Dial Down the Noise: Limiting Electromagnetic Interference

By Paul Golata, Mouser Electronics
 

Back when I was a teenager, I got my first job to save enough money to buy a stereo. After several months of saving, I took my money down to an electronics store and bought an AM/FM stereo system with a turntable, 8-track tape player, and cassette player. I even went out and bought a variety of rock records, an electric guitar, and an amplifier. From that day on, all my parents seemed to say was, "Turn down that noise!"

Electromagnetic Interference

Well, I still create quite a racket, but as a technology specialist, now, I deal less with issues involving audible noise levels than I do with keeping various electronic systems and designs from generating spurious electromagnetic inference (EMI). EMI is known for unwanted electrical signals that permeate from an external source into an electronic circuit. Electromagnetic induction, electrostatic coupling, or conduction are responsible for generating EMI. This article focuses on how properly designed electronic solutions can assist engineers with "dialing down the noise": Limiting EMI in automotive and industrial applications so that interference is not an issue.

Automotive

An area experiencing the dramatic impact of the need to control EMI is that of automotive innovation. The automotive marketplace is expected to double approximately over the next 15 years as transportation transforms. Part of this transformation will be a byproduct of the electronics industry and its ability to provide new services that have not previously existed. These innovations will rely upon four key technologies that are shaping the future. The ACES acronym reflects the key technologies:

• A: Automation
• C: Connectivity
• E: Electric
• S: Shared

Standards: CISPR

The automotive industry commonly works according to the Comité International Spécial des Perturbations Radioélectriques-that is, the International Special Committee on Radio Interference-(CISPR) standards. The CISPR 25 Class 5 test standard is the automotive norm. The rapid and ongoing proliferation of electronics employed in automobiles in the form of systems, standard and optional features, and accessorized components along with the movement toward hybrid and fully electric vehicles is propelling this market.

EMI Sources

Both internal and external sources generate EMI. Because the number of electronic solutions within a vehicle is consistently increasing and they must operate as an organic whole that is void of excessive EMI disturbances to other systems, minimizing individual and collective EMI-producing sources is paramount. Internal and external EMI-producing sources of a vehicle may include: An airbag, Bluetooth(R) technology devices, controller area network (CAN) bus interfaces, collision avoidance systems (e.g., light detection and ranging (LIDAR)/radar), cruise control, direct current (DC) motors, electronic braking systems, an electronic control unit (ECU), fuel control systems, gaming systems, garage door openers, a global positioning system (GPS), ignition systems, an in-vehicle safety and security system, infotainment systems, power steering, radio (AM/FM/satellite), remote door entry, tire pressure monitors, and the like.

EMI Problems

Modern automobiles utilize switching power supplies that produce EMI and expose surrounding systems to this unwanted interference. The ever-increasing quantity of switching power supplies within the automobile coupled with decreasing physical size, proximity, the need for high electrical efficiency, the need to operate under intense thermal stress, and the need for long lifetimes all call for electronic products that can meet these harsh environmental demands.

As autonomous vehicles start to come to market, EMI issues will become ever more important. The reason is that the safety of the entire vehicle and its occupants will be dependent upon proper system operation as human drivers yield their control, travel, and safety to the vehicle.

Reduction

A proverb attributed to American polymath Benjamin Franklin (1705–1795) states, "A gram (ounce) of prevention is worth a kilogram (pound) of cure (1735)." The proverb conveys the reality that electronic designers should consider EMI issues long before everything is integrated together, because if they do not expend the proper amount of thought on the front end of a design, it will cost a great deal more to make corrections on the back end.

Electronic engineers have three primary ways they typically work to reduce EMI. The first step may include ensuring that electronic components or systems carry the proper electromagnetic (EM) shields. Proper EM shielding involves adding conductive or magnetic materials into the design to act as barriers against spurious radiation fields. Applying a Faraday shield is a common approach. Secondly, engineers can utilize electronic filtering. Filters act to block unwanted frequencies while letting the desired frequencies pass with little to no effect. The final EMI-reducing method is to build in proper electrical grounding. Grounds provide a pathway for electricity to pass into earth (ground), which is a point of low electrical potential (voltage). Grounds help to reduce excessive accumulation of undesirable electrical charges.

Let's now turn to look at how two completely distinct electronic components may work in collaboration to dial down the noise and limit EMI issues in automobiles.

A Solution: Power Management Integrated Circuits

Analog Devices (ADI) is the leading global, high-performance analog technology company dedicated to solving the toughest engineering challenges. Acknowledged industry-wide as the world leader in data conversion and signal conditioning technology, ADI enables customers to interpret the world around them by intelligently bridging the physical and digital with unmatched technologies that sense, measure, power, connect, and interpret. To that end, ADI has introduced a family of products that can solve a variety of problems, showing itself as an excellent solution for applications requiring low EMI.

To lower EMI, ADI has invented their patented Silent Switcher® technology which enables impressive EMI-resistance performance in high-frequency, high-power supply scenarios. Silent Switcher technology splits the single, largest, powered hot loop into two smaller, powered hot loops that are strategically positioned to cancel each other's magnetic field, acting in the same manner as a Faraday shield.

Silent Switcher® 2 architecture is the next generation of this technology. It simplifies board design and manufacturing by incorporating external hot and warm loop capacitors as the internal bypass capacitors on the inside of an integrated circuit (IC), thus reducing radiated EMI. Internalizing the capacitors lowers the EMI through the minimization of noisy antenna sizing. Silent Switcher parts are also monolithic regulators, providing high-level integration through integrated metal-oxide semiconductor field-effect transistors (MOSFETs). All these implementations lead to higher efficiency, especially at higher switching frequencies.

The ultralow EMI of the Silent Switcher 2 design eliminates printed-circuit-board (PCB) layout sensitivity, and one way to reduce solution costs is to minimize the number of required PCB layers, though this will yield performance tradeoffs. For instance, designers do not expect a 2-layer board solution to produce an equivalent EMI performance of a 4-layer solution. In general, Silent Switcher 2 technology yields excellent EMI performance with 2-layer boards and even single-layer boards, which can greatly reduce manufacturing costs.

Additionally, Silent Switcher offers optional spread-spectrum modulation, which also helps to lower EMI. Silent Switcher and Silent Switcher 2 regulators meet the demanding EMI-emissions requirements of automotive environments.

Powering Systems on Chips

System-on-chip (SOC) devices are found throughout today's automobiles. Infotainment systems, cameras, collision avoidance systems, safety and warning devices, and sensors are processing ever increasing levels of information. Processing must be done in systems that are efficient yet self-contained, without additional electrical power, but it requires high efficiency. Because SoCs are doing so much, they are regularly consuming power that can make managing EMI difficult.

New, high-technology vehicles may contain a large variety of Arm cores, digital signal processors (DSPs), video and graphics processors, along with ancillary support devices. Each of these components requires reliable power that likely include the following voltage rails: 5V, 3.3V, 1.8V, 1.2V, 1.1V, and 0.8V.

To support the current levels of SoCs demand, a switching power controller with external MOSFETs is the traditional choice over monolithic power devices. Monolithic devices are compelling because their internal MOSFETs minimize cost and solution size, but their traditionally limited current capability and thermal issues typically limit their use. The LT8650S is a new family of monolithic, step-down Silent Switcher regulators that possess the current capability and thermal management features to support SoCs.

LT8650S

Let's consider ADI's (Power by Linear™) recent release of the LT8650S - Dual Channel 4A, 42V, Synchronous Step-Down Silent Switcher 2 with 6.2µA Quiescent Current (Figure 1), designed for applications that include general purpose step-down and automotive and industrial supplies. The LT8650S dual-channel, synchronous, monolithic, Silent Switcher 2 regulator offers SoC applications with a wide input voltage range, exceptional EMI-resistance performance, and a small-solution size while providing multiple high-current outputs and fast transient responses.

Figure 1: This diagram depicts ADI's (Power by Linear) recent release: The LT8650S Dual-Channel 4A, 42V, Synchronous Step-Down Silent Switcher 2 with 6.2µA Quiescent Current. (Source: Analog Devices)

It has two channels-each capable of supplying 4A simultaneously with up to 6A available on either channel. It has an ultralow-quiescent-current BurstMode® operation of 6.2μA IQ when regulating 12VIN to 5VOUT1 and 3.3VOUT2 along with a low-output ripple of <10mVP-P. The part provides high efficiency (94.6 percent at 2A, 5VOUT from 12VIN at 2MHz and 93.3 percent at 4A, 5VOUT from 12VIN at 2MHz) at low-output currents, a forced continuous mode that allows for a fixed switching-frequency operation over the entire output-load range and a spread spectrum operation, which can further reduce EMI emissions. LT8650S's optional external VC pins empower optimal loop compensation for a fast transient response. The VC pins can also allow for current sharing. LT8650S triggers a fast minimum switch-on time of 40ns. It operates with a wide input-voltage range of 3 to 42V and an adjustable and synchronous permissible switching frequency (fsw) of 300kHz to 3MHz in a small 4 × 6mm, 32-pin, low-profile quad-flat no-leads (LQFN) package.

Automotive SoCs place heavy demands on the power supply's load-transient response. It is not uncommon to see a load-current slew rate at 100A/µs for peripheral power supplies and higher for core supplies. Regardless of the changes in load, the power supply must minimize the output-voltage transient. A fast switching frequency such as the 2MHz capability of the LT8650S family helps speed transient recovery. Faster switching frequencies correspond to faster dynamic responses with proper loop compensation.

Power Inductors Support Low EMI Solutions

Coilcraft is a leading global supplier of magnetic components such as high-performance radio-frequency (RF) chip inductors, power magnetics, and filters. In addition to being a supplier of a large selection of standard components, Coilcraft is also a recognized leader in designing and building custom magnetics to fit a customer's exact electrical requirements for applications in various industries, including automotive and industrial. Coilcraft is considered as a preferred supplier by engineers because of its reputation for quality, reliable delivery, engineering support, and the superior performance of their products, which are all critical in markets such as automotive and industrial.

Coilcraft offers three popular styles of high-performance, molded, power inductors, which are designated as the XEL, XAL, and XFL series within the Ltra product family. The XEL series and XAL series are both innovative, composite material, power inductors designed to offer a very soft current saturation profile. The XFL series offers a soft saturation profile, but it is more similar to the traditional ferrite assembled parts and drops faster than XAL and XEL. XFL offers the lowest direct current resistance (DCR), low core losses, and a low profile. The new XEL has lower alternating current (AC) loss compared to the XAL, while XFL has the lowest total power loss (Figure 2, multi parts). For high-switching-frequency power converters, which are designed to withstand high peak current, XEL is the best choice.

Figure 2: The tables provide a view of important differences including the inductance versus current relationship as well as the power losses versus ripple current relationship between the XAL, XEL, and XFL series. (Source: Coilcraft)

ADI choses to use Coilcraft's power inductors for its LT8650S demo board (DC2407A) design. The fixed inductor ADI choice was the XFL5030-102ME, 1µH, AEC-Q200-a member of the Coilcraft XAL/XFL Power Inductors family. This family of parts got selected to work with the new generation of Silent Switcher 2 chips due to their smaller size and higher efficiency at higher frequencies. A variety of factors made this an excellent decision by the ADI designers.

Inductor Technology Drives Lower EMI

Coilcraft XAL/XFL Power Inductors get produced with Coilcraft's Ltra technology. Ltra technology is a unique way of producing an inductor using magnetic materials, such as powdered iron (Fe),to form a solid, combination body shape through a molding process. This technology effectively eliminates any wasted space between windings, which results in a higher saturation current and lower DC resistance in the same size part.

The market trend is oriented to design DC/DC converters with increased switching frequency and the target of reducing the inductance value required as well as the size and cost of the power inductor. Together with this target, there is also the need to increase component efficiency. With the switching frequency increase, the frequency related losses and core losses-AC losses-become relevant and then a request can occur for specific parts designed to operate at a high switching frequency. The Ltra technology composite material core provides better efficiency at a 2MHz switching frequency.

Ltra products are designed with large terminations to offer the best possible solder-joint strength and thermal management. Large terminations can also carry high current ratings without hot spots, maintaining a cooler part and increasing long-term reliability. The compact size of these components allows for a closer displacement on the PCB, reducing the overall solution size.

Ltra is designed to offer very robust parts to the market. Their rugged shape with strong resistance to mechanical and vibration stress make them suitable for harsh environments like automotive, industrial, and portable electronics applications. Ltra offers superior temperature stability. The variation of saturation that curves over a temperature range from -40°C to +125°C is below 3 percent. These products are AEC-Q200 grade 1 qualified.

Some inductor core materials exhibit a negative property called thermal aging. When thermal aging happens to inductors, the efficiency gets worse with the application of time and temperature. The efficiency loss causes more heat, which again lowers the efficiency leading to more heat and so on. Inductors and circuits can overheat beyond acceptable ratings-sometimes with disastrous effects. Coilcraft Ltra materials embody special engineering to eliminate thermal aging concerns.

Ltra technology also reduces radiated EMI using effective shielding: The presence of magnetic resins throughout the coil body ensures the best possible reduction of harmful EMI emissions, and the fringing flux that is well contained in the coil body allows the placement of components and current traces to be very close.

Coilcraft's scale of parts within their power inductor family allows for an ample selection options. This empowers a designer to select different inductors based upon the specific design under consideration. With different sizes, optimized parameters, and varying power levels each style offers unique performance benefits.

These power inductors offer low DCR. DCR indicates the resistance (ohms) of an inductor as a result of the resistance of the wire used in the winding.

These power inductors also provide sufficient saturation current (ISAT). ADI does not typically max out the current limit per channel due to thermal de-rating-which for this device is 4A per channel with both channels running simultaneously and only 6A per channel when only one channel is on.

Conclusion

Today's automotive and industrial applications demand electronic solutions that provide high efficiency, high frequency, and low EMI. A specific solution example highlighted how ADI's Silent Switcher 2 regulators and Coilcraft's power inductors are products working together to deliver the low EMI solution you need. The only noisy emissions in your car that you need to worry about now are the kids in the backseat screaming, "Are we there yet?"