EMC is interpreted as electromagnetic compatibility. For PCB power supply design, the design of EMC is very important.
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The basic framework of EMC design:
The transmission source generates a transmission coupling path to transfer the transmitted energy to the receiver, and the transmitted energy is processed in the receiver to generate an intended or unexpected action. Interference can result if the received energy causes unintended effects on the receiver. According to the three elements of EMC, there are three ways to solve EMC problems:
1) To suppress the emission of the emission source, the steeper the voltage or current changes, the more obvious the interference. For example, electric sparks can generate electromagnetic waves with rich spectral components. Digital electronics are represented by pulses. In general, the higher the frequency of the source signal, the more efficient the communication path. The rise or fall times of digital signals must be slowed down. Suppressing the high-frequency spectral components of the radiation source will greatly reduce the efficiency of the communication path, thereby reducing the signal level at the receiver.
2) Make the coupling path as ineffective as possible. Coupling is mainly divided into conduction coupling, radiation coupling, capacitive coupling and inductive coupling. Putting electronic equipment in a completely closed metal protective box can effectively cut off the communication path, but this method does not suitable for use. Metal protective boxes also inevitably have cable entry and exit interfaces, which is far from ideal in practical applications. In addition, protective measures are costly. A direct conductive communication path through a metal conductor is actually more efficient than a radiative communication path, thus putting an obstacle on the conductive communication path. This extra long cable forms the electrical loop of the dipole antenna, forming a loop antenna. The longer the cable and the larger the area of the loop, the more efficiently the electromagnetic energy can be transmitted or received. The connection of cables, circuits and internal parts of the equipment will cause radiation or electromagnetic interference in the circuit. Therefore, long wires should be avoided as much as possible on the communication path, and the area of the current loop should be controlled.
Through the above analysis can provide our basic design ideas. Let the radiation source not flow through the equivalent antenna model or the equivalent loop path that flows through is the shortest/equivalent return flow area is the smallest; optimizing the current value of the equivalent radiation impedance Rr reduces the radiation energy.
For clock systems and high-frequency signal systems, the differential mode radiation and the return area of the signal are the key to the design.
Layout strategy for PCB design of high-frequency circuit & low-frequency switching power supply circuit:
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The direction of the current, the high-frequency current takes the loop with the lowest inductive reactance; not the shortest path! The high-frequency signal return path returns through the mirror image! Switching power supply layout and wiring should choose the smallest loop! For high-frequency and high-speed signals, the design of double-sided and multi-layer copper-clad ground structures is to meet the minimum return area of key signals; for the EMI radiation of switching power supply systems, it is necessary to optimize the switching noise loop area and reduce the high-frequency noise current intensity. Design ideas!
3) The receiver is insensitive, reducing the sensitivity of the receiver to noise while retaining useful functions. An effective method is to use error correction coding in the digital receiver. Although there is unexpected electromagnetic energy incident on the receiver, But error-correcting codes allow receivers to function in the presence of potentially interfering signals.
Problems in the design case of EMC:
1. The interface filter is too far away from the interface,
2. The interface filter components are not arranged vertically in a row
3. The power filter components are not arranged together
4. The input and output circuits of the switching power supply are too large
Design ideas: When the switching power supply board is wired on the PCB, control the loop area of the input rectification filter loop, power loop, output rectification loop, and output filter loop. The loops should be small and the wiring should be short!
EMC Layout design points:
(1) Sensitive signals are kept away from high-current signals, especially frequency signals, and do not run in parallel.
(2) The wiring of the current loop should be as small as possible.
(3) The analog ground and power ground need to be separated and can be connected together through vias or star points.
(4) Multi-channel IC power supply can be connected in parallel to single-point grounding to reduce crosstalk.
(5) The control loop is separated from the power loop.
(6) The decoupling capacitor needs to be placed close to the IC power pin.
(7) Multi-stage filtering is done on the power supply, and the smaller capacitor is close to the pin of the chip.
(8) Anti-reverse and clamp protection devices are placed close to the connector.
(9) Do a good job of isolation design for high voltage ground and low voltage ground.
(10) Pay attention to the spacing of safety traces.
