COVER STORY
The search for cooler, high power and scalable POL regulators in small footprint packages yield results
Afshin Odabaee, business unit manager, µModule Power Products, Linear Technology Corporation, talks about 3D packaging architecture and how clever component placement resolves thermal issues
T
he following statements will surely upset DC/DC IC and circuit designers, but the truth is that it’s truer today than a few years ago. With all due respect to their complex minds and rich experiences in the art and science of designing with Bode plots, Maxwell’s equations and concerns for poles and zeros to finally devise an elegant DC/DC converter circuit, IC designers often scape dealing with that one last dreaded physics topic: heat. It’s the packaging engineers’ job. And the packaging engineers now have far more influence in the merits of a DC/DC POL (point-of-load) regulator’s thermal performance, especially those with high power housed in small packages. A POL regulator generates heat because no voltage conversion is 100 per cent efficient (yet). Then there is the question of how hot the package becomes due to its construction, layout, and thermal impedance. Thermal impedance of the package not only raises temperature of the POL regulator, it also increases the temperature of the PCB and surrounding components as well as contributing to the complexity of a system’s heat removal arrangements. The removal of the heat from the
package when it is assembled on a PCB is done by two major means: 1) When surface mounted, the heat is conducted into the copper PCB layers, spreading the heat from the bottom of the package
2) The cool airflow removes heat from the top of the package, or more precisely, the heat is transferred to the cooler fast air molecules in contact with the surface of the top of the package. Of course, there are methods of passive and active heatsinking, which for simplicity of discussion, are considered subsets of the second category. So, more copper, more PCB area, thicker PCB layers, spreading out placement of components on the PCB, bigger and faster fans are some good ideas from the tool kit of thermal management to keep the whole system including the DC/DC POL regulators operating within a safe temperature value. Good ideas perhaps, but are there other approaches to help with thermal management of small but high power POL regulators? Although at first some or all the steps mentioned earlier have a significant impact in keeping a system cooler, applying these thermal remedies may diminish a system’s
or end product’s selling competitive edge. The end product may become larger because of the intentional separation of components on the PCB, audibly noisier because of the number of fans and faster streams of airflow entering and exiting hot circuits, and perhaps finally, rendering the end product inferior in a market that companies constantly compete to win on the merits of compactness, computational power, data rates, efficiency, and cooling costs. 28-20nm and sub-20nm digital devices are burning more power to deliver better performance, while equipment suppliers battle each other with faster, smaller, quieter and more efficient innovations. Behind the excitement of new digital technology prowess, there is also the analog and power technology’s struggle to deliver more power in smaller packages with minimal contribution to the rise of a system’s overall temperature. A POL voltage regulator with higher power density seems like a good choice: It’s smaller but higher power.
Judging POL regulators by the power density numbers are …. for the rookies 40W per cm2
to be better than a 30W per cm2
(or cubed) POL regulator has regulator.
Marketers use power density to sell their products and system designers each year demand higher power-density regulators to position their next faster, smaller, quieter and more efficient products against their competitors. Should higher power density numbers be a deciding factor in choosing a “better” POL regulator?
First, forget the power density number and study the data sheet for the POL regulator. Find the thermal derating curves. A well-documented and characterised POL regulator should have many such graphs specifying output current at different input voltages, output voltages and airflow speeds. In other words, the data sheet should show the output current capability of the POL regulator under your circuit’s conditions so you can judge the regulator by its thermal and load current abilities. Does it meet the requirement of your system’s typical and maximum ambient temperature and airflow speed? Remember, output current derating relates to the thermal performance of the device. The two are closely related and equally important. Second is the efficiency; not first, second. Efficiency alone is misleading and gives an inaccurate representation of the thermal characteristics of a DC/DC regulator. It is needed to calculate input current and load current, input power consumption, power dissipation, junction temperature, etc. But, to make better sense, an efficiency number should be studied along with the output current derating and other thermal data related to the device and its package.
The last factor to consider is the ease of cooling the POL regulator. The package thermal impedance values provided in the data sheet are key to simulate and calculate rise in junction, ambient and case temperatures of the device. Because much of the heat in surface mount packages flow from the
Figure 1: 3D or vertical packaging technology for high power POL regulator modules elevates the inductor and exposes it to airflow as a heatsink. The rest of the DC/DC circuitry is assembled on substrate under the inductor keeping PCB area small and improving thermal performance of the package
8 February 2017 Components in Electronics
www.cieonline.co.uk
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