Power converter applications - heat radiation in design

Along with the development of the switching power supplier industry, AC/DC and DC/DC power converters are now being widely adopted in a comprehensive range of applications. As technologies and techniques improve, the devices are getting smaller with higher power densities and superior performance. So the effect of heat radiation has been and is critical in most designs and field applications.

Typically, AC to DC power converters, because they use electrolytic capacitors, are particularly sensitive to heat. Long-term operation in high temperatures will significantly reduce the lifetime of electrolytic capacitors and may lead to potential problems or safety issues. Appropriate heat radiation design will extend the lifetime of a power converter and reduce the possibility of failure or safety issues. This article will discuss heat radiation design in power converter applications.

Typical heat radiation design applications

Natural convection

Limited by size, the majority of high-power density and miniature power converters, such as Mornsun products, use natural convection as their heat radiation method.

There are three ways to use heat radiation in natural convection applications:

• Free air convection dissipated from the surface of converter.
• Radiated heat from the converter to surrounding objects, or through the bottom of converter to the PCB.
• Conduction via the converters pins to the PCB.

Practical tips for natural convection include:

• Ventilation: With converters that use natural convection and radiation as the major heat radiation method, ventilation is very important. It’s important not to surround the converter with any large objects and even make through holes on the PCB to enhance convection if necessary.
• Arrangement of heat source: if there is more than one heat source in the application, keep each of them at distance to avoid mutual radiation from heating the power converter.
• Design - mounting the PCB: mounting the PCB is one of the critical considerations. The heat dissipation and management if purposely designed - eg, increasing the size of the copper printed in the primary loop - lower the component density.

Using with a heat sink

In some cases where natural convection may not be sufficient for heat dissipation in the application, using a heat sink is a simple way to further lower the converter’s surface temperature.

Potting materials used in converters are usually heat-conducting materials - silicon or resin. In some situations using natural convection, the heat-resistance between converter case and the ambient environment is far higher than it is between converter case and inside components; inside of the converter, the heat distribution is even. The majority of heat is dissipated from converter surface into the air. The effectiveness of heat dissipation is determined by the converter’s surface area. Adding a heat sink to the converter is the way to enlarge its surface area so that more heat is dissipated from the surface through natural convection and radiation.

Practical tips for using a heat sink include:

• The heat sink is best made of a material with good heat-conducting performance. Aluminium and copper are two such materials.
• The heat sink should be ribbed or have fins (to increase the surface area), with a matte surface and black finish.
• The heat sink should have a thick base, ribbed on the longer dimension.
• The converter with the heat sink is best positioned where the converter case and ambient environment has the greatest temperature difference. Use heat-conducting material in between the converter case and heat sink so they can efficiently transmit the heat.

Forced convection using a fan

In some instances, the application may need further assistance with heat dissipation. After adding a heat sink, fan-forced convection may be required.

Cooling fans can significantly lower the surface temperature of power converters in critical applications. However, a big fan can occupy a large space in the equipment, so size, speed and performance selection is important. High RPM fans with strong pressure performance can make a lot of noise, and vice versa. Size and speed both affect the fan’s capability of heat dissipation. Taking these factors into consideration, a good fan ought to be a compromised combination of performance and adverse impacts.

When installation the fan, it should be positioned to blow towards the heat sink on the converter, producing a vertical air current so there is a constant air flow through the fins and grooves of the heat sink.

Conclusion

The above three methods are typically what needs to be considered for heat dissipation design in power converter applications and choosing the most practical and efficient solution for your application. Correct assessment and application of dissipation with or without a heat sink or fan will ensure your project has a long and reliable lifespan.

For more information on heat radiation design applications, contact Mornsun Power product distributors such as DLPC (ANZ/Oceania), Fairmont Marketing (Victoria and South Australia) and Fero (New Zealand).