The design trade-offs that must be addressed during the design of a DC-DC converter circuit are simplified by integrating the inductor into the power management device. This article focuses on the design issues that are simplified by including an integrated inductor optimized for a range of electrical and layout applications as part of the power management solution. The inductor design is considered particularly troublesome since its design can affect performance relative to efficiency, stability and EMI requirements as well as dictate circuit layout and overall footprint. Finally, integration of the inductor guarantees system performance that yields a more turn-key experience for system developers.
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.
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.
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.