That Troublesome Inductor
DC-DC converter design has often been called an art more than a science. One key reason for this is the troublesome inductor. There are many aspects of the inductor that make it a difficult component with which to design. These include a lack of standards, temperature variability of key performance parameters, unpredictability of the thermal environment, and sensitivity to layout.

No Standards
There are no technical standards that regulate or dictate uniform inductor performance. Each vendor has its own unique core material formulation, wire winding technique, wire type, electrode material, etc. Contrast this with a MLC capacitor where the dielectric material properties, voltage tolerances, and physical dimensions are standardized. When you purchase a 10 μF X5R 10V 1206 MLC capacitor from any number of vendors, the fundamental properties will be the same. The only variability will be slight differences in equivalent series resistance (ESR) and equivalent series inductance (ESL). There are no such standards and hence no such commonality for inductors. For any given inductance value, the designer is faced with a myriad of choices in DC loss characteristics, AC loss characteristics, small signal parameters, saturation current, physical dimensions, and so on. There are many degrees of freedom in choosing the inductor, each with its implications for performance. Understanding the impact of each trade-off is something one learns through years of trial and error -- emphasis on trial and on error.

Temperature Effects
Another aspect of the inductor is the potentially high degree of variability in many of the performance factors over temperature. DC loss characteristics will increase with increasing ambient temperature. The AC loss characteristic is a function of temperature for some magnetic materials with varying minima temperatures for different grades of the same generic materials. Saturation flux density will decrease with temperature for some magnetic materials, whereas it will remain constant over a wide temperature range for other magnetic materials.

The primary source of the temperature dependence of DC loss comes from the increase in resistivity of the wire with increasing temperature. The resistivity of copper, for example, increases by 0.393 percent per °C; aluminum and silver have essentially the same temperature coefficient of resistivity. The DC resistance of the winding would be 20 percent greater at 75°C than it would at 25°C. The increase in DC resistance with increasing temperature would result in a corresponding increase in increasing temperature rise due to the increased power dissipation for the same DC current.

AC inductor loss can come from either the conductor winding, or the magnetic material. AC loss in the conductor comes from a combination of skin effect and proximity effect. Both of these losses decrease with increasing temperature. This occurs because as resistivity increases, skin depth increases by the square root of that change in resistivity. AC power loss in some magnetic materials, such as powdered iron, sendust, and metal films, is relatively constant with temperature, whereas AC loss with magnetic materials such as NiZn or MnZn ferrite can be a function of temperature. The core loss minima can occur anywhere between 25°C and 120°C, depending on the specific material.

Finally, the saturation flux density will decrease with increasing temperature over normal operating temperature ranges for some magnetic materials. What this can mean is that if a converter is designed such that it operates near saturation at room temperature (saturation current is usually specified at 25°C) operating at higher ambient temperatures may result in saturation of the inductor, allowing for large peak currents to flow through the correspondingly low AC impedance of the inductor. The larger AC currents correspond to large RMS currents that will cause higher than expected conductor loss in the inductor. Good design practice requires characterization at elevated temperatures, which is not always done by the manufacturer.

Unpredictable thermals
Unpredictable Thermal Environment
Aside from the obvious issues arising from the temperature affects, there are two issues that are not initially obvious. The first is that it is very difficult to obtain this level of detailed thermal performance characteristics from the inductor vendor. In many cases, they will not have gone into this level of characterization themselves. However, the implications for the converter design are immense. The second, and equally, if not more important issue, is that the power supply designer likely will not have control over the thermal environment to which the inductor is exposed. Local ambient temperature can be quite high if the power supply is placed next to a heat-sink for example. Or, a large ASIC in close proximity may cause heating of the ground plane underneath the inductor. Therefore, calculating the temperature rise of the inductor may be meaningless if a significant portion of the temperature rise comes from external conducted or radiated heat transfer into the inductor.

Sensitivity to Layout
We have spent a lot of time talking about the performance characteristics of the inductor, we now move into the better-known difficulties of this troublesome component -- the sensitivity to layout and placement of the inductor. One has to pay very careful attention to noise generated by the switch-node to avoid introducing conducted noise into the printed circuit board (PCB). Even more important is the impact of layout on the input and output AC current loops, since these are the primary source of radiated EMI. Both conducted and radiated noise can couple into the feedback circuitry and affect control loop stability. Improper layout is often the culprit in EMI issues and in loop stability.

Consider the switch-node, the common point of the high-side and low-side power MOSFETs. The voltage at this point slams between the input voltage and ground. Rapid pulsing of this node can give rise to induced EMF in the PCB immediately adjacent to the node. Parasitic capacitances between the switch-node and other nodes of the PCB can generate unwanted currents that can lead to potential EMI problems.

The input and output current loops are the primary source of radiated noise in the converter design. Someone with RF design experience would recognize these currents flowing in these loop paths as a "loop antenna." The radiation efficiency of these "loop" antennas is related to their physical dimension relative to the AC frequency. To minimize the radiation efficiency, it is critical to make these loops as small as possible. The input current loop is generally not impacted by the placement of the inductor unless poor controller IC package design, or PCB IC density forces sub-optimal component placement. The output AC current loop, however, is directly affected by placement of the inductor. Care must be taken in the layout of the inductor and output capacitor to minimize this current loop and to prevent parasitic effects in the AC current return path.

Integration to the rescue!
Integration to the rescue
Integration of the inductor effectively removes this troublesome component and its associated challenges from the power supply designer's list of headaches. The result is a solution with the smallest possible footprint, the lowest possible part count, and one that delivers guaranteed turn-key performance. Best of all, from the designer's perspective, integration of the inductor makes the power supply design experience more akin to designing in a linear regulator.

Complete Characterization
Integration of the inductor removes the burden of characterization, and the associated variability, from the designer and places this on the power semiconductor vendor. The new generation of power semiconductor manufacturers has resident expertise in magnetic materials and in inductor design and characterization. Integrated in the power device itself, the inductor is designed to achieve optimization over the entire range of operating conditions including input/output voltages, operating frequency, load current, DC loss, AC loss, thermal dependencies, temperature rise, saturation current, and allowable flux leakage. The burden of core material, core structure, winding construction needed to optimize the design falls on the manufacturer of the DC-DC converter. Since the silicon manufacturer would also have a more in-depth characterization of the silicon parameters than would be publicly available, it allows for a "super optimization" of the converter. The result is predictability in all aspects: electrical, thermal, and noise/EMI.

Optimal Package Layout
Bringing the inductor inside the package allows for noise and EMI optimization of the subsystem layout. Careful attention can be given to the placement of the input and output power pins relative to the ground pins. This provides for the smallest possible AC input and output current loops. Impedance between the noisy switch-node and the inductor can be controlled and minimized thereby reducing the potential for the conducted emissions from the switching voltage.

Part Count Reduction
Many available converters have most or all the compensation network external to the controller. This is necessary due to the wide variability of characteristics in the inductor and the inability to optimize over a wide range of operating conditions. Figure 1 shows a typical non-integrated solution beside an equivalent integrated solution. In the case of the non-integrated inductor part, the external components include the input and output capacitors, analog input filter capacitor, numerous compensation components, high-side MOSFET boot-strap, and of course, the inductor. The total part count is 14 external components. Note that this solution features integrated MOSFETs. Had the MOSFET switches not been integrated, the part count would go even higher with more chance for variability.

Figure 1. Comparison of typical buck converter external component requirement (left) with that of an equivalent integrated inductor converter (right)

Integration of the inductor provides for a higher degree of control over the converter design. Complete characterization of the inductor allows the design to be optimized for the desired operating conditions. Full system characterization means that the compensation network can be designed and optimized and then implemented in silicon. Referring to the integrated inductor circuit diagram in Figure 1, we see that the external part count has come down from 14 external components to just three. For devices with internal soft-start, the external part count is just two components. Small footprint
Footprint Reduction
One of the key things that integration enables is the dramatic reduction in solution footprint and solution height. Since essentially all the converter components are contained inside the package, the packing density is very high. A best-in-class external inductor solution like that shown in Figure 1 will have a footprint of approximately 550mm² for an output power of 10W or about 12W/in². The equivalent integrated solution will have a total footprint of 180mm² or about 36W/in². This gives a difference in power density of 3X, or a footprint that occupies about one-third the board area.

Guaranteed System Performance
One of the issues raised in the preceding discussion was the problem of the unpredictability of the thermal environment, and how this makes performance prediction very difficult. Integrated inside the package, the inductor is effectively insulated from the high variability of local ambient temperature by the encapsulating molding compound. The thermal behavior inside the package is well understood and the temperature rise and the material behavior can be predicted and accounted for in the circuit design.

Another key to why integration of the inductor really makes a difference is in the guarantee of overall power supply system performance. As previously enumerated, integration allows the solution to be optimized from end to end based on a complete characterization of the inductor, and of the silicon, over the expected operating conditions. The compensation is precisely tuned for the inductor design, the MOSFET, and for the otherwise impossible to predict interconnect parasitics. Once the design is completed, the converter, complete with inductor, is tested as a whole unit. This optimization, coupled with solution final test, guarantees power supply performance making switch-mode DC-DC converter design as close to a turn-key experience as possible. Design Example
Design Example
The EN5312QI 1A DC-DC converter with integrated inductor is chosen to illustrate the design-in process. The pin configuration of the part is structured for optimal input and output capacitor placement. A separate pin is given for input ground and for output ground. This particular device uses a 3-pin voltage ID or VID configuration to program the output voltage. This allows the designer to choose from one of seven preset output voltages or an external divider can be used.

Proper design of an integrated inductor DC-DC converter is simple and straight forward. As with any high frequency device, care must be taken in layout of the ground planes. To simplify the process, the following steps are recommended.

1. The input power, output power, and ground planes should be laid out such that the input and output capacitors can be placed as close to the converter IC as possible. Figure 2 shows the power plane configuration.
2. Separate ground planes should be used for the input and output filter sections. The ground pours should be as thick as possible but should not contact each other.
3. The input ground plane should be used to connect the ground terminal of the input capacitor to the input ground pin on the IC package. This ground should not be connected to any other device with the following exception: small vias should connect the input ground plane to the system ground. This will reduce input ripple due to the high frequency content of the switching waveform rising and falling edges.
4. Several small through vias should connect the output ground plane to the system ground plane. This reduces output ripple that may be caused by switching spikes.

Figure 2. Ground and power plane configuration for EN5312QI

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Figure 3. Completed 1A DC-DC converter design. "Just add the caps"

Conclusions
DC-DC converter design can be a very frustrating and time-consuming process. Lack of standards and high variability in inductor properties make optimizing the design difficult, and prediction of performance a trial and error process more than an engineering science. Unpredictability in the thermal environment also adds to this uncertainty. Inductor placement affects the converter noise/EMI and can affect regulator stability.

Integration of the inductor allows for a complete system optimization, a dramatic reduction in solution footprint and profile, a reduction in part count, better control and predictability of EMI, and guaranteed system performance.

About the author
Michael G. Laflin is the Director of Marketing for Enpirion. You can t contact Mike at: mlaflin@enpirion.com
For more information on Enpirion go to www.enpirion.com