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Reducing EMI in automotive environments

Posted: 24 Jan 2014     Print Version  Bookmark and Share

Keywords:electromagnetic interference  EMI  DC/DC  switchmode regulators  LT8614 

The success or failure of every power supply relies on the printed circuit board layout. It sets functional, electromagnetic interference (EMI) and thermal behaviour. While switching power supply layout is not a "black" art, it can often be overlooked in the initial design process. Nevertheless, since functional and EMI requirements have to be met, what is good for functional stability of the power supply is also usually good for its EMI emissions as well. It should also be note that good layout from the beginning does not add any cost, but can actually provide cost savings, eliminating the need for EMI filters, mechanical shielding, EMI test time and PC board revisions.

Moreover, the potential problem for interference and noise can be exasperated when multiple DC/DC switchmode regulators are paralleled for current sharing and higher output power. If all are operating (switching) at a similar frequency, the combined energy generated by multiple regulators in a circuit is then concentrated at one frequency. Presence of this energy can become a concern especially if the rest of ICs on the PC board as well as other system boards are close to each other and susceptible to this radiated energy. This can be particularly troubling in automotive systems which are densely populated in and are often in close proximity to audio, RF, CAN bus and various radar systems.

Addressing switching regulator noise emissions
In an automotive environment, switching regulators usually replace linear regulators in areas where low heat dissipation and efficiency are valued. Moreover, the switching regulator is typically the first active component on the input power bus line, and therefore has a significant impact on the EMI performance of the complete converter circuit.

There are two types of EMI emissions; conducted and radiated. Conducted emissions ride on the wires and traces that connect up to a product. Since the noise is localized to a specific terminal or connector in the design, compliance with conducted emissions requirements can often be assured relatively early in the development process with a good layout or filter design as already stated.

Radiated emissions, however, are another story. Everything on the board that carries current radiates an electromagnetic field. Every trace on the board is an antenna, and every copper plane is a resonator. Anything, other than a pure sine wave or DC voltage, generates noise all over the signal spectrum. Even with careful design, a designer never really knows how the bad the radiated emissions are going to be are until the system gets tested. And radiated emissions testing cannot be formally performed until the design is essentially complete.

Filters are often used to reduce EMI by attenuating the strength at a certain frequency or over a range of frequencies. A portion of this energy that travels through space (radiated) is attenuated by adding metallic and magnetic shields. The part that rides on PCB traces (conducted) is tamed by adding ferrite beads and other filters. EMI cannot be eliminated but can be attenuated to a level that is acceptable by other communication and digital components. Moreover, several regulatory bodies enforce standards to ensure compliance.

Modern input filter components in surface mount technology have better performance than through-hole parts. However, this improvement is outpaced by the increase in operating switching frequencies of switching regulators. Higher efficiency, low minimum on- and off-times result in higher harmonic content due to the faster switch transitions. For every doubling in switching frequency, the EMI becomes 6dB worse while all other parameters, such as switch capacity and transition times, remain constant. The wideband EMI behaves like a first order high pass with 20dB higher emissions if the switching frequency increases by 10 times.

Savvy PCB designers will make the hot loops small and use shielding ground layers as close to the active layer as possible. Nevertheless, device pin-outs, package construction, thermal design requirements and package sizes needed for adequate energy storage in decoupling components dictate a minimum hot loop size. To further complicate matters, in typical planar printed circuit boards, the magnetic or transformer style coupling between traces above 30MHz will diminish all filter efforts since the higher the harmonic frequencies are the more effective unwanted magnetic coupling becomes.

Solution to these EMI issues
The tried and true solution to EMI issues is to use a shielding box for the complete circuit. Of course, this adds costs, increases required board space, makes thermal management and testing more difficult, as well as introduces additional assembly costs. Another frequently used method is to slow down the switching edges. This has the undesired effect of reducing the efficiency, increasing minimum on-, off-times, and their associated dead times and compromises the potential current control loop speed.

Linear's recently introduced LT8614 Silent Switcher regulator is touted to deliver the desired effects of a shielded box without using one, and so eliminates the above mentioned drawbacks (figure 1). The LT8614 also has an IQ of 2.5µA operating current. This is the total supply current consumed by the device, in regulation with no load.

Figure 1: The LT8614 Silent Switcher.

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