Silence of the amps
28 September 2009
Ultralow EMI DC/DC regulator system meets EN55022 Class B Standard.
There are many categories of EMI. An engineer has to worry about both susceptibility and emissions. Susceptibility refers to 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 people’s products.
In general, a designer worries most about emissions. With few exceptions, most systems operate in an environment where the emissions of each product are required to 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 so that no one has to worry about susceptibility.
How we failed
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 aluminium 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 localised 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 increase a design’s chances of 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 10 dB lower than class A.
Class A devices are used in commercial, industrial or office environments. Class B devices are residential. An example of a class A device would be 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 upon the other components that make up 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 (Translation: International special committee on radio interference).
To date, there are three products that have radiated EMI emissions compliant with CISPR 22 class B: LTM8020, LTM8021 and the LTM8032.
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.
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 emission 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 parts listed above, the chosen test specification is CISPR 22 class B.
Of the three products under discussion, the LTM8032 is purposely built for low EMI. It is rated for up to 36 VIN, and 10 VOUT at 2 A. It was tested in MET Labs’ 5 metre 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 assembled unit is placed atop an all-wooden table. The all-wood construction ensures that the test setup 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 heatsink, 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 – they are not relevant to this discussion.
Figures 3a and b give the LTM8032 emissions plots for maximum power out, 10 V at 2 A, for 24 V and 36 V 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 Figure 3 through 6 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 upon 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.
There are two traces in the plot, one 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 10 W out, 5 V at 2 A, from 12 VIN. Once again, the emissions are very low.
Two other parts are also CISPR 22 class B compliant, the LTM8020 and LTM8021. The LTM8020 is rated for up to 36 VIN and up to 5 VOUT at 200mA, while the LTM8021 is rated for 36 VIN, 5 VOUT at 500 mA. These two devices were tested in MET Lab’s 10 metre chamber. This chamber is a bit noisier than the 5 metre 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 through 6.
The DUT configuration is similar to the LMT8032. They are assembled on circuit cards with no bulk caps and only the minimum required ceramic caps. 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 12 V. The output power is 1 W, 5 V at 200 mA. Emissions spectrums for the LTM8021 are given in Figure 6 for input voltage of 12 V. The output power is 2.5 W, 5 V at 500 mA.
An innovative family of DC/DC µModule regulators has been designed for noise sensitive electronics 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, and complete DC/DC circuit in a surface mount package. This family of DC/DC µModule regulators brings a peace-of-mind to all system designers concerned with noise. The LTM8020, LTM8021 and LTM8032 are quiet and likewise, this article ends with little chatter!
David Ng is a Design Manager for Linear Technology.
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