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Distributed-power architecture is common in today’s telecom infrastructure and in computing, networking systems, and industrial equipment. Consequently, such systems need high-power DC/DC converters to step-down high-input DC voltages to low-DC-output voltages in order to drive a variety of ICs, ASICs, and other such semiconductor devices spread across printed circuit boards (PCBs). With more functionality and features integrated on these semiconductor chips, high-density, high-performance, and high-power step-down DC/DC converters are needed to drive them. In short, due to space constraints, these power converters must deliver up to 100 W or more from packages that occupy a small PCB footprint with the ability to handle the dissipated power.
In other words, these simple-to-design integrated power modules must offer the capability to deliver up to 100 W or more with high-conversion efficiency from a compact package without requiring any heatsinks.
This article will explore these integrated power modules and focus on their thermal performance without airflow or a heatsink. Measured board temperature with airflow and without airflow will be compared to show that the thermally-enhanced packages are designed to efficiently manage the dissipated power.
There are a few suppliers who cater to such needs. One of them is Intersil. The supplier has prepared a fully encapsulated step-down power module, which is housed in a thermally enhanced quad flat-pack no-lead package (QFN) package. Designated ISL8225M, it offers high efficiency and low-thermal resistance to enable full-power operation without heatsinks or fans. A higher-current version with pin compatibility is the ISL8240M.
The ISL8225M incorporates components such as resistors, capacitors, inductors, and control ICs
in a single QFN package that offers features such as good thermal and electrical conductivity, low weight, and small size. The package’s good electrical and thermal conductivity is attributed to copper lead-frame with exposed copper thermal pads (Figure 1). Additionally, the copper-lead frame and multi-component assembly are over-molded with polymer mold compound to protect the devices inside the package, which is surface mountable. In addition, the thermal-molding compound, coupled with large copper pads, permits even heat distribution while enabling efficient heat transfer from the package to the PCB. The leaded package also allows for easy pin access for testing.
Figure 1: The ISL8225M QFN package’s good thermal conductivity is attributed to copper lead-frame with exposed copper thermal pads.
As a result, this fully-encapsulated step-down switching power supply can deliver up to 100 W output power from a small 17 mm square PCB footprint. Its design enables the power module to operate at full-load across a wide temperature range. The two 15 A outputs may be used independently or combined to deliver a single 30 A output. While the power module supports input voltage of 4.5 VDC to 20 VDC, the output voltage ranges from 0.6 VDC to 7.5 VDC. Also, with the single-resistor modification, the output voltage can be easily adjusted to different voltages. Likewise, the higher-current version ISL8240M is rated to offer a single output of 40 A or two 20 A outputs each. Its input- and output-voltage ratings are similar to that of the ISL8225M.
The ISL8225M QFN package offers typical junction-to-ambient thermal resistance θJA of approximately 10°C/W at natural convection (~5.8°C/W at 400LFM) with a typical 4-layer PCB. To estimate the module-junction temperature, use equation 1.
Tjunction = P × ΘjA + Tambient
In the equation Tjunction is the module internal maximum temperature (°C), Tambient is the system ambient temperature (°C), P is the total power loss of the module package (W) and θJA is the thermal resistance of module junction to ambient. According to Intersil, the high efficiency of the module and low-thermal resistance permit full-power operation without heatsinks or fans. In addition, the QFN package with external leads permits easy probing and visual solder inspection.
To demonstrate the power-handling capability of the module, Intersil conducted a thermal test on an evaluation board labeled ISL8240MEVAL4Z, which is a 4-layer PCB with top and bottom 2 oz. copper and 1 oz. copper for internal layers (Figure 2). The input voltage is 12 VDC and output is 1 VDC at 30 A load current. The switching frequency is 500 kHz. The measurement is made at room temperature with no air flow. The thermal image of the module as captured on the PCB is shown in Figure 3. It is observed that the maximum temperature of the package is 70.2°C, which is well within the operating temperature range of the power module.
Figure 3: Thermal image of ISL8240M at 30 A load current with 1 V output voltage.
Under similar input- and output-voltage parameters, the module’s thermal image was captured after the load current was increased to 40 A. The thermal image, as depicted in Figure 4, shows that the module package runs at a temperature of 93.8°C, which is well within its operating temperature range of -40°C to 125°C.
Figure 4: Thermal image of ISL8240M at 40 A load current with 1 V output voltage.
Since these power modules offer automatic current sharing and phase interleaving, up to six modules can be paralleled for higher output current capability. To show that at least three modules can be paralleled without any using a heatsink or a fan for higher output current, Intersil has tested the thermal performance of these modules in parallel configuration. For that, three ISL8225M modules are connected in parallel to deliver an output current of over 90 A at 1 VDC output. The input voltage is again 12 VDC. The thermal image for the three modules in parallel is shown in Figure 5. It is observed that all three modules are well within the rated operating temperature range without using any heatsinks or fans.
Figure 5: The thermal image for three ISL8225M modules connected in parallel.
In essence, for space-constrained applications, manufacturers like Intersil offer integrated-power modules that can deliver high-output power from a thermally-enhanced compact package without needing a heatsink or airflow. In addition, these modules can be easily connected in parallel to realize significantly higher output power, again without adding heatsinks or a fan.For more information on the products discussed in this article, use the links provided to access product pages on the Hotenda website.