In today’s world of electronics, the trend has been to make things smaller and smaller, the assumption being that—if they are smaller—they will be more convenient and more readily adopted. But is this true? The first iPhone was truly a marvelous tiny phone that combined features unheard of to that point. But today’s version is nearly twice the size—not because of technology, but because people demand something they can see and use. The flat screen television has taken a similar route. The incredibly thin screen has only resulted in making the viewing area much larger. A feat that could not be achieved with a cathode ray tube (CRT)-based product.
Following this trend, magnetic components like inductors have also been shrinking. Remembering our physics, that inductance is based on volume, shrinking the volume reduces the inductance. This has been countered by using higher switching frequencies. To date, no phone or tablet on the market has a bulge in the back because the magnetics were too large. Cameras are a different topic.
All components have inefficiencies that result in power losses. These losses result in heat that must be dissipated by either conduction, convection, or radiation. As components get smaller, their surface area decreases, meaning they must have lower losses to match otherwise they will overheat. In magnetic materials, as the operating frequency increases, the losses increase exponentially. On the winding side, a rectangular wave excitation contains many high frequency harmonics that result in skin and proximity effects within the coil, increasing its effective resistance. You get the picture, smaller hotter parts with less cooling area. So, how is an engineer to decide on an inductor?
What is needed is a method that accounts for each inductor’s real-world losses. However, this is complicated by the fact that each application is unique in both its electrical, mechanical, and thermal constraints. Furthermore, each magnetic components manufacturer uses their own unique tests to define product performance, thereby making it difficult to compare parts on an equal basis. This is particularly important for rated current and saturation current.
Standard testing of rated current
The fact that every inductor vendor uses their own method to determine the rated current is complicated by how the inductors are mounted and the thermal conduction the mounting provides. With small inductors, the mounting can provide useful heat sinking, reducing the operating temperature and extending the current carrying capacity. Put another way, this allows the winding to carry more current, increasing the energy storage capacity, provided that the core does not saturate.
To help our customers, Würth Elektronik has added testing in accordance with IEC 62024-2, “High frequency inductive components – Electrical characteristics and measuring methods – Part 2: Rated current of inductors for DC-to-DC converters.” This standard provides a transparent and recognized method to determine rated current. To distinguish the new measurements from historical values, the new result is called “performance current.” Würth Elektronik datasheets now contain both values and provide notes as to which specific “class” of the standard each inductor was tested under.
The designer now has a more complete picture of the inductors’ capabilities which include the inductance without bias (L), the currents where the inductance drops by a specified percentage due to bias (ISAT,30%), and the currents that cause a 40°K temperature rise under typical (IR) and performance conditions (IRP,40K). However, numbers are hard to visualize, and the heating conditions are based solely on DC currents.
Simulation based on real-world measurements
Real applications have high frequency AC ripple currents that increase losses beyond the DC bias. The impact of these ripple currents on the winding and core losses is difficult to predict accurately by calculation. To address this problem, Würth Elektronik has measured its inductors under real-world conditions in a buck converter circuit over a wide range of frequencies, duty cycles, and bias conditions.
By measuring total losses this way, all the influences of construction and material are considered. The result of this rich data set is available through the online tool REDEXPERT. The user simply enters the basic operating parameters, and a list of suitable inductors appears complete with losses under those conditions based on measurement. This includes charts that show the inductance roll-off as bias increases, as well as temperature rise under a selected ambient temperature.
Sliders on the charts allow the user to obtain intermediate values along the curves. The charts can show the characteristics of many inductors making it easy to compare several potential devices to each other. Engineers often need to document their choices and share them with others. A simple click provides a URL that can be sent to colleagues, allowing them to reproduce the same results. Obtaining samples involves only a simple click-and-drag operation.
Backed by real data, the versatility and convenience of REDEXPERT allows designers to make smart determinations with confidence in the results and without complex calculations. That’s the goal and purpose of support from online design tools.