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Analysis of key points of EMI radiation design in automotive electronics

Release Time:2019-01-25 Edit the author:金致卓 Reading:753

As you all know, the layout of a printed circuit board determines the success or failure of all power supplies, determining functionality, electromagnetic interference (EMI), and performance when heated. The layout of the switching power supply is not magic, it is not difficult, but it may often be overlooked during the initial design phase. However, because both functional and EMI requirements must be met, arrangements that are beneficial to the stability of the power supply are often beneficial in reducing EMI emissions, so it is better to do it later. It should also be mentioned that designing a good layout from the start does not incur any cost, and in fact saves money because there is no need for EMI filters, mechanical shielding, time-consuming EMI testing, and modification of the PC board.

In addition, when multiple DC/DC switching mode regulators are connected in parallel to achieve current sharing and greater output power, potential interference and noise issues may worsen. If all regulators operate at similar frequencies (switches), the total energy produced by multiple regulators in the circuit is concentrated at one frequency. The presence of this energy can be a concern, especially if the PCs and other ICs on other system boards are close together and susceptible to this radiant energy. In automotive systems, this problem can be particularly troublesome because automotive systems are densely packed and often close to audio, RF, CAN buses, and various radar systems.

Respond to switching regulator noise radiation problem

In automotive environments, switching regulators are often used in places where heat dissipation and efficiency are important to replace linear regulators. In addition, switching regulators are typically the first active component on the input power bus and therefore have a significant impact on the EMI performance of the entire converter circuit.

There are two types of EMI radiation: conductive and radiative. Conductive EMI depends on the wires and circuit traces that are connected to a product. Since the noise is limited to a particular terminal or connector in the design of the solution, then with the good layout or filter design described above, it is often possible to ensure compliance with conducted EMI requirements early in the development process.

However, radiated EMI is another matter. All components carrying current on the board radiate an electromagnetic field. Each trace on the board is an antenna, and each copper plane is a resonator. In addition to a pure sine wave or DC voltage, any signal produces noise that covers the entire signal spectrum. Even after careful design, the designer will never really know how severe the radiated EMI will be before the system is tested. Also, it is not possible to formally conduct a radiated EMI test until the design is basically completed.

The filter can attenuate the intensity at a certain frequency or over the entire frequency range to reduce EMI. Part of the energy propagates through the space (radiation), so metal shields and magnetic shields can be added to attenuate. The portion of the PCB trace (conducted) can be controlled by adding ferrite beads and other filters. EMI cannot be completely eliminated, but can be attenuated to acceptable levels for other communications and digital components. In addition, several regulators enforce standards to ensure compliance with EMI requirements.

New input filter assemblies using surface mount technology perform better than through-hole assemblies. However, this improvement is offset by an increase in the operating frequency of the switching regulator switch. Faster switching produces higher efficiency, shorter minimum on and off times, and therefore higher harmonic components. When all other parameters, such as switching capacity and switching time, remain constant, EMI is degraded by 6 dB for every doubling of the switching frequency. Broadband EMI behaves like a first-order high-pass filter, if the switching frequency is increased by a factor of 10. , it will increase the radiation by 20dB.

Experienced PCB designers will design the hotspot loop to be small and keep the shielded formation as close as possible to the active layer. However, the device pinout configuration, package construction, thermal design requirements, and package size required to store sufficient energy in the decoupling component determine the minimum size of the hotspot loop. To complicate matters, in a typical flat-panel printed circuit board, magnetic or transformer-type coupling above 30 MHz between traces will offset all filter efforts because the higher the harmonic frequency, the unwanted magnetic coupling Become more effective.

A new solution to these EMI problems

The solution to a reliable and truly EMI problem is to place the entire circuit in a shielded box. Of course, doing so increases the cost, increases the board space required, makes thermal management and testing more difficult, and resul

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