bottom of the package to the PCB, layout guidance and discussions about thermal measurements must be articulated in the data sheet to eliminate surprises later during system prototyping.
Guiding heat from the inside of the package to the top and into the air
A high-power switching POL regulator depends on an inductor or transformer to convert the input supply voltage to a regulated output voltage. In a non-isolated step-down POL regulator, the device uses an inductor where the inductor and the accompanying switching elements such as MOSFETs produce heat during DC/DC conversion. About a decade ago, a new packaging advancement allowed an entire DC/DC regulator circuit including the magnetics to be designed and fitted inside a molded plastic, called modules or SiP, where much of the heat generated inside the molded plastic has to be routed to the PCB from the bottom of the package. Any conventional attempt to improve heat removal capability of the package contributes to a larger package such as attaching a heatsink to the top of the surface mount package. However, as recent as three years ago, an innovative module packaging technique took advantage of available airflow to cool itself. A heatsink was integrated into the module package and over molded. Designed in a unique shape, the heatsink was connected to the MOSFETs and inductors, the heat generators, inside the package and the other end of this heatsink was a flat surface exposed on top of the package. With this new packaging and on- board heatsinking technique, the device could be cooled quickly with some airflow as the air removed heat from the top of the package where the flat surface of the heatsink was in contact with the air.
POL module regulator with stacked inductor as heatsink The size of an inductor in a POL regulator depends, among many factors, on voltage, switching frequency, current handling and its construction. In a module approach where the DC/DC circuit including the inductor is overmolded and encapsulated in a plastic package and resembles an IC, the size of the inductor dictates the thickness, volume and weight of the package. The inductor is also a heating element contributing to the overall internal temperature of the POL module regulator. The integrated heatsink in the package, discussed earlier, conducts heat from the MOSFETs and inductor to the top of the package and is greatly helpful in quickly transferring internal heat to outside of the package on top and finally to air, a cold plate or a passive heatsink. However, this technique is useful for smaller size, lower current inductors where they easily fit inside within the plastic mold compound of the package. For higher power POL regulators that depend on larger and higher current inductors, placement of the magnetics inside the package forces other components of the circuit to be pushed away thus expanding the PCB footprint of the package. A larger footprint means a heavier package. To keep the footprint small and to further improve heat dissipation, the packaging engineers have developed another trick: go vertical, stack or 3D (Figure 1).
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Figure 2: LTM4636 uses stacked inductor as heatsink to achieve impressive thermal performance in small footprint area
3D packaging with exposed stacked inductor: keep footprint small, increase power, improve heat dissipation Small PCB footprint, more power and better thermal performance; all three are possible simultaneously with 3D packaging a new method in construction of POL regulators (Figures 1 and 2). The LTM4636 is a µModule regulator with on-board DC/DC regulator IC, MOSFETs, supporting circuitry and a large inductor to decrease output ripple and deliver load currents up to 40A from 12V input to a precisely regulated output voltage ranging from 3.3V to 0.6V. Four LTM4636 devices can current share to provide 160A of load current. The footprint of the package is only 16mm x 16mm. If you do the numbers, power density is very good. But, let’s not be fooled by this number. The benefits that this µModule regulator brings to system designers is in its thermal performance, a combination of its impressive DC/DC conversion efficiency and heat dispersion ability. To keep the footprint small, the large footprint inductor is elevated and secured on two copper lead frame structures so that the rest of circuit components can be soldered under it on the substrate. If the inductor were to be placed on the
substrate, the µModule regulator could have easily occupied more than 1,225mm2 of the PCB vs its small 256mm2
footprint.
This technique rewards system designers with a more compact POL regulator layout but it has another great benefit - it demonstrates good thermal performance. The stacked inductor in the LTM4636 is not overmolded (encapsulated) with the plastic compound but the rest of the components are. The inductor is conveniently exposed to the air and with its smooth corners and raised structure, the air flows more readily around and on top of it (minimal flow blockage).
The LTM4636 is a 40A capable µModule regulator benefiting from 3D packaging technology also referred to as Component-on-Package (CoP, Figure 2). The body of the package is an overmolded 16mm x 16mm x 1.91mm BGA package. With the inductor stacked on top of the molded section, the LTM4636’s total package height, from the bottom of BGA solder balls (144 of them) to the top of the inductor, is 7.16mm.
In addition to dissipating heat from the top, the LTM4636 is designed to efficiently disperse heat from the bottom of the package to the PCB. It has 144 BGA solder
balls with banks of them dedicated to GND, VIN
and VOUT where high current
flows. Collectively, these solder balls act as a heatsink to the PCB. The µModule is optimised to dissipate heat from the top and bottom of the package. At 12VIN
, 1VOUT with full load current of
40A (40W) and standard 200LFM airflow, the LTM4636 has only 40°C rise over the ambient temperature (25°C-26.5°C). The derating numbers are shown in Figure 3. At 200LFM, the LTM4636 delivers full current of 40A up to 83°C ambient temperature. Half current, 20A is at very high ambient temperature of 110°C ambient. This means that the LTM4636’s load current delivery is less affected by the ambient temperature when some airflow is available.
160W, scalable 4 x 40A µModule POL regulator with thermal balance One LTM4636 is rated for 40A load current delivery. Two devices in current sharing mode (or parallel) provide 80A and four of them up to 160A. The current mode architecture of the LTM4636 is responsible for precision current sharing among the multiple blocks of 40A each. A precision current sharing, in return, allows for a balanced heat dissipation among two, three or four devices in parallel. Because of this ability, no one device will be overloaded or overheated. Another feature of this device is its ability to operate out-of- phase to reduce output and input ripple current which in turn reduces the number of input and output capacitors.
Conclusion Choosing a POL regulator for a densely populated system requires scrutiny beyond voltage and amperage ratings of the device. Evaluation of its package’s thermal characteristics is essential as it determines an equipment’s cost of cooling, cost of PCB and size. An advancement in 3D, also referred to as stacked, vertical, CoP, allows high power POL module regulators to fit on small PCB footprint but more importantly allows efficient cooling. The LTM4636 is the first series of µModule regulators to benefit from this stacked packaging technology. As a 40A POL µModule regulator with stacked inductor as heatsink, it boasts between 95 per cent to 88 per cent efficiency, only 40°C rise and occupies only 16mm x 16mm of PCB area.
Figure 3: Full current of 40A delivered up to 83ºC ambient, 200LFM
www.linear.com/LTM4636 Linear Technology (UK) Ltd Tel: 01628 477066
Components in Electronics February 2017 9
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