Advances in power supply packaging

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-moulding. 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 minimisation 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 standardised 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 ('programme' for plan) 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 optimised for an operating frequency of around or below 500kHz. To facilitate the required size reduction and increase of power density, an increased switching frequency up to 2MHz and above is necessary in order to minimise magnetic physical volume. The relatively recent availability of wide-band-gap (WBG) semiconductor devices operating ideally at higher frequencies in excess of 5MHz, 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 10MHz 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.