There are three types of passive components in electronic circuits: resistors, capacitors, and inductors, among which inductors may be the most peculiar in principle. The phenomenon of inductance was discovered by Michael Faraday and Joseph Henry in the 1830s: Faraday discovered that a changing magnetic field can induce current; Henry independently studied the phenomenon of "self induction", which refers to the induction of current in a conductor within itself.
Before people fully understood electromagnetics, it was a mystery that simply winding a wire into a coil could change its electrical properties. In the early days of radio, DIY enthusiasts would use a magnetic rod or cardboard tube only a few inches long, wound with dozens of turns of wire to make tuning coil inductors, to assemble transistor radios.
The schematic symbol of an inductor is based on its physical appearance design (Figure 1). The types of inductors include hollow, iron core, and variable.
Figure 1: Inductors (right figure) were initially formed by wires wound around a hollow tube or iron core; The corresponding principle symbols are shown in the figure (left image). (Image source: Hackatronic. com)
Inductance is a characteristic of a conductor, and due to the effect of its magnetic field, the current passing through the conductor often changes. Therefore, inductors are sometimes referred to as chokes because they can "choke" changes in current. The relationship between inductance (L) and the rate of change of voltage (V) and current (I) can be expressed by a simple equation: V=L (dI/dt).
Although wound coil inductors are still widely used, they are no longer suitable for many circuits today. They may be too large, unable to provide the required values, exhibit undesirable parasitic effects, have high DC resistance (DCR), and exhibit performance degradation at higher frequencies. Compared to early DIY radio enthusiasts, ready-made wound inductors for radio frequency (RF) applications with dimensions less than 1 square millimeter (mm2) are now available for purchase.
Modern inductors for power converters
Although inductors have made significant progress, even enhanced coil inductors have shortcomings in performance and size for modern circuits. Modern power inductors are precision components that have been carefully modeled, with their core and secondary parameters fully defined, and their properties optimized according to different application priorities.
In addition, suppliers have developed new materials to meet the needs of different switching power supply topologies, such as single ended primary inductor converters (SEPIC), Ć uk converters (named after their inventor Slobodan Ć uk), and various buck boost configurations.
Most of these inductors use advanced ferrite and powder materials, and their characteristics have been carefully tuned. These inductors have extremely low DCR (significantly improving the inductance Q value - the standard value for measuring inductance performance) and smooth inductance roll off. The latter refers to the degree to which the actual inductance value decreases or "rolls off" due to magnetic core saturation as the DC current increases, similar to the frequency response roll off of a filter.
Inductors used in power supplies typically also need to have relatively high rated current handling capabilities, typically in the tens of amperes. This parameter is not defined by a single value, but by multiple values such as root mean square current (Irms), peak current (Ipeak), and saturation current (Isat). The inductors provided by the manufacturer will have different rated current combinations and other top-level parameter references to meet the priority requirements of various topology structures.
The manufacturer has also developed advanced materials and surface mount technology (SMT) (Figure 2) that can withstand the associated heat without compromising performance or reliability. Shielding type helps minimize radio frequency interference (RFI) issues in sensitive applications.
Figure 2: High power SMT inductors can now provide various surprising small sizes without affecting performance. (Image source: Eaton)
The HCM/HPAL molded inductor series from Eaton Electronics Division reflects the advancement and differentiation of these converter optimized inductors. Both series use advanced inductor materials, which are characterized by durability, high current, and low EMI. Their molded structure can provide soft inductance roll off in various rated current ranges.
HCM and HPAL series devices come in various sizes, but their volumes are relatively small.
To ensure reliability and robustness, the rated operating temperature of HCM/HPAL devices is -55 to 125 ° C (ambient temperature plus self temperature rise), and they contain rust inhibitors that help prevent surface rusting due to humid environments (MSL level 1).
The HCM series uses advanced pressed iron powder with excellent Isat performance, which can be seen in two representative devices, HCM0503V2-R68-R and HCM0503V2-4R7-R. HCM0503V2-R68-R is a 680 nanohenries (nH), 8 milliohms (m Ω) DCR unshielded inductor with an operating frequency of up to 1 megahertz (MHz). Its size is only 5.7 × 5.4 × 3.0 mm, with a rated current of 10 amperes (A) (Irms)/12 amperes (Isat). HCM0503V2-4R7-R uses the same package size, but is suitable for situations that require higher inductance. It is a 4.7 µ H, 47 m Ω unshielded device with a rated current of 4.1 A (Irms)/6 A (Isat).
In contrast, HPAL inductors use alloy powder to achieve lower DCR and higher Irms while maintaining lower core losses. The power range of this series of inductors is from 0.15 μ H to 10 μ H, and the current is from 4.5 A to 40 A. It has electromagnetic (EMI) shielding function, which is crucial in certain applications. Example devices include HPAL1V0630-R47-R (a 470 nH, 4.1 m Ω inductor rated at 18 A (Irms) and 20 A (Isat)) and HPAL1V0630-8R2-R (an 8.2 µ H, 55 m Ω inductor rated at 5 A (Irms) and 5.5 A (Isat)).
The graph in Figure 3 shows the roll off relationship between the nominal inductance, DC current, and temperature of the HPAL1V0630-8R2-R inductor.