Thermal Management
sinking from both the top and bottom, facilitating the use of a metal chassis or a BGA heat sink. This form factor promotes excellent thermal dissipation with or without airflow. Figure 6 shows an infrared (IR) thermal image of the top of the LTM4611
demonstrating a power- loss of 3.5W with no airflow, tested on a lab bench, converting a 5V input to a 1.5V output at 15A. The hottest surface temperature measures around 65°C. In contrast to Figure 6, Figure 7 shows an IR thermal image of the top of the LTM4611 demonstrating a power loss of only 3.2W with no airflow, tested on a lab
Figure 5. Thermograph of four parallel LTM4601 with BGA heat sinks & 400LFM right-to-left airflow in a 75ºC ambient chamber (12VIN to 1.0VOUT at 40A)
dropout switching regulation) with a very high 15A load current. No linear regulator could offer low voltage dropout at 15A. And if it could, it would easily burn 4.5W despite the fact that computed efficiency is 83% (=1.5V/1.8V). The LTM4611 has only 3.2W power loss with 0LFM and only 65ºC as its hot spot, which is negligible. These numbers allow a system designer to construct a very compact circuit because the constraints for heat removal are minimised. If the efficiency of a
Figure 6. Top thermal image of an LTM4611 regulator producing 1.5V at 15A from a 5V input. Power loss is 3.5W. No-airflow bench testing results in a 65ºC surface temperature hotspot.
bench, converting a 1.8V input to a 1.5V output at 15A. Hotspot locations, not their magnitude, are slightly changed from the positions seen during operation at 5V input
Tech clip
For many DC/DC µModule regulators, to prove the thermal performance, in addition to showing the efficiency and output power de-rating curves, quick 45 second Tech Clip videos are extremely useful tools in understanding thermal behaviour of a device. Figure 7 is an example of Tech Clip for the LTM4611. An infrared camera was used to record progression of temperature variation on the surface of the LTM4611 as it becomes warmer during operation. Pay a special attention to the vectors used to measure temperature on the surface of the LTM4611 (marked as 1 and 2). Ambient temperature is measured at 31.5ºC.
Blue is the lowest temperature and yellow indicates hotter areas. Note that when observing colours to determine temperature, the spectrum of the colours (blue to yellow to white) indicate gradient of a temperature and not the absolute value.
The Figure 7 Tech Clip is tested with 1.8Vin with 1.5V output (extremely low
www.cieonline.co.uk
DC/DC regulator is impressive or acceptable then study the thermal impedance of the package. Try to understand thermal (temperature) behaviour of the product under different operating conditions. A high efficiency conversion value can be misleading when thermal management is a
concern. A simple power dissipation calculation to determine an approximate temperature of the junction is a wise idea. Look for the manufacturer’s extensive thermal data such as thermal images and de-rating curves. A quality DC/DC regulator solution, especially in a modular form, should convince you of its performance with data, images and maybe video clips.
Figure 7. Tech Clip Video for 1.8VIN, 1.5VOUT at 15A Output Load, 3.2W Power Loss with 0LFM & 65ºC Surface Temperature.
Linear Technology |
www.linear.com
Afshin Odabaee is Product Marketing Manager and Alan Chern, Assistant Applications Engineer, both Power µModule Products, and Jason Sekanina is a Design Engineer, Power µModule, Linear Technology
Components in Electronics March 2012 11
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