Packaging of a High Power Density Point of Load Converter

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Virginia Tech

Due to the power requirements for today's microprocessors, point of load converter packaging is becoming an important issue.   Traditional thermal management techniques involved in removing heat from a printed circuit board are being tested as today's technologies require small footprint and volume from all electrical systems.  While heat sinks are traditionally used to spread heat, ceramic substrates are gaining in popularity for their superior thermal qualities which can dissipate heat without the use of a heat sink.  3D integration techniques are needed to realize a solution that incorporates the active and components together.  The objective of this research is to explore the packaging of a high current, high power density, high frequency DC/DC converter using ceramic substrates to create a low profile converter to meet the needs of current technologies.

One issue with current converters is the large volume of the passive components.  Increasing the switching frequency to the megahertz range is one way to reduce to volume of these components.  The other way is to fundamentally change the way these inductors are designed.  This work will explore the use of low temperature co-fired ceramic (LTCC) tapes in the magnetic design to allow a low profile planar inductor to be used as a substrate.  LTCC tapes have excellent properties in the 1-10 MHz range that allow for a high permeability, low loss solution.  These tapes are co-fired with a silver paste as the conductor.  This paper looks at ways to reduce dc resistance in the inductor design through packaging methods which in turn allow for higher current operation and better heavy load efficiency.  Fundamental limits for LTCC technologies are pushed past their limits during this work.  This work also explores fabrication of LTCC inductors using two theoretical ideas: vertical flux and lateral flux.  Issues are presented and methods are conceived for both types of designs.  The lateral flux inductor gives much better inductance density which results in a much thinner design.

It is found that the active devices must be shielded from the magnetic substrate interference so active layer designs are discussed.  Alumina and Aluminum Nitride substrates are used to form a complete 3D integration scheme that gives excellent thermal management even in natural convection.  This work discusses the use of a stacked power technique which embeds the devices in the substrate to give double sided cooling capabilities.  This fabrication goes away from traditional photoresist and solder-masking techniques and simplifies the entire process so that it can be transferred to industry.  Time consuming sputtering and electroplating processes are removed and replaced by a direct bonded copper substrate which can have up to 8 mil thick copper layers allowing for even greater thermal capability in the substrate.  The result is small footprint and volume with a power density 3X greater than any commercial product with comparable output currents.  A two phase coupled inductor version using stacked power is also presented to achieve even higher power density.

As better device technologies come to the marketplace, higher power density designs can be achieved.  This paper will introduce a 3D integration design that includes the use of Gallium Nitride devices.  Gallium Nitride is rapidly becoming the popular device for high frequency designs due to its high electron mobility properties compared to silicon.  This allows for lower switching losses and thus better thermal characteristics at high frequency.  The knowledge learned from the stacked power processes gives insight into creating a small footprint, high current ceramic substrate design.  A 3D integrated design is presented using GaN devices along with a lateral flux inductor.  Shielded and Non-Shielded power loop designs are compared to show the effect on overall converter efficiency.  Thermal designs and comparisons to PCB are made using thermal imaging.  The result is a footprint reduction of 40% from previous designs and power densities reaching close to 900W/in3.

Buck Converter, High Power Density, High Frequency, Low Temperature Co-Fired Ceramics, Ceramic Substrate, Integration, Electronic Packaging