When I first entered the world of professional engineering, I didn’t think much about inductors. A basic inductor is a very simple passive component that’s been around for a long time, and furthermore, analog designers are often attempting to eliminate these components; in this article you can read about the undesirable characteristics of inductors and see some clever op amp–based circuits that provide a second-order filter response without the need for inductance.
However, I have gradually learned that there is still a lot of design effort dedicated to inductors, even outside of the RF domain. Why? Because they play a central role in the functionality of switch-mode voltage regulators, and nowadays, switch-mode voltage regulators are everywhere.
Inductors and Switching Frequency
There are various decisions and details that come into play when you’re designing a switching regulator, but the choice of switching frequency is a dominant consideration. Many aspects of the converter’s performance—including transient response, EMI, size, cost, and efficiency—are influenced by switching frequency.
It seems to me that the general trend these days is toward higher-frequency converters, despite the fact that higher switching frequencies are associated with lower efficiency, which leads to reduced battery life in portable devices.
This gives you an example of the relationship between switching frequency and efficiency. This particular plot is taken from the datasheet for a step-down regulator made by Linear Tech/Analog Devices.
I suppose this is due in large part to the fact that the reduced size and cost associated with higher switching frequency is a compelling requirement in many applications. I recently wrote about a new DC/DC converter from STMicroelectronics that operates at frequencies up to 2 MHz and uses complex control circuitry to achieve high efficiency and low quiescent current while maintaining compatibility with space-constrained systems.
As designers seek to increase switching frequency and also satisfy demands for low cost, small size, and low radiated EMI, they will need inductors that are optimized for this environment. Ideal inductors are not negatively affected by higher switching frequencies; they simply exhibit an impedance that increases steadily as frequency increases. Real inductors, however, are not so simple.
The HA74 Series
These new inductors from TT Electronics are intended for space-constrained, high-frequency DC/DC converters, with a specific emphasis on automotive applications. They are AEC-Q200 certified and operate over the rather impressive temperature range of –55°C to +155°C.
The HA74 line is intended to provide high power density and high efficiency in a small-footprint, low-profile package. They’re recommended for switching frequencies as high as 3 MHz.
Real inductors have resistive characteristics that result in lost power, and this lost power reduces the efficiency of the overall regulator circuit. Despite the fact that we often think of an inductor as having a DC resistance, various aspects of an inductor’s resistive behavior are influenced by frequency.
I’m certainly not an expert on inductor physics, so when it comes to the details I’m going to rely on an app note from Texas Instruments. According to Application Report SLVA157, magnetic hysteresis loss, eddy-current loss, skin-effect losses, and radiation losses are all frequency-dependent components of the total loss created by an inductor.
The datasheet for the HA74 series is rather brief, and I don’t have detailed information on the characteristics that make these inductors suitable for high-frequency applications. All I know for sure is that they have a “low-loss metal-alloy core composition,” and I’m assuming that the materials and the winding configuration are optimized for low loss at higher frequencies.
If you take a look at the basic buck converter shown below, you can see that the current flowing to the load must pass through the inductor.
The inductors used in switching regulators must be able to handle the required current without excessive heating or saturation. The HA74 devices are rated for currents ranging from 1.2 A to 2.9 A. The following plot conveys the effect of high currents on the temperature and inductance of the HA74-054R7, which is the part that has the 2.9 A current rating.
Plot taken from the HA74 datasheet.
What challenges have you faced when trying to optimize DC/DC converter circuits for a given application? Feel free to leave a comment and let us know.