Power-supply engineers balance a number of factors in designing for modern systems. Design requirements such as output-voltage control and ripple, as well as current capacity, are primary concerns, but factors such as board area, materials cost, and noise emission are what distinguishes a highly successful design from the rest of the pack.

Switch-mode DC/DC converters are a viable alternative to linear regulators due to their size, weight and efficiency advantages, especially as the switching frequency is increased. At higher switching frequencies, lower values of reactive components can be specified to achieve the required output characteristics. This increase in frequency can have the undesirable effect of increasing EMI if good design and layout practices are not followed.

Increased switching frequency has the positive effect of reducing the required inductance value, making the integration of the inductor into the converter practical, yielding a significant reduction in the board real-estate of a switching power supply. This has also enabled the use of distributed Point-of-Load (POL) power conversion in systems, especially those with low-voltage/high-current devices, or a number of components with incompatible power-supply requirements. Distributed power conversion also reduces the effects of board layout on supply-voltage integrity over a centralized approach.

The major concern with non-isolated, board-mounted converters at high switching frequencies is radiated EMI, which can be a combination of two types:

Differential mode EMI
Differential-mode radiation is caused by high-frequency current loops on the circuit board, and the resulting radiated E field is proportional to the product of f², A, and I, where f is frequency, A is the area of the current loop and I is the current in the loop. Thus, as the frequency spectrum of the current is extended, the area of the current loop must be reduced to limit differential-mode radiation. The high level of integration in packaged converters permits very small internal loop areas and facilitates minimizing the areas of the critical loops that extend outside the package.

Common mode EMI
Common-mode radiation, perhaps the more difficult type of EMI to control, is caused by common-mode current flowing on busses on the circuit board and interconnecting cables. It is proportional to the product of f, I_c, and L, where f is frequency, I_c is the common-mode current level, and L is the effective radiating length of the bus or cable. Unlike differential-mode radiation, the conductor lengths L cannot typically be minimized by the designer, and the only parameter that can be readily controlled is I_c, by using proper layout (e.g. ground plane) and filtering.

To complement its family of integrated power converters, Enpirion has developed design and layout guidelines that lead to minimal common-mode current and acceptable common-mode radiated EMI levels for even the most challenging board designs. The primary source of radiated electrical noise is current flow in the input-loop filter, but such noise can also be made worse if there is coupling of high frequency noise into the output loop.

Recall from your college electromagnetics course that currents flowing in a closed path, i.e. a loop, act as an efficient radiator of electromagnetic energy. Maximum radiation efficiency occurs when the loop dimension is on the order of one-half wavelength. To minimize the radiation efficiency, that is to reduce radiated noise, the loop needs to be made as physically small as possible by using the following methods:

• Input filter capacitor should be placed as close to the DC/DC converter package as possible.
• Careful attention should be paid to the output filter section as well. An integrated inductor DC/DC converter will help minimize the loop dimension and provide excellent isolation between input and output AC current loops.
• Be mindful of parasitic inductances in the board traces, solder joints, and component pads. Do not use thermal reliefs in the capacitor pads. High impedances in the primary or intended loop paths may send the currents out onto the PCB, following much larger physical paths and coupling into other circuits. Remember that high-frequency currents follow the path of least impedance, not least resistance!

Horizontally polarized radiated EMI measurements (combined differential and common mode) for Enpirion's 3A EN5336 module and 6A EN5366 module on evaluation boards are shown in Figure 1 and Figure 2, along with the CISPR22, Class B specification. All levels are referenced to a 3 meter distance. Vertically polarized emission is not shown, but is as good--or better--than the horizontally polarized emission. Both evaluation boards exhibit at least 10 dB of margin, and both modules are switching at about 5 MHz.

Figue 1: Radiated EMI from the Enpirion 3A EN5336 evaluation board operating with V_in=5V, V_out=1.5V and I_out=2.5A.
(Click on image to enlarge)

Figure 2: Radiated EMI from the Enpirion 6A EN5366 evaluation board operating with V_in=5V, V_out=1.5V and I_out=5A.
(Click on image to enlarge)

Proper board layout alone will not achieve the noise immunity required for today's RF, analog and high-speed digital applications, where switcher noise can limit data throughput and cause failure in spectral compliance. A high level of integration in the power converter itself is necessary in order to localize the high-frequency noise inside the package, rather than on the PCB.

In the case of Enpirion's line of ultra-integrated DC/DC converters, the inherent shielding and compact construction of the integrated inductor reduces the opportunity for radiated noise to couple into the traces of the circuit board. In fact, integration of the magnetics virtually eliminates the board-level design and layout issues normally associated with switch-mode DC/DC converters.

With the technological advancements found in power converters today, designers do not have to view EMI as a trade-off for efficiency. As the data indicates, excellent EMI performance is quite achievable at 5 MHz. The key is careful attention to circuit design and proper board layout, and vendors such as Enpirion have made a considerable effort to define the layout rules and make them available to customers in application notes. Using compact, ultra-integrated switch-mode converters significantly reduces parasitic effects that can harm loop stability, and makes board layout much simpler. All of this enables easier and faster incorporation into various applications to meet demanding EMI requirements.

About the author
Ali Ghahary is applications engineering manager at Enpirion, Inc, Bridgewater, NJ. He has nearly 20 years of experience in analog-circuit design in the power-electronics field, and he holds two patents. He earned his Bachelor and Master of Science degrees with honors from Virginia Polytechnic Institute (Virginia Tech).