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An ultra-low profile power supply for non-isolated point-of-load applications (2006)

Using power supplies in ever-smaller spaces is becoming a difficult task but one that needs to be achieved to reduce overall size for the end user. The main limitation in using a power supply in a cramped environment is thermal. A heat sink has an inherently large size since its raison d'etre is large surface area to improve convection efficiency to the ambient. This large size is highly detrimental to installation in a cramped environment.

If the heat sink is omitted, a second thermal limitation makes itself known: the low-thermal-conductivity PCB will not dissipate heat effectively and hotspots will form quickly. In many circumstances, a large cooling fan will then be required to keep the active components from overheating. Insufficient cooling will also cause the capacitors to overheat which will modify their electrical characteristics, resulting in lower efficiency and possible thermal runaway of the circuit.

A third thermal constraint is the airflow. In a cramped environment, it is often difficult to have a high airflow rate to keep temperatures in check. The limited amount of space will limit the maximum theoretical air flow due to pumping losses. Also, in many applications, natural convection is the desired cooling mechanism but this requires large board area for adequate cooling due to the substrate hotspots mentioned above.

At CPES we have developed a system design method called Stacked Power that addresses all these issues. The main limiting factor in conventional power supplies is the low-thermal-conductivity PCB. In order to resolve this, we use an aluminum nitride Direct-Bonded Copper (DBC) ceramic substrate. The devices are embedded inside the ceramic to reduce size since components can then be stacked on the top and bottom. A special inductor was designed using Low Temperature Co-fired Ceramic (LTCC), which has also been designed with a low profile: Less than 2mm for 60nH at 20A. This LTCC inductor is used as a chassis layer for the active DBC layer which is stacked on top. The entire power supply has an ultra-low profile of 5mm including output capacitors and driver chips. The highly-conductive substrate distributes heat extremely effectively so that maximum convection cooling is possible. The module in Figure 1 has been demonstrated to run at full load of 20A in natural convection conditions.

This work has attracted the attention of Sharp electronics for use in high-performance LCD TV displays where natural convection is the only viable option.
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