Does the mounting orientation of the SMPS inductor affect radiation?

The spectrum of EMI emissions from a switched-mode power supply (SMPS) is a function of many parameters, including hot loop size, switching speed (slew rate) and frequency, input and output filtering, shielding, layout, and grounding. One potential source of radiation is the switch node, called SW on many schematics. The SW node copper can be used as an antenna to emit noise from fast and efficient high power switching events. This is the main source of radiation for most switching regulators.

By: Keith Szolusha, Application Director, Analog Devices | Gengyao Li, Application Engineer | Frank Wang, EMI Engineer

The spectrum of EMI emissions from a switched-mode power supply (SMPS) is a function of many parameters, including hot loop size, switching speed (slew rate) and frequency, input and output filtering, shielding, layout, and grounding. One potential source of radiation is the switch node, called SW on many schematics. The SW node copper can be used as an antenna to emit noise from fast and efficient high power switching events. This is the main source of radiation for most switching regulators.

The amount of copper at the top SW node should of course be minimized to limit the antenna size. With a monolithic switching regulator (power switch inside the IC), the SW node goes from the IC all the way to the Inductor, leaving a short trace on the top layer. By using a controller (power switch external to the switch controller IC), the SW node can be independent of the switch and away from the IC. The SW node copper is connected to one side of the inductor in buck and boost switching topologies. The layout of layer 1 SW nodes in the XY plane of the PCB or on inner layers is tricky due to the numerous performance parameters involved (see Figure 1).

Does the mounting orientation of the SMPS inductor affect radiation?
Figure 1. SW node highlighted in the XY plane of layer 1 on the DC3008A LT8386 low EMI LED driver


Figure 2. The white stripe on a Coilcraft XAL inductor is a sign of a short coil lead because the coil lead is not visible.
It indicates the orientation of terminals and short leads. Connect high dv/dt here for lowest EMI.

Inductor Geometry

Of course, the SW node also extends vertically (in the Z plane) when the inductor terminals are considered. The vertical orientation of the inductor terminals may increase the antenna effect and radiation of the SW node. Furthermore, the internal inductor windings may not be symmetrical. Even though the symmetrical termination of the inductor indicates that there is a symmetrical structure hidden in the package, the polarity indication on the top of the component says otherwise. Figure 2 shows the internal winding structure of the Coilcraft XAL inductor series. Flat wire windings start at the bottom of the element and end at the top, so one terminal ends up being much shorter than the other in the Z plane.

Also, an inductance with exposed SW nodes on the sides can be worse than one with shielded vertical metal, as shown in Figure 3. The board designer can choose the inductor with the fewest vertical exposed terminals to reduce EMI, but what about the orientation of the two inductor terminals and their relative impact on radiation?

Radiation reflects the truth

The low-emission performance of the board under test is the result of a combination of IC emission performance and layout considerations. Even with low-e monolithic ICs, the layout must be handled with care, taking into account the installation of critical radiating components. To demonstrate this, we examined the effect on the board of the orientation of the main inductor L1 of the LT8386 demo circuit (see Figure 4). In this case, inductor manufacturer Coilcraft specifies that the short terminals of the XAL6060 series inductors are marked with a white line above the component. Standard CISPR 25 conducted emission (CE) and radiated emission (RE) tests in an EMI chamber show that the orientation of this inductor (see Figure 5) can significantly affect performance.


Figure 3. Pay attention not only to orientation, but also to inductor terminal type on EMI sensitive designs


Figure 4. SW node highlighted in DC3008A LT8386 low EMI LED driver schematic.
Place the short side terminals in direction 1 and direction 2 to compare the full radiation results.


Figure 5. Coilcraft XAL6060-223MEB inductor directional radiation test with DC3008A LT8386 LED driver. L1 direction 1 (left), the short terminal is on the SW node; L1 direction 2 (right), the long terminal is on the SW node. The radiation results are shown in Figures 6 to 8.

Figures 6, 7, and 8 show that the radiation performance of the DC3008A is directly affected by the direction of L1 on the demo circuit, with no changes to other components. Specifically, for direction 1 – that is, the short side terminals are placed on the SW node, the low frequency RE (150 kHz to 150 MHz) and the FM band CE (70 MHz to 108 MHz) have lower EMI. The 17 dBµV/m to 20 dBµV/m difference in the AM band cannot be ignored.

Not all inductors are “created equal”. Winding orientation, terminal shape, shape of terminal connections and even core material may vary. Magnetic and electric field strengths that differ in core material and structure may play a role in altering the inductive radiation. However, this case study has revealed an aspect that needs attention that we can turn into an advantage.


Figure 6. Radiated emissions show that the direction of the inductance on the DC3008A has a significant effect on the results.
With short-side terminals attached to the SW node to minimize the SW antenna (red), the radiated emission (RE) is significantly improved.


Figure 7. The short-side terminal of the inductor is attached to the switching node. The current probe method improves conducted emission (CE) (>3 MHz) compared to the other polarity.


Figure 8. Short-Side Terminal of Inductor Attached to Switch Node, Voltage Method Conducted Emission (CE) Improvement (>3 MHz) Compared to Another Polarity

Inductance without polarization indication

Orientation is easy if the inductor manufacturer points out the difference in internal terminal size with silkscreen front markings or dots. If you choose one of these inductors for your design, it is wise to mark it on the PCB silkscreen, on the installation drawing, or even on the schematic. Unfortunately, some inductors have no polarization or short terminal indication. The internal winding structure may be nearly symmetrical, or there may be known structural differences. There’s no malicious intent here – manufacturers may not be aware of this particular mounting orientation distinction inherent in their products. Regardless, we recommend evaluating the radiation of selected inductors in both directions in a certified chamber to ensure repeatable high-performance measurements.

Sometimes there is no external marking, and the mounting orientation of the inductor is inevitably arbitrary, but the inductor is still required because of other parameters. For example, Würth Elektronik’s WE-MAPI metal alloy power supply inductors are small in size and very efficient. Its terminals are located only on the bottom of the housing. Each component has a dot near the top of the WE logo, but the data sheet does not specify that as the starting point for winding indications (see Figure 9). Although this may cause some confusion at first, the element has a fairly symmetrical internal winding structure and should perform the same in both mounting orientations. Therefore, the point on the top of the IC does not have to be indicated on the mounting silk screen. However, if used in circuits where EMI is critical, it is wise to test in both directions to confirm performance.

Another example: Würth WE-XHMI

We tested the DC3008A with a high-performance Würth inductor, with the point on the top of the package and the starting point of its winding indicated in the data sheet (see Figure 10). For the LT8386’s form factor and current requirements, the 74439346150 15µH inductor is a good fit. Again, for comparison with Coilcraft, we mounted this inductor in both orientations for radiated testing (see Figure 11).

The result (see Figure 12) is similar to a Coilcraft inductor. The radiation results show that the mounting orientation of the inductor has a significant effect on the radiation. In this case, direction 1 in Figure 11 is clearly the best direction with the lowest radiation. The lower frequency AM band (RE) and FM band (CE) radiation in direction 1 is clearly better.


Figure 9. The WE-MAPI inductor data sheet does not give a winding start point, but there is a winding start point on the label on the top of the component.
These inductors may not have direction-dependent radiation effects, but should be confirmed by testing.


Figure 10. The top marking on the WE-XHMI series inductors indicates the starting point of the winding


Figure 11. Würth 74439346150 (“WE 150”) inductor directional radiation test with DC3008A LT8386 LED driver. L1 direction 1 (left), the starting point of the short terminal of the winding is on the SW node; L1 direction 2 (right), the long terminal is on the SW node.

The radiation results, shown in Figure 12, indicate that the winding start point should be connected to the SW node for best results.

Dual Switch Node Buck-Boost IC (Results To Be Continued)

Obviously, the direction of the inductor has an effect on the radiation in a single switch node boost LED driver. We can assume that the SW node of the boost regulator has the same characteristic radiation since the power conversion and switching elements in the voltage regulator and LED driver circuit are the same.

We can also assume that the buck regulator has a similar SW node design priority to minimize antenna effects at the inductor terminals. However, since the SW node of the buck regulator is closer to the input side of the converter, follow-up work may help to determine whether the effect of the inductor direction in the RE and CE regions is the same as that of the boost regulator.

For a two-switch node buck-boost converter, there is a bit of a dilemma. Commonly used buck-boost converters such as those in the LT8390 60 V synchronous 4-switch buck-boost controller family have important low EMI characteristics such as SSFM and a small hot loop architecture. A single inductor design does not clearly reveal the effect of inductor orientation on radiation. If the short terminal is placed on one SW node, the long terminal acts as an antenna on the other SW node. Of these designs, which direction is the best? What happens when all four switches are toggled in a 4-switch work area (VIN close to VOUT)?

We will explore this issue in a future article – testing EMI of a 4-switch buck-boost controller with two SW nodes in different inductance directions. Leave it to you to think: maybe there are more than two options for this topology, 180° apart?


Figure 12. Radiated and conducted emissions show that the mounting orientation of the Würth 74439346150 high-performance inductor has a significant effect on the radiated results

in conclusion

The orientation of the inductor in a switching regulator is important. When measuring radiation, pay attention to the inductance direction and its repeatability – know how the chosen inductance differs in these respects, test in both directions, and if the direction cannot be determined, clearly inform about possible installation pitfalls Circuit board production department. It may be possible to improve radiation by simply rotating the inductor by 180°.

About the Author

Keith Szolusha is Director of Applications at Analog Devices in Santa Clara, California, USA. Keith has been with BBI Power since 2000, focusing on boost, buck-boost and LED driver products, while also managing the EMI chamber for power products. He is a graduate of the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts, with a BS in Electrical Engineering in 1997 and an MS in Electrical Engineering in 1998, specializing in technical writing. Contact information:[email protected]

"The spectrum of EMI emissions from a switched-mode power supply (SMPS…