EMI: Lurking in the Background, As Always

03.01.2010 // Posted by: Murray Slovick // Posted in: Articles, New Technology

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A funny thing happened as I was thinking about a topic for this month’s column. Toyota’s once unassailable reputation for quality was facing a series of knee-buckling challenges, the most widespread of which involved unintended acceleration in the Toyota Camry, the company’s most popular line of vehicles.

Since the issue first surfaced Toyota has settled on a mechanical fix: the company will reinforce the pedal assembly, thereby eliminating what it termed “friction that sometimes causes the sudden acceleration to occur.” Toyota will repair more than two million vehicles with sticking gas pedals and nearly five-and-a-half million with "defective" floor mats.

Nonethless, whispers continue to circulate regarding a possible electromagnetic interference (EMI)-problem within the Camry’s electronic throttle- control system. Toyota says it has not found any evidence that EMI is a cause of its unintended acceleration difficulties but the National Highway Transportation Safety Administration (NHTSA), which has carried out EMI tests on Toyotas in the past without finding evidence of any safety issues, has announced that it was launching a fresh and more detailed investigation into the possible effects of EMI on electronic throttle-control systems.

NHTSA is looking into this because EMI generated from a range of devices, including cell phones, satellite radio and microwave towers, conceivably could initiate or replicate errant signals, causing an incorrect answer to be generated from an MCU or other automotive control system by randomly powering a gate up or down.

EMI causes sleepless nights on the part of system designers because its causes and cures are not all that well understood. It’s a complex problem, usually with no single “magic” all-encompassing solution. One thing we do know: the more electronics you have in a vehicle, the more noise you tend to get. With the increasing amount of electronic equipment being used together in areas where they can affect each other, the possibility of electromagnetic interference cannot be dismissed out of hand.

If you are a design engineer, it pays to know about EMI since the EMI standards and tests that must be passed can be very rigorous and failure can result in substantial project delays. So the following is a quick review of EMI causes and solutions, since understanding any problems is the first step toward solving it.

The Basics

In its most basic form EMI is current flowing in the wrong place. It comes about from a disturbance that affects an electrical circuit due to either electromagnetic conduction or radiation emitted from an external source. Conducted EMI is caused by the physical contact of the conductors while radiated EMI is caused by induction (with no physical contact of the conductors).

EMI generators can be devices such as transmitters that intentionally radiate electromagnetic fields or devices that unintentionally produce electromagnetic fields, such as motors, welders and common household appliances. Digital circuits are a potential source of emissions due to their handling of periodic waveforms. In a digital environment noise can be generated by load transients, switching circuits, power supplies, and external cabling.

Almost any transitions with sharp edges, such as clocks, produce electromagnetic radiation. What’s more, internal clocks can interact with data streams, which, in turn, can interact with neighboring systems and potentially oscillate to cause further EMI at different frequencies than the initial problem signal.

Other highly repetitive signals, such as buses, also can cause EMI if they are not decoupled, terminated or filtered properly. These repetitive digital signals act like multiple hidden EMI transmitters, while cables and board traces act as multiple hidden EMI antennas.

Passives to the Rescue

EMI suppression and receiver noise reduction can be achieved effectively with efficient filtering methods. The whole purpose of using filters in electronics is to reject unwanted energy by diverting current from an undesired path—either to prevent emissions from getting too far from the chip or to prevent external interference currents from getting to the chip. It should be noted that filtering addresses EMI symptoms, but does not deal with EMI susceptibility.

A traditional EMI filter consists of passive components: capacitors, inductors, or resistors used singly, or combined together to form a filter network. You can use a capacitor to divert current, elements in series (resistors, ferrites, inductors) to block current on the line or a combination of both. EMI filters and/or chip ferrite beads serving in an EMI noise suppression capacity are available from a wide range of suppliers, including AVX, Dearborn, Kemet, Murata, Panasonic, Spectrum Advanced Specialty Products, TDK and Vishay, among others.

Due to their relatively low cost, availability and wide range of values, capacitors are often the main devices utilized to reduce EMI. Bypass and decoupling capacitors on each active device (connected across the power supply, as close to the device as possible) have long been used to reduce the amount of noise generated on circuit boards.

EMI filters can be found in configurations such as feedthrough capacitors in L-, Pi-, LC- and T-filters. Feedthrough capacitors are a good choice when the impedance connected to the filter is high. They provide a built-in ground path for noise across the signal line, and can function as in-line filters for digital circuits. The configuration of the feedthrough allows the signal to be fed through the device, with the capacitance filtering EMI noise to ground.

EMI can be effectively suppressed by installing filters on the clock line, bus lines, and power supply line. High frequency clock signals generate high frequency noise and the noise and signal frequencies may be close to each other. Therefore, an EMI filter with high and steep attenuation should be used. In addition, use of an EMI suppression-enabled clock IC can result in a reduction of system radiated EMI of 10 dB of more.

In power electronics demand for power converters giving ever-higher power densities and efficiencies makes EMI control very challenging. Higher power densities make the converter’s EMI filter susceptible to internal fields and demands for fast switching speeds of switching devices to minimize switching losses and improve efficiency can increase EMI significantly.

Tackle Layout First

Even the best filtering scheme will not overcome poor circuit board layout. Indeed, it is good practice to implement layout fixes before filtering, since a layout fix can be done without cost. Multilayer boards should be used whenever possible, with large power/ground planes embedded to lower the EMI generated by the system. A minimum of four layers is recommended to accomplish a low EMI PCB design. In all multi-layer boards can offer up to a hundred-fold EMI improvement over two layer boards.

Shielding is usually a last resort after other techniques have failed, because of the added expense of shielding components. A shield is essentially a conductive barrierthat will limit (or contain) the electric field and magnetic field of concern. Shield types vary from conductive paint to metal covers. The principle is straightforward: electrical and magnetic fields cannot penetrate a well sealed and grounded metallic enclosure.

However, discontinuities in the attenuation can occur where there are windows, seams, vents or I/O connections poking through the shield. Increasing the size of the opening in the metal case causes the shielding effect to decrease.

In a car, the GPS navigator, for instance, is usually shielded with a metal case to prevent noise radiation from the unit, as well as limit noise entering into the system.(It doesn’t help that car navigation systems most often are located very near the car stereo unit, which is easily interfered with by noise). Noise from a car navigator can radiate from the interface cable as well as via the connector port openings in the metal case. As a preventative measure EMI filters are installed on all lines of each interface cable to suppress noise conduction through the cable.

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