Power 4. Efficiency
Efficiency varies with load and input voltage and as technology allows higher and higher values, small differencesmake bigger percentage changes to the actual power dissipated by a converter. An improvement in efficiency from96 to 98 per cent for example, is a reduction in converter dissipation ofmore than 50 per cent.
than a partwithmarginally better efficiency at heavy load. If the load has fast variations, the converter should be selected
to have appropriate dynamic response. All regulated converters will display a transient change in output voltage with load swings depending on the rate of change of current, which can be quite difficult to predict. Extra capacitance on the output of the converter will improve the response but beware of any data sheet limitations on capacitance. If exceeded, the convertermay not start up correctly andmay even be unstable.
7. Isolation
Isolation is specified for various reasons: to break ground loops, to help with EMI suppression, to allow converter outputs to be electrically referenced to different points and to provide safety isolation. If a convertermanufacturer states an isolation voltage with no claimtomeet any agency standard other than functional, itmust be assumed that this voltage is amanufacturing test or flash voltage only, not a working voltage.
Efficiencies can vary across the load range depending on the product
The peak efficiencies of the best designs fromdifferent manufacturers tend to converge at similar values, however, light load efficiencies can be dramatically different. These values can affect the ability tomeet energy efficiency standards during light load ‘standby’ conditions.
5. Thermalmanagement
Typical converters will be rated for operation to 85°C, with or without derating. Note fromthe data sheet at which location the temperature is defined, which could be a set distance away with the converter in a particular orientation or it could be a baseplate temperature for a brick style product. The efficiency of the converter will itself affect the local ambient by dissipatingmore or less heat unless the environment temperature is actively controlled by, for example, a thermostatically controlled fan.
Some brick converters have options for a baseplate to be fitted
which lowers the temperature rise of the part, especially if the plate ismated to a cold wall or a finned heatsink.
6. Nature of the load
If the load on the converter has a lowduty cycle itmay be an overall energy saving to select a converterwith better low-load efficiency
8. Agency standards
Formost purposes, functional isolation would be adequate, but for isolation tomeet an agency standard for personal safety, the converter specifiermust consider higher grades such as basic, supplementary or reinforced. All these terms have specific meanings within the safety standards and the correct grade depends onmany considerations such as working voltages, pollution degree of the environment, overvoltage category of the supply, the end application of the equipment and other isolation elements in the system. Specialist skills are necessary to select the appropriate isolation grade.
9.Mechanical format
The popular brick converters are available in smaller sizes with increasing power density over time. Note that each halving of brick size, for example from half brick to quarter brick is typically only a 60 per cent reduction in footprint. Encapsulated low power DC/DC converters are also available in a variety of formats, again with increasing power density over time.
10. Environment
Apart from thermal management mentioned earlier, the user should consider other factors such as shocks, bumps, vibration and humidity. The DC/DC converter should have qualification data available proving compliance with international specifications. Surface mount parts should be rated for moisture sensitivity level (MSL) and their ability to withstand lead-free reflow soldering processes.
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A baseplate lowers the temperature rise of brick converters
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