Low-Cost Conducted Emissions Filtering in Switched-Mode Power Supplies

Charles Grasso and Bert Downing

An often overlooked EMI-suppression mechanism available for flyback power-supply design saves space while minimizing filtering cost.

The power-supply engineer faces two contradictory standards requirements. Furthermore, the conflict is exacerbated when the engineer is designing in a cost-competitive environment. In order to meet conducted emissions requirements, the engineer normally employs common-mode filtering techniques that incorporate capacitors to ground. However, safety standards strictly govern the amount of leakage current that a product can generate at customer-accessible locations—and leakage current is a direct function of the capacitors used in the filter design.

This article examines the flyback power-supply design from an electromagnetic compatibility (EMC) perspective, and it identifies and describes a critical, often overlooked emissions artifact. In addition, it describes an easy yet commonly neglected suppression mechanism that can dramatically reduce the overall cost of power-supply filtering.

The Design Engineer’s Challenge

A manufacturer of electronic products has legal responsibilities to meet EMC and safety standards applicable in the product’s market. For information technology equipment (ITE) manufacturers, the EMC standards are becoming harmonized in accordance with the average and quasi-peak limits described in CISPR 22. The safety requirements for ITE are based on the end use of the product and strictly control the leakage current limits for various types of product. For example, a personal computer, under the scope of UL 60950, is allowed 3.5 mA of leakage current, whereas a piece of audiovisual equipment, according to UL 6500, is allowed only 500 µA of leakage current.

Thus, the power-supply engineer is presented with a design challenge. If the product under design can be marketed to the home, the radiated and conducted emissions profile must meet the more stringent Class B limits. However, if the product is something like audiovisual equipment, the safety leakage requirement of 500 µA applies and sets the upper value of any common-mode filter capacitors that can be used in the filter design. An added complication is that the consumer-sector ITE industry is highly cost-competitive. With the ac filter representing a significant percentage of the overall power supply design, designers expend considerable effort to maximize ac filter performance at minimum production cost.

The use of switched-mode power supplies (SMPSs) has become ubiquitous in low-cost consumer electronics. By contrast with linear power supplies, an SMPS offers efficiencies up in the 80% range and a very compact size for the power generated. The ac or dc input voltage generally is chopped at high frequency—typically in the 120-kHz range, but higher frequencies up to the MHz range are possible—rectified, and finally filtered to provide the required outputs.

Selection of the convertor topology is an important aspect of controlling emissions. In terms of EMC, the Cuk convertor, a convertor based on capacitive energy transfer that uses a resonant circuit to achieve zero-voltage/current switching, is usually regarded as the quietest solution. The flyback convertor, which uses isolated inductive storage, is by and large regarded as the least expensive but noisiest topology for power-supply engineers to deal with.¹ It remains the favored design for use in cost-sensitive consumer electronics. A typical flyback configuration is shown in Figure 1.

Figure 1. Block diagram of a flyback power supply.

The ac supply is fed through an electromagnetic-interference (EMI) filter to a full-wave bridge rectifier and smoothed with a bulk capacitor. A primary side switching circuit, typically a metal oxide semiconductor field-effect transistor (MOSFET), using a variable-duty cycle opens and closes the connection to the transformer primary. When the MOSFET is closed, the smoothed dc is applied to the transformer, and when the MOSFET is opened, the back electromagnetic force in the transformer couples to the secondary and the output is rectified and filtered. The pulsed primary waveform produces a high-voltage, high-spectral-content waveform that is the primary source of the conducted emissions profile.

The EMI filter employed to comply with the emissions requirements adds components, takes up board space (a hidden cost), and can represent a significant fraction of the overall cost of the design. For example, in a 25-W $6 power supply, 75 cents for filtering is a whopping 12.5% of the cost.

Flyback Conducted Emissions

The various sources of conducted emissions in switched-mode power supplies have been thoroughly documented.^2–4 They chiefly involve parasitic coupling, snubber networks, and trace and component placement. While parasitics do play an important role in conducted emissions, the flyback topology contains an often overlooked source of common-mode emissions that is a by-product of the design and that can have a dramatic effect on the common-mode conducted emissions profile, even at low frequencies.

It is generally accepted that differential-mode emissions are predominant below 1 MHz and that common-mode emissions come to the fore above 1 MHz. However, the flyback noise source generates common-mode emissions from the supply at frequencies of less than 1 MHz. These consist primarily of harmonics of the fundamental switching frequency.

How this additional source of common-mode noise contributes to the conducted emissions profile of an SMPS can be seen in the diagram in Figure 2. In the flyback design, the primary switching ground is not connected directly to secondary ground or earth ground because the line side of the ac input would be connected to ground and thus would create a short circuit. Therefore, the primary ground has a radio-frequency (RF) potential above earth ground (identified in Figure 2) as common-mode voltage (V_cm) developed across the common-mode impedance Zt. This V_cm results in a common-mode current (I_cm(t)) with two components. Part of the common-mode current flows through the capacitance of the transformer and into the dc secondary (I_cm(int)). The remainder (I_cm(ext)) flows out of the power supply and into the line impedance stabilization network (LISN), where the interference is measured and compared against commercial emissions standards.

Figure 2. Block diagram of the common-mode current flow.

The data displayed in Figure 3 show how V_cm can affect the emissions profile of an SMPS. The chart on the left (Figure 3a) is the conducted emissions profile of a power supply designed as a Class II product (with no ground wire). This profile reflects the limits used before FCC conducted emissions were changed to harmonize with CISPR 22 in July 2004. Figure 3b shows the effect of adding the ground wire—the low-frequency emissions increase by 34 dB. More notably, this effect was seen on a power supply that had an input filter with a knee frequency at 37 kHz and theoretically provided more than 60 dB of attenuation at 450 kHz.

Figure 3. The effect of adding a ground wire to a 75-W switched-mode power supply is apparent from a comparison of the conducted emissions profile without the ground wire (a) and with the ground wire (b).

An analysis of how and why the jump in low-frequency emissions occurs can be performed with reference again to Figure 2. In a flyback SMPS design, the power ground is floating with respect to chassis ground, creating a local RF potential, V_cm, that excites the primary switching loop with harmonics of the switching circuit. The RF current generated by V_cm is split between the primary and secondary impedance loops of the power supply, with values in proportion to the loop impedances. The secondary (internal) loop consists of the primary-to-secondary capacitance of the transformer transferring I_cm(int) to the secondary and returning to V_cm through the secondary capacitances and the parasitic impedance between power ground and chassis ground. The primary (external) loop consists of the primary circuit, the line cord, and the LISN. The return path is along the green wire for the closed loop.

Now, if the green wire is removed, the product has a two-bladed wire cord and no ground pin. Figure 2 shows that external loop is broken, minimizing the common-mode current caused by V_cm measured at the LISN.

Based on the preceding analysis, if the common-mode impedance Zt is minimized, then V_cm will be reduced and Icm minimized, with a follow-on reduction in the degree of attenuation required by the ac filter. This allows the ac filter to be made from smaller, cheaper components.

Zt is reduced by adding a capacitor between the chassis ground and the power ground. The importance of this bridge capacitor should not be underestimated. Its importance can easily be missed when the ac filter can easily cope with the common-mode noise generated by V_cm. Typically, this occurs within a design environment where requirements pertaining to ac leakage current are relatively relaxed, as large values of capacitance can be used to reduce the cutoff frequency of the ac filter. However, when low leakage-current requirements are mandated, such as in the medical or entertainment industries, meeting the challenges of a low cost objective becomes harder because the majority of the filter cost is in the magnetics. Reducing the common-mode capacitors in a filter usually results in a compensatory increase in the magnetics.

The solution is to maximize the value of the bridge capacitor. There are, however, limitations on the design. Any bridge capacitor crossing a safety barrier has to be rated for that application by a safety agency. These are designated as Y capacitors (Y caps, for short) and typically are available to about 4700 pF.

Design Implementation

A 25-W flyback convertor based on the Viper53 off-line switcher solution developed by STMicroelectronics (Plan-les-Ouates, Switzerland) was redesigned in accordance with the principle just described (see Figure 4). This power supply was slated to be incorporated in more than 1 million units of equipment; hence, minimizing the cost of EMC would be beneficial to the bottom line. The sensitive nature of the design allows only the basic elements to be shown here, but the detail presented is sufficient for describing the nature of a development investigation that was undertaken.

Figure 4. Basic design of a 25-W flyback power supply that was redesigned to reduce the choke T1 value and eliminate the Y capacitors C1 and C2 by maximizing the bridge capacitor.

The design started with T1 as a 20-mH choke; capacitors C1 and C2 were 3300 pF, and C3 was 1000 pF. The final configuration had T1 as a 14-mH choke; C1 and C2 were eliminated, and C3 was boosted to 3300 pF. Reducing the value of the common-mode choke allowed the use of an already existing smaller part, which resulted in a significant cost savings of 30 cents. An additional 2-cent savings was realized with the elimination of the two Y capacitors, C1 and C2. This made for a total cost savings of 32 cents on an original filter cost of 75 cents.

Under the pressure of development, no effort was made to find the lower bound of value for the common-mode choke. Although finding the lowest possible inductance value might be of some academic interest, it does not make sense economically. Different inductance values in the same core tend to be very similar in price. Maximum cost benefits are achieved when smaller bobbin sizes can be used. In addition to the cost benefits and improved conducted emissions results it generated, the redesign dropped the leakage current from 340 µA to a noteworthy 120 µA, which is well under the 500-µA limit set by UL 6500 for audio and video products.

During the design study, it was discovered that higher values of bridge capacitance actually made the conducted emissions worse at frequencies greater than 10 MHz or so. An investigation of the board revealed a long connection between the capacitor and secondary/chassis ground. Now, it takes only 20 nH of inductance (or 1 in. of trace) to resonate with 4700 pF at 16 MHz, so, clearly, the board layout and capacitor installation play a significant role in the effectiveness of the C3 capacitor in reducing common-mode voltage. Data comparing conducted emissions performance before the fix was made with the improvement following the fix are presented in Figure 5.

Figure 5. A comparison of conducted emissions data for a 24-W flyback convertor before any improvements were made (a) and after maximizing the bridge capacitor and optimizing filter components (b).

With the use of low-cost single-layer boards for power-supply designs ever increasing, keeping the capacitor connection inductance to a minimum is a challenge. A suggested layout is depicted in Figure 6, illustrating the important design principle that the connection between chassis ground and dc ground be made as close to the bridge capacitor location as possible.

Figure 6. Recommended layout for a flyback convertor designed with a bridge capacitor whose capacitance value has been maximized.

Conclusion

Accurately identifying a source of common-mode noise often results in a low-cost EMI-filtering solution, an advantage in a cost-competitive market environment. This article has analyzed the effect of an often overlooked source of conducted emissions and proposed a new approach that affects printed-wiring-board layout techniques while reducing filter cost.

Although it is generally accepted that, below 1 MHz, differential-mode emissions predominate while, above 1 MHz, common-mode emissions do, the design study described here clearly shows that low-frequency common-mode emissions do exist and must be accounted for.

References

1. WD Kimmel and DD Gerke, “The Hidden Schematic: EMC Threats in Medical Power Supplies,” Compliance Engineering 18, no. 8 (2001): 54–58.
2. S Chand and N Hasan, “A Practical Approach to Conducted Noise Compliance,” ITEM Update, (2002).
3. SW Mee and JE Teune, “Reducing Emissions in the Buck Converter SMPS,” IEEE EMC Society Newsletter, no. 199 (2003): 49–53.
4. G Spiazzi, A Zuccato, and P Tenti, “Analysis of Conducted and Radiated Noise of Soft-Switched Flyback DC-DC Converter,” in Proceedings of the International Telecommunications Energy Conference (Piscataway, NJ: Institute of Electrical and Electronics Engineers, 1996), 297–304.

Charles Grasso is a senior compliance engineer and Bert Downing is a power supply engineer with EchoStar Communications Corp. (Englewood, CO). Grasso can be reached at 303-706-5467 or charles.grasso@echostar.com. Downing can be reached at 303-706-4851 or bert.downing@echostar.com.