basic frequency composition range of SMPS conducted interference is 90 – 100 MHz.

Figure 2 DC/DC step-down converter: switching frequency = 2MHz

Conducted EMI measurement when the input and output pins use 10 ?F filters.

There are two types of conducted interference: differential mode interference and common mode interference. Differential mode interference signals appear between the input terminals of the circuit, such as signal and ground. The current flows through the two input terminals of the same phase. However, the current input of No. 1 is the same as No. 2, but in the opposite direction (differential reference). The load of these two input ends forms a voltage that changes with the strength of the current. This voltage change between trace 1 and the differential reference creates interference or communication errors in the system.

Common mode interference occurs when you add a ground loop or poor current path to the circuit. If there is an interference source, common mode current and common mode voltage are formed on trace 1 and trace 2, and the ground loop acts as a common mode interference source. Both differential mode interference and common mode interference require the use of special filters to deal with the adverse effects of EMI interference.