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Posted on 05 July 2019

Minimizing Electromagnetic Interference When Powering Densely Populated Systems

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Ultralow EMI DC/DC Regulator System Meets EN55022 Class B Standard

Systems incorporating sophisticated high frequency functionality are consuming more power and are increasingly assembled on denser circuit boards. Higher power consumption and close proximity of components increase the risk of point-of-load switching regulator’s electromagnetic energy interfering with RF circuitry.

By Afshin Odabaee, μModule Product Marketing Manager, Linear Technology Corporation

 

The noise from switching DC/DC regulators encompasses both conducted and radiated noise. The conducted noise travels over printed circuit board (PCB) traces and can be attenuated with filters and proper layout. Experienced system designers resolve this issue by adding input and output filters such as ferrite beads (pi-filters) and via careful layout of the PCB. Often a linear post regulator is used after a switching regulator’s output to filter some of this energy. This is a common practice when powering RF power amplifiers, for example.

The radiated noise, also referred to as electromagnetic noise, travels through air (space) and is often more difficult to tame. This must be resolved at the source and the source can be as obvious as multigigabit transceivers or as elusive as DDR memory or the overlooked DC/DC switching regulator.

Raise Some Noise

There are many categories of EMI. An engineer has to worry about both susceptibility and emissions. Susceptibility refers the amount of noise that can be thrown at the design without malfunction or destruction, such as ESD spikes, AC riding on a DC line and even lightning strikes. Emissions refer to the amount of noise that the design throws out at other products.

In general, a designer worries mostly about emissions. With few exceptions, most systems operate in an environment where the emissions of each product must not exceed some predefined level. In theory, if each product complies with these emission levels, the noise level running throughout the system is low enough that there is no worry about susceptibility.

Switching DC/DC regulators, by nature, dissipate energy. It is the strongest at the switching frequency of the regulator (switching of gate of MOSFET, for example, is one source). Depending on the frequency, the harmonics or the strength of the energy, a DC/DC switching regulator can disrupt data integrity or at times prevent a system from passing EMI standards such as EN55022 or CISPR22 class B or A.

Often, systems engineers who have been burned by previous last minute EMI issues, decide to over-filter a regulator’s circuit. The fear of not passing a test due to EMI noise is far greater than the cost or wasted PCB area. And no one can blame them for their concern.

There are several common methods to alleviate noise. Here is a list:

Bypassing

Bypassing is used to reduce the flow of high switching current especially in high impedance PCB traces. It is often accomplished by shunting the path by a capacitor.

Decoupling

Decoupling in a power supply circuit refers to the isolation of two circuits on a common line. As was mentioned before, low pass filters are very effective.

di/dt

Know the sources of fast derivative current sink or source and determine their return paths. Make sure to bypass all of them.

Layout

Make sure your small signal, ground and power planes are properly placed. Keep small signals and power planes separated from one another. Minimize inductance in your traces.

Shields

To contain and reduce emanated energy from a DC/DC regulator (if it’s “noisy”), you may need to add metallic shields around the circuit. Use shielded inductors.

Adjust the frequency

Does the switching regulator have an adjust pin, PLL (phase lock loop) or SYNC pin to set the switching frequency to a desired value? It’s a good idea to choose a switching regulator with PLL capability. It may come in handy later on during final testing of your board.

Spread Spectrum Frequency Modulation (SSFM)

Some modern switching regulators come with an on-board SSFM feature. Or you can buy an SSFM clock generator if the regulator lacks this function. With SSFM you can reduce the energy level by spreading it across wider frequency range, thus preventing a strong level concentrated at a particular frequency value. Be sure that the switching regulator has SYNC or PLL capability.

Let someone else worry about it

If the switching regulator circuit is designed cleverly, its layout optimized and most importantly already tested under strict industry EMI standards, then someone else has already done the job. These products do exist.

We Failed the EMI Test

There are three little words that design engineers dread: “We failed EMI.” There are four little words that are even worse: “We failed EMI again.” Many a seasoned engineer is scarred with dark memories of long days and nights in an EMI lab, struggling with aluminum foil, copper tape, clamp-on filter beads and finger cuts to fix a design that just won’t quiet down.

There are two types of 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.

Radiated emissions 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, no one really knows how the bad the radiated emissions are until the system gets tested, and radiated emissions testing cannot be formally performed until the design is essentially complete.

So what is a design engineer to do? One approach is to use parts that are pre-tested and known to have low emissions. Using these “verified and certified” parts greatly increases design success.

In the United States, radiated emissions and testing are regulated by the Federal Communications Commission. The most commonly encountered specification is the Federal Code of Regulation (CFR) FCC Part 15. CFR FCC Part 15 regulates all radio frequency devices, whether or not they are intentional emitters. It defines two classifications of unintentional radiating digital devices, A and B. Class B is stricter, defining limits around 10dB lower than class A.

Don’t get confused by the term “digital device.” In the FCC’s eyes, a digital device is anything that generates and uses timing signals of frequency greater than 9kHz. Today, that covers a lot of products, including most switching power supplies.

Class A devices are used in commercial, industrial or office environments. Class B devices are residential. An example of a class A device is a mainframe computer, seldom seen in a home. A monitor, while certainly used in offices, is also used in private homes, so it is a class B device.

In order to be useful in a class B device, a component should radiate less noise than the specified limit. How much less is dependent on the other components in the system. If the device emits more than the class B limit, some means must be devised to reduce the noise, such as shielding or slew rate limiting.

Meeting EMI Regulatory Standards

In Europe, allowable electromagnetic emissions are generally defined by EN55022. Another commonly encountered specification is CISPR 22, which comes from the international agency Comite International Special des Perturbations Radioelectriques (International Special Committee on Radio Interference). These two specifications are similar to FCC part 15, defining similar (but not identical) limits and dividing them into the two A and B emissions classes.

In today’s modern designs, switching power supplies can make a significant contribution to the radiated noise coming from a system. To date, there are three products that have radiated EMI emissions compliant with CISPR 22 class B: LTM8020, LTM8021and the LTM8032 ìModule® DC/DC regulators.

Each of these units was tested at the MET Labs facility in Santa Clara, California. MET Labs is accredited by numerous agencies, including NIST and A2LA for EMI testing. A complete listing of MET’s credentials is given on their website: www.metlabs.com/pages/emcaccred.html

Radiated emissions testing is highly regulated, and the test method specifications are very detailed. There is no means by which a design engineer can influence the measurement technique or method. When asking a lab to perform radiated emissions testing, an engineer chooses only the test specification; the lab handles the rest and the design engineer is not invited to participate in the measurement process. In the case of the LTM8000 series µModule devices listed above, the chosen test specification is CISPR 22 class B.

Of the three products under discussion, the LTM8032 is built specifically for low EMI. It is rated for up to 36VIN, and 10VOUT at 2 Amps. It was tested in MET Labs’ 5 meter chamber set up as shown in Figure 1. The LTM8032 is mounted on a circuit board with no bulk capacitance installed. The input and output capacitance are the minimum ceramic values specified in the data sheet for proper operation.

The test set-up. The power source, a linear lab grade power supply, is on the floor

The assembled unit is placed atop an all-wooden table. The all-wood construction ensures that the test set-up does not shield or shadow noise emanating from the device under test (DUT). The power source, a linear lab grade power supply, is on the floor. The load for the LTM8032, with its heat sink, is also on the table top.

Measuring EMI from the LTM8032

Before measuring the emissions from the LTM8032, a baseline measurement is taken to establish the amount of ambient noise in the room. Figure 2 shows the noise spectrum in the chamber without any devices running. This may be used to determine the actual noise produced by the DUT. Ignore the red lines in the Figure 2 graph, as they are not relevant to this discussion.

LTM8032 baseline - Ambient noise in the 5 meter chamber (no devices operating)

Figures 3a and b give the LTM8032 emissions plots for maximum power out, 10V at 2A, for 24V and 36V inputs, respectively. There is a slight discrepancy to note between the spectrum plots and the CISPR 22 class B limits. The CISPR 22 class B limits shown in Figures 3 through 7 are for quasi-peak measurements, which take the peak noise emissions and calculate the integral average of the noise signals over time. The time of the averaging is based on the frequency at which the noise is detected. The noise measurements in Figures 2 through 6, however, are simply peak measurements, as indicated in the upper right corner of the spectrum plot, so the design margin indicated in the plots is even greater than what is graphically indicated. A copy of this report is available on www.linear.com/umodule.

LTM8032 emissions for 20 Watts out, 24Vin

LTM8032 emissions for 20 Watts out, 36Vin

There are two traces in the plot, one each for the vertical and horizontal orientations of the test lab’s receiver antenna. The LTM8032 easily meets the CISPR 22 class B limits by a wide margin.

Figure 4 shows the emissions at 10Watts out, 5V at 2 Amps, from 12Vin. Once again, the emissions are very low.

LTM8032 emissions for 10 Watts out, 12Vin

Two other parts are also CISPR 22 class B compliant, the LTM8020 and LTM8021. The LTM8020 is rated for up to 36Vin and up to 5Vout at 200mA, while the LTM8021 is rated for 36Vin, 5Vout at 500mA. These two devices were tested in MET Lab’s 10 meter chamber. This chamber is a bit noisier than the 5 meter chamber, as shown in Figure 5. As in the case of the LTM8032, the red lines are the quasi-peak limits, while the spectrum plot displays the peak measurements. The actual noise margin is greater than what is shown in Figures 5 and 6.

LTM8020 emissions for 12Vin, 5Vout at 200mA

The DUT configuration is similar to the LTM8032. They are assembled on circuit cards with no bulk capacitors and only the minimum required ceramic capacitors. They are mounted on a wooden tabletop, along with the load, and the power source is on the floor.

Emissions spectrums for the LTM8020 are given in Figure 6 for input voltages of 12V (data for 24V and 36V inputs are available at www.linear.com). The output power is 1W, 5V at 200mA.

LTM8021 Emissions for 12Vin, 5Vout at 500mA

Emissions spectrums for the LTM8021 are given in Figure 6 for input voltage of 12V. The output power is 2.5W, 5V at 500mA.

Summary: Ultralow EMI, Low Heat Dissipation and Compact DC/DC Systems-in-a-Package Solve EMI Issues in RF Systems

An innovative family of DC/DC μModule regulators has been designed for noise sensitive electronic systems such as RF systems that are concerned with EMI. These devices have been tested by a certified test lab for EMI evaluation.

These μModule regulators provide ultralow noise performance with high efficiency, compact package and a simple design similar to a linear regulator because of:

• Shielded inductors
• Careful layout
• On-board filters
• Controlled MOSFET gate drive
• Low input and output ripple
• Complete DC/DC circuit in a surface-mount package

This family of DC/DC μModule regulators brings peace-of-mind to all system designers concerned with noise. The LTM8020, LTM8021 and LTM8032 are quiet and provide complete power supply solutions for wireless systems.

 

 

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