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Thermal considerations of power converters

Supplier: Benbro Electronics
29 July, 2010

When installing power converters into equipment racks, system integrators and electronic designers need to take into account a number of thermal considerations to ensure the optimal performance of the power converter and hence, the entire system.

Good thermal practice allows the power converter to operate within an ambient temperature below that of the rated maximum specification. It also contributes to the power converter's ongoing reliability and extends its life as well as that of the entire equipment system rack as a whole.

Each power converter is self-heating and this coupled with the heat generated from the power supply and surrounding elements means that the operating temperature of the power converter often exceeds the product manufacturer's specified maximum.

All power product manufacturers such as Astrodyne specify the maximum operating temperature of their products.

In a typical 150W AC/DC switching power supply with a 75% efficiency, a 20ºC rise in internal temperature can be expected. Coupled with a 25ºC ambient, this raises the internal temperature of the components to 45ºC. Once maximum ambient is reached, power converters typically derate the output at 3%/ºC, resulting in rapid output power rolloff above the manufacturer's rated maximum ambient.

At 70ºC ambient, power output drops by 30%, resulting in an effective constant output capability of 105W. The mean time between failures (MTBF) is also severely downgraded by elevated temperature operation as shown in the graph below.

To effectively control and manage the thermal environment of power converters, system integrators and electronic designers need to be aware of several temperatures including:

  1. The actual ambient immediately around the converter under normal operation (including self-heating contribution),
  2. The manufacturer's temperature rating for the converter (when properly mounted and installed), and
  3. The maximum system ambient operating specifications of the overall system itself.

These combined factors will allow a designer to ascertain the temperature envelope under which reliable power converter operation is assured. If these cumulative temperatures are expected to exceed the operating specifications of the converter, then a designer can use various heat dissipation and control methods to control the temperature.

According to John Bennett, Director Engineering at Benbro Electronics Pty Ltd, a number of methods can be used to disperse heat. These include:

  1. Additional heat sinking, either through mounting the unit to an outer case wall as a dissipating surface, or by use of finned heat sinks on the device itself.
  2. Using fans to provide forced air cooling to dissipate heat.
  3. Operating with a derated output power level, or oversizing the converter to conform to the manufacturer's power derating curve specification.
  4. Heat can also be dissipated by locating the converter away from other heat-generating parts of the system.

"By using any or a combination of these methods designers can control the temperature and help assure the full performance of the power converter product over the anticipated system operating range," explained John.

"It is paramount that system integrators and designers not underestimate or overlook the thermal considerations of power converters if they want to ensure optimal reliability and performance of both the converter and the system," added John.