Trends in power supply packaging – advances in component integration
Customers are pushing for increasing power per board and implementing ever more silicon on the PCB with the result that increasing processing density in high-end server designs will continue to have an impact on future power systems. The power demand per board in ICT data servers has increased from 300 W in the early 1980s to more than 1kW today – and it is anticipated that power of 3‑5 kW per board will be required by 2020. Current DC/DC power converter solutions and technologies are not adequate at these power levels.
Today, a 1kW DC/DC converter in a quarter-brick format is a reality with power density figures that could not even be imagined a few years back. Could a 1kW eighth-brick be possible in the near future with advanced packaging and highly integrated components?
This article takes a look at where the power industry is going in terms of component integration, thermal management and more than doubling DC/DC power converter density compared with current state‑of‑the‑art technology.
3D Packaging
Today’s DC/DC power converter bricks are still dominated by planar two-dimensional PCB constructions, but customer applications requiring smaller footprints, lower profile devices and reduced parasitic impedances are driving technology for high-density 3D packaging.
The use of 3D packaging technology is limited in these high power bricks, but there are promising developments in embedding both active and passive components, and PCB vendors see this as a major opportunity to move up the value chain. This will include chip stacking, package stacking and component embedding through over-molding. Very important in this area is the integration of magnetic materials with the ultimate solution being integration of the magnetic component on the semiconductor wafer.
The most common technique in 3D packaging is that of embedding components (both active and passive) within the PCB. Embedding of components in the PCB construction offers the power designer significant gains in footprint reduction, enhanced cooling possibilities and, for example, the positioning of drivers in close proximity of switching devices. This will facilitate performance and efficiency improvements by minimization and precise control of interconnect parasitic impedances in high-frequency switching circuit designs. Subsequent 3D assembly of additional components will further contribute to the required footprint reduction and magnetic component size reduction.
Embedded components offer significant advantages to power designers. However, support from the silicon industry is essential and a supply chain with standardized requirements and qualification tests for active and passive components will be necessary. Given an appropriate infrastructure, embedded technology will be an important contributor to improving power density in high-power applications. The Hermes program funded by the EU has successfully demonstrated that a size reduction of 40 % in high-volume power converters is feasible. It is expected that magnetic isolation, coupled with embedding could provide reinforced isolation. Feedback paths, through integration with control could also become magnetic, leading to the possibility of a chip-scale isolated DC/DC power converter solution.
Components
Switching frequency for high-power DC/DC converters have generally been optimized for an operating frequency of around or below 500 kHz. To facilitate the required size reduction and increase of power density, an increased switching frequency up to 2 MHz and above is necessary in order to minimize magnetic physical volume. The relatively recent availability of wide-band-gap (WBG) semiconductor devices operating ideally at higher frequencies in excess of 5 MHz, such as GaN and GaAs switching FETs, has been an enabler for higher switching frequencies. New DC/DC converter topologies may even increase switching frequencies well into the 10 MHz range. This in turn drives the requirement for packaging with reduced parasitic components, which can be achieved using 3D integration techniques.
The availability of embedded components in the PCB will enable the reduction of parasitic impedances required to take advantage of WBG devices at higher frequencies, and will help facilitate significant improvement in footprint and efficiency for high-power DC/DC converters. However, the higher switching speed is dependant on the availability of low-loss high-frequency magnetic materials innovation leading to commercially viable power transformer and inductor solutions on the market for high-volume production.
There are several viable technology paths to higher frequency integrated magnetic components including advanced magnetic-core designs and core materials, as well as air-core designs, which may enable significant improvements in efficiency and power density. The availability of several viable approaches for miniaturized magnetics, including air-core designs without the dependence on core material characteristics, also provides flexibility in production methods and the possibility to use different 3D integration techniques such as embedded windings in multi-layer PCB and multi-layer ferrite substrate with integrated active Cu layers (see figure below).
Today these new techniques are limited to lower power converters, but further improvements in core materials and expansion into products with increasing output current will be enabled by improved processes for magnetic materials. Even the ultimate goal in 3D integration with the magnetics embedded in the semiconductor wafer and the possibility of complete monolithic system integration would be possible in the future.
Thermal Management
Advancements in components and packaging technologies are making power ratings higher and higher so that the power density, in terms of watts per cm3, is now an order of magnitude higher than that of older technology used some 15 years ago. The latest power bricks in the market, such as Ericsson’s high-power-density 864 W quarter-brick module, offer an impressive 37 W/cm3 (600 W/in3), which put very high demands on efficient internal thermal management.
Because electronics components such as semiconductor devices are sensitive to high temperatures, it is essential to make sure the components in high-power-density bricks are cooled properly and are operated at a reasonable temperature. Unless the heat transfer mechanism is extremely efficient, the power system design and reliability can be jeopardized.
The primary cooling mechanisms that could be used to cool electronic equipment are conduction and convection. Component power dissipation (Pd, comp) and component junction‑to‑case thermal resistance (Rth, J-C) for each critical component becomes extremely important as they determine the actual junction temperature that limits the DC/DC converter’s thermal performance, i.e. the maximum case temperature allowed at maximum output power.
The temperature difference between the component junction (or core) and case can be calculated using the following equation
ΔTJ-C = Pd, comp x Rth, J-C
Consequently, advanced cooling technologies and techniques for improvements of thermal performance of emerging 3D packaging assemblies aiming for double the power density (75 W/cm3 or 1200 W/in3) compared with current state-of-the-art, are absolutely crucial and will ultimately determine the feasibility of higher power densities regardless of any improved component technology.
Many standard components are not suitable for high density or 3D designs due to insufficient thermal performance. Other thermal design challenges met when delivering extreme levels of power from DC/DC converter assemblies include high-current distribution, connector technology, assembly on 45-layer boards for example, and the inadequacy of even greatly enhanced traditional cooling techniques such as existing air-blown convection.
Over-molding is likely to continue as a technology used for improving thermal performance, but it is also clear that thermally enhanced packaging for all power components including magnetic components and capacitors to allow cooling from at least two opposite surfaces will be required, together with the use of improved thermal materials, processes and cooling techniques. An example of such 3D packaging is shown below.
Instead of mounting power components on a PCB with die attach or thermal interface material, the components are held on a temporary carrier and a heat sink is electroformed around them. Components of various sizes and thicknesses can be integrated on the same board, called an Integrated Thermal Array Plate (ITAP). When the carrier is removed, the bottom and top surfaces of the components are co-planar and facilitate a defined and optimised thermal connection. Up to 50 % improvement in thermal resistance, i.e. 50 % higher power dissipation for a fixed TJ, may be achieved compared with conventionally packaged components with epoxy or solder attach.
Through-Silicon-Vias (TSV) will also be a potential solution in stacked chip solutions in combination with liquid cooling techniques. Also here experimental results indicate that a 50 % improvement of thermal performance is achievable. The replacement of solder and thermal grease with sintering in assemblies using Direct Bond Copper (DBC) technology is another technique that could improve the thermal performance significantly.
Other potential cooling techniques include both liquid conductive cooling of certain high-power components and forced air convection cooling of medium- and low-power dissipating components. The use of passive liquid cooling technologies (e.g. heat pipes) is likely to become more widespread to deal with local hot-spots. Thermal spreading combined with convection air cooling may have an extended life due to reduced thermal impedances in component packages achieved through improved chip attachment techniques, but active liquid cooling technologies (e.g. pumped and two phase boiling) could be be required for the most demanding high-power dissipation components in high-power-density DC/DC converters.
The 1 kW eighth-brick
There will be an on-going 3D packaging and IC type chip-scale development but on a larger scale, including integration of power magnetics that will increase the power levels far beyond today’s non-isolated buck converters. The use of planar magnetics is already widespread and the power converter assemblies will probably be over-moulded to improve thermal performance.
However, development of 3D packaging and other embedding technologies required to double the power density cannot be driven by the DC/DC power converter industry alone. Significant investments by companies in the high-volume automotive and motor drive industry will be needed. It will also require support by the power component industry providing suitable components and standardized specifications and qualification tests.
Increasing processing density in server designs will certainly continue to have an impact on future DC/DC converter power density. The power demand per board in ICT data servers has increased significantly in recent years and it is anticipated that 3-5 kW per board will be required in the near future. In addition, the equipment is required to occupy less floor space, which will imply higher overall power density.
The development of 3D packaging and other embedding technologies promise significant improvements in power density and thermal management. Both experimental results and full-scale production shows the potential of achieving double power density combined with thermal management solutions for keeping the component core temperatures within specified levels for reliable operation, facilitating a 1 kW eighth-brick in the near future. The main hurdle will be the development of low-loss very high frequency (>5 MHz) magnetic designs and core materials.
About the author
Marketing and Communication Director
Patrick Le Fèvre’s career has been focused on power products and sustainability since 1982 when he started with a start-up called Micro-Gisco. Le Fèvre joined Ericsson in June 1996 and was promoted to Head of Marketing and Communication in 2001. Patrick Le Fèvre is also involved in several environmental forums.
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