As DC-DC converters continually increase in power density, careful thermal management is essential to maximise their usable power. Efficiency, although important, is not the only factor. Removal of dissipated heat is also essential, because product reliability and operational life relate inversely to operating temperature. Therefore, robust power system design calls for thermal management that effectively handles load and environmental demands in practice as well as in theory.
A converter’s conversion efficiency is the ratio between output and input power. This efficiency depends on its operating parameters, particularly input voltage and output current as well as ambient temperature. Therefore, good thermal management allows for the worst case efficiency expected within an application’s particular operating parameters. Consider, for example, a Vicor Maxi device with 89% practical worst-case efficiency operating at 500 W full load. This dissipates 61.8 W of power. The converter’s thermal impedance in free air is 4.9˚C/W, so the dissipation creates a temperature rise of 302.8˚C. This is much higher than the specified maximum operating temperature of 100˚C – a difference too big for elimination by improved efficiency alone.
The converter therefore requires a heatsink to reduce its baseplate’s thermal impedance. The extent of this reduction depends on allowable temperature rise, which is the difference between the baseplate and ambient maximum temperatures. For example, for an ambient 55˚C, the allowable temperature rise is (100-55) = 45˚C. Divide this by the maximum power dissipated – 61.8 W – to obtain a maximum allowable thermal impedance target of 0.73 ˚C/ W. This is the sum total of all thermal impedances in the system. Reducing any of these makes heat dissipation easier to achieve.
To maximise robustness and longevity, a derating factor of 0.75 should be applied whenever possible. In the circumstances above, this would result in a more desirable target thermal impedance of 0.55˚C/W.
Thermal impedance is minimised by natural or forced convection cooling, or by conduction cooling. Natural convection can be maximised by maximising surface area, either by using heat sinks or by mounting modules on heat-conducting surfaces. In some applications, forced air may be necessary to keep the device temperature within limits. Vicor recommends conservative designs, irrespective of the cooling method used; ideally these should be verified by direct measurement, as obstructions, eddies and airflows can all hinder airflow. This can significantly reduce actual cooling capacity compared with that predicted by theoretical design.
If plenty of space is available, adding a heat sink large enough to achieve the required cooling is easy. Mostly, however, commercial pressures dictate that space is at a premium in terms of height as well as area. Therefore, low profile heatsinks offering reasonable thermal characteristics become essential. Vicor offers such heatsinks for their Maxi, Mini and Micro converters, which add only 0.125 in (3.2 mm) to the module baseplate by shifting the cooling pins to the side of the module.
With today’s high power density applications, heat dissipation as well as efficient design is essential to maintain safe converter operating temperatures and device reliability. This can be achieved successfully by using special-purpose Vicor heatsinks, or with custom designs.