Integrating Inductors into SMPS Power Supplies

Inductors play a fundamental role in the design and development of switch-mode power supplies (SMPS). All three primary SMPS topologies − step-down (buck), step-up (boost), and step-up/down (buck-boost or inverter) − include an inductor along with a MOSFET switch, a diode, and an output capacitor. The on and off states of the MOSFET divide the SMPS circuit into a charge phase and a discharge phase, and the inductor, which stores electrical energy in the form of a magnetic field, behaves accordingly, enabling the energy transfer from source to load during each switching cycle. Without the inductor, the current would drop to zero when the switch is open. But while passive components (inductor and capacitor) are essential to SMPS modules they can occupy up to 30 per cent of the volume of the system and also significantly impact cost. So development work is ongoing.

Maximum efficiency is a prime consideration for all SMPSs. Commercially available products switch at frequencies up to several megahertz and can have efficiencies of more than 95%. It has been known for some time that increasing the switching frequency can reduce the size of passive components and the inductance needed can be lowered significantly (≤ 50 nH) as well, which raises the possibility among researchers of embedding small inductors in the Printed Circuit Board (PCB). A number of inductor winding technologies exist to provide the optimum inductive solution for different requirements, so the use of spiral and toroid inductors have been investigated for this purpose. While discrete air core solenoid inductors find common use with high switching frequency designs, a review of the literature reveals that they don’t seem to be as popular for use as embedded inductors.

Among the benefits that arise from increased switching frequency include higher power density, decreased cost, reduced weight and faster transient response. The increased frequency, however, also leads to several new challenges including greater power loss. SMPS losses are found largely in the switching components (MOSFETs and diodes) and, to a lesser extent, in the inductors and capacitors of the SMPS circuit. The inductance of solid ferrous cores and powdered ferrous cores, which can better store energy than air cores, changes with operating frequency and as the frequency climbs losses increase.

So far it has proven difficult to miniaturize on-chip inductors while maintaining adequate inductance and performance. One penalty of higher switching frequency is that it results in more EMI/RFI produced as fast-switching currents of the charge/discharge loops produce magnetic fields. Attempts at using air-cored inductors found that they contribute to EMI becoming more of an issue because the cores don’t feature the closed magnetic fields of ferrous cores and hence allow stray radiation to escape.

Given that the core materials that are normally used to get high inductance in inductors have unacceptably high core losses at higher frequencies, new core materials or ways of designing the components therefore have to be developed.

To that end, researchers at the University of California Berkeley (with sponsorship from the Semiconductor Research Corporation) have focused on advancements in on-chip inductors by using insulated nano-composite magnetic materials as the filling material to shrink the size and improve the performance of high frequency on-chip inductors, thereby enabling a new wave of miniaturized electronics and wireless communications devices.

Typically, large areas (up to millimeters in diameter) are required to construct on-chip inductors with adequate inductance for circuit applications. This large area requirement contributes losses due to parasitic effects between the spiral coil and the semiconductor substrate. According to the UC Berkeley researchers, initial results demonstrate a significant enhancement in inductance of up to 80 percent, which corresponds to at least a 50 percent shrink of the on-chip inductor size. The new inductor fabrication technology offers the additional advantage of extending the operational frequency range, which is currently limited by eddy current losses.

In the meantime, some vendors have been able to shrink the inductor such that it can be integrated with the rest of the power module into a reasonably compact package. For example, the LXDC line of micro DC/DC converters from Murata Americas the smallest full DC/DC converter in the world − utilizes a unique ferrite substrate to embed the power inductor within the ferrite substrate. By utilizing this structure, the IC can be mounted directly above the power inductor coil with almost no pattern length to diminish leakage radiation noise. The I/O connections are also routed through this same ferrite substrate to create a function similar to ferrite beads that significantly reduces conductive noise.

The concept of mounting the IC on a shielded inductor also is utilized in the newest TI Simple Switcher nano module voltage regulators, which employ a packaging size of only 2.5 x 3 x 1.2 mm. A complete solution that requires only an input capacitor, an output capacitor, a VCON capacitor, and feedback resistors it occupies just 35mm2 of board space. The nano module comes in 8-pin LLP footprint package with an integrated inductor. The nano modules operate at a fixed 2 MHz switching frequency.

Altera Enpirion’s Power SoC 5V and 12V DC-DC Step-Down Converters integrate MOSFET switches, small-signal control circuits, compensation and an integrated inductor in a 12x13x3mm QFN module. The Altera Enpirion device helps engineers by offering greatly simplified board design, layout and manufacturing requirements. In addition, overall system level reliability is said to be improved given the small number of components required with the Altera Enpirion solution.