Sinrace Power Supply Technology (Shenzhen) Co., Ltd.

In the United States and Europe, Class A and Class B limits of FCC (Federal Communications Commission) and VDE (European association for electrical, electronic, and information technologies) standards govern conducted-noise emissions. In the United States, the FCC requires compliance with Class A for equipment operating in factory settings and Class B—the stricter standard—for equipment destined for home use. In Europe, all countries require that equipment for both home and factory use meets the VDE Class B standard.

Most switching power supplies today operate in the frequency range of 100 kHz to 1 MHz. Usually, the dominant peaks in the conducted-noise spectrum reflect back to the power line, corresponding to the fundamental switching frequency and its harmonic components. Conducted emissions standards, such as EN55011 and EN55022, set quasi-peak and average limits on the conducted noise that reflects from the input of converters or power-supply systems back to the source over the frequency range of 150 kHz to 30 MHz. To comply, all of the conducted noise—the peaks in the spectrum—must fall below the specified limits.

Power supply manufacturers most often construct EMI (electromagnetic-interference) filters in a single package with configurations similar to those in Figure 1. An EMI through-hole filter has a common-mode choke and line-to-ground Y capacitors plus two additional inductors and a line-to-line X capacitor; Z1 provides transient protection. This filter configuration provides sufficient insertion loss to comply with the Level B conducted-emissions limit. Nevertheless, power-supply designs often use capacitors, inductors, and active and passive filters to reduce or attenuate the amount of both common- and normal-mode conducted noise.

The 48V-input dc/dc converter has differential mode capacitor C1 on the input. This single-mode, 120-µF, 100V electrolytic capacitor ensures low input impedance, high stability, and good transient response, and it serves as an energy reservoir for the converter. To reap the most benefit, designers should place the capacitor as close as possible to the input pins of the module.

The module and the single capacitor provide a starting point for the EMI-reduction effort. The spectra of Figure 2a show the harmonic content of the noise and the EMI limits, A and B levels, for this converter-and-differential-mode capacitor combination. Testers made these measurements at 100% load nominal line for a 48V, 150W dc/dc converter. With only this differential-mode capacitor, the converter is clearly not meeting the limits, but the power component's developers did not design it to meet any specific EMI limits.

The effect of adding bypass capacitors to the converter-and-differential-mode-capacitor combination is rather dramatic. Notice the bypass capacitor on each input pin to the baseplate, which is ground, as well as on each output pin to the baseplate. These electrolytic capacitors are 4700-pF, 100V Y capacitors that are common in the industry. They effectively attenuate the type of noise that the power component generates.

The 48V design with 100% load generates a little higher noise than would, for example, a 3.3V design with a 50% load. Nevertheless, the spectrum of Figure 2b shows significant improvement. Even with the addition of a 27-µH differential inductor, L1, the 48V design is still not compliant at the lower frequencies, in which noise is still present above the B limits.

Figure 2d shows the next stage of adding a common-mode choke. This configuration eliminates the differential-mode choke, because the common-mode choke has differential-mode inductance. The common-mode inductor accentuates the capabilities of the Y capacitor, because it provides high impedance to common-mode noise that a filter must direct out of the converter; therefore, the noise follows the path of least resistance to ground, which is through these Y capacitors.

The spectrum of the 48V converter peaks just over the top of the B limit; thus, the 48V converter design needs a little more filtering. The noise spectrum of a 3.3V converter with a common-mode filter would be less than the B limit, at both 50% and full load.

Conducted EMI compliance in telecommunications is emerging as an important application for active filters for dc/dc converters. In 2003, PICMG (Peripheral Component Interconnect Manufacturing Group) ratified a new specification for telecom blades—PICMG 3.0, or more commonly, ATCA (Advanced Telecom Computing Architecture). The specification requires dc-powered blades to meet the EN 55022B limits for conducted EMI. Filtering at the blade level ensures interoperability between blades and reduces the amount of bulk dc filtering that each equipment rack requires.

Furthermore, the trend toward smaller devices with more functions in less space continues in the electronics industry. As available space shrinks, the potential interference between devices increases, as systems cram more functions into densely packed blades and racks. The control of conducted EMI becomes an even more important design task as frequencies rise and voltage levels fall.

Telecom blades are not exempt from the trend toward denser packaging coupled with higher performance. The ATCA PICMG 3.0 specification supports a 2.5-Tbps backplane bandwidth in a standard 19-in. rack. A fully populated ATCA rack can have 14 blades in a 19×21×15-in. volume. To support more functions, each blade can use as much as 200W of dc power. EMI compliance becomes more difficult, because the standard requires each blade to supply its own power from a –48V dc redundant input. Onboard dc/dc conversion, in the form of bricks or discrete converters, generates conducted and radiated EMI on each blade. Unlike compact PCI, in which most of the dc power converts in the blade itself, EMI control becomes a nightmare under these circumstances.

To minimize blade-to-blade and rack-to-rack interference, ATCA blades must provide onboard filtering for conducted EMI. PICMG 3.0 states that each blade must meet the conducted noise specification of EN5022B. A blade-level approach to filtering minimizes interference between cards. PICMG 3.0 also requires the rack to meet an overall conducted EMI standard. By controlling EMI on card with "distributed" filters, the filter for the rack can be smaller. The filter for a fully populated ATCA rack needs to support almost 60A of dc current. Inductors to support this current are large, but controlling EMI onboard helps to keep these inductors as small as possible.

Active EMI filters are available that attenuate conducted-mode and differential-mode noise over 150 kHz to 30 MHz, which conducted emissions standard EN55022 (CISPR22) requires. Their 7A rating supports multiple dc/dc-converter loads up to a pc-board temperature of 60°F; they find use on 48V dc bus, which span 36 to 76V dc, in real-world telecom environments.

Unlike passive-design approaches, active filtering reduces the volume of the common-mode choke, resulting in a low-profile, surface-mount device. This situation occurs primarily because active EMI filters reduce the size of the inductive elements, allowing an EMI filter to reside in a 1×1×0.2-in. package. This size saves valuable board real estate, and the reduced height enhances airflow in blade applications, helping blade designers recover some of the lost space that EMI control requires. The 7A rating of active filters easily handles the current required for a 200W ATCA blade.

Figures 4a and 4b, respectively, demonstrate the "before" and "after" overall dc/dc-converter noise profile when using an active filter. The plots were taken using the standard measurement technique and set up according to CISPR (International Special Committee on Radio Interface) 22. The results show the total noise spectrum for a standard dc/dc converter under load, compared with the EN55022 Class B quasipeak-detection limit. They reveal that an active filter effectively reduces the total conducted noise spectrum to well below the required limits.