Posted on 06 February 2015

Power Supply in Inductor

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Inductor and packaging to optimize the POL

Point of load regulators (POL) are very widely used in distributed power systems to deliver the final voltage required by the load, particularly for high performance ICs and ASICs. In most cases, these POLs utilize the Buck Converter switching topology to achieve high density and high efficiency performance. Of all the components within the POL the main energy storage component, the inductor, is by far the largest.

By Dave Cooper, Applications Manager, Power Modules; Sumida Corporation

Whereas IC manufacturers and others have focused on improving the performance of the ICs, capacitors and packaging, little progress has been made on improving the largest component in the POL which is the Inductor. To achieve the best performance of the POL for the available space, we focus on the inductor and the packaging to optimize the POL performance.

As IC voltages have continued to drop there is a need for increasing numbers of separate voltage rails, with corresponding increase in the number of POLs on a typical card. Equipment density continues to increase, and there is therefore more and more incentive to reduce the physical size of the POL. Over the past few years the performance of semiconductors has continued to improve to the point where MOSFETs are now almost perfect switches. Capacitors have also improved dramatically as a result of new dielectrics as well as improved manufacturing techniques. Meanwhile, improvements in magnetic components have been much slower, and there are no dramatic breakthroughs on the horizon. The result is a POL where most of the space is allocated to the inductor.

The traditional POL using discrete components on a daughter board is giving way to low profile integrated modules with all components mounted internally, sometimes called Power Supply in Package, or PSiP, and a growing number of suppliers are offering POLs in this format. Output current available from PSiP devices varies from a few mA up to perhaps 20A, with the most common requirements being between about 5A and 10A.

Power Supply in Package (PSiP)

Most PSiP manufacturers use a packaging design approach where the control chip, FET switches and inductor are mounted alongside each other on a small circuit board or lead frame and are then encapsulated in some type of potting compound or injection molded plastic housing, as illustrated in Figure 1.

Inductor alongside control chip

This packaging approach is perhaps convenient for the manufacturer and can work well in larger sized PSiP devices. However in small size, high power density PSiP devices it sacrifices performance it two ways: first the inductor has to be made small enough to fit inside the encapsulated package, which results in lower inductance and/or higher DC resistance; and secondly, the plastic packaging is not optimized for thermal conductivity, leading to hot spots and potentially limiting operating thermal range and reliability. The PSiP efficiency is reduced significantly versus larger POLs and further size reduction becomes impractical.

Power Supply in Inductor (PSI2)

For a new range of high power density, low profile POLs with high efficiency, Sumida has taken a different approach. Instead of mounting the inductor alongside the other components, all components are mounted inside the inductor. The entire package is made from magnetic material rather than inert plastic fillers, allowing the largest volume for the inductor coil and the lowest possible DC resistance. We call this approach Power Supply in Inductor, or PSI2. Figure 2 illustrates the concept.

Components inside inductor

A small cavity in the underside of the inductor allows space for the controller, FETs and other small components on the circuit board, which forms the base of the module and has the LGA pads on the bottom. The inductor itself utilizes a custom core as well as a custom coil, and is carefully optimized to maximize the use of the available volume. With the correct choice of magnetic materials and coil design, this technique can offer very high POL efficiency even in a small, low profile package.

12V input, 6A POL module using PSI2

Using the PSI2 technique, a 12V input, 5V output POL with a 6A current rating can be integrated in a package only 9mm x 15mm x 2.8mm high. Efficiency is over 94% at full load, with only about 1.8W losses. In contrast, a competing part with the same package size and utilizing the typical PSiP structure shown in Figure 1 was measured at about 91% efficiency under the same load conditions, with losses of about 2.8W. The comparison is shown in Figure 3.

Measured power loss at 12V input, 5V/6A output

The reduction of losses by almost 35% is very significant, both from the point of view of heat dissipation in the product and also because of the energy savings. Large numbers of POLs are used in computing, telecommunications and industrial applications and every watt of energy reduction can save the end user as much as $20 (*) in electricity costs over the product lifetime.

Other advantages of PSI2

This packaging approach offers other advantages as well as efficiency. The magnetic material is a much better thermal conductor than typical plastics used for potting, so that the package surface temperature is very uniform with no hot spots. Overall temperature was measured to be about 10°C cooler than the competing part under identical load conditions. A second competitor’s module showed similar temperature rise to the Sumida part, but in a significantly larger package size (15x15mm rather than 9x15mm). Figure 4 shows an infrared image of the three parts under identical load conditions.

Thermal comparison

The magnetic material also provides some shielding for EMI, and although there is very little EMI generated by this type of low voltage, non-isolated device the improvement can be important in some cases. And unlike a separate shield, it comes with no added cost.

Future direction

The first modules made using the PSI2 technique are rated for 6A maximum current, with two series available: SPM1004 with 12V input and SPM1005 with 5V/3.3V input. Output voltages range from 5V down to 0.6V, covering all standard IC power rail requirements. Sumida plans to extend the same packaging approach to higher currents and higher input voltages, and potentially also to modules with constant current output for LED driver applications.


Sumida’s PSI2 technique offers significant benefits in high density Point of Load modules and represents the next step in POL integration. By maximizing the performance of the inductor some of the limitations of a small package size can be overcome, avoiding the compromise of efficiency that is otherwise necessary.

(*) IEEE study on the Worth of a Watt


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