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Director of business development,Murata Power Solutions UK, Paul Lee, offers ten points to considerwhen choosing an isolated board-mount DC/DC converter
There has never been a wider choice of DC/DC converters for boardmount applications. Products fall into twomain categories: open frame ‘brick’ converters and encapsulated types in various mechanical configurations.
Brick converters are categorised as fractions of a standard-sized
brick, with sizes down to 1/32nd or 0.75 by 0.9in. The parts are generally open-frame and are rated up to around 400Wfor a half brick with forced air. Options are often available to add baseplates for conduction cooling to a cold wall or heatsink assembly.
Encapsulated board-mount DC/DC converters aremore often
used at low power and are normally characterised for convection cooling, typically for industrial
environments.Mechanical form- factors range fromthrough-hole SIP and DIP packages to surface mount types. Parts with similar electrical performance are often available in all three formats.
limits. To ensure the best output voltage accuracy with simple unregulated converters, itmay be better to specify at 75 per cent of rated load.
2. Input voltage
Nominal input voltages may be relatively fixed but spikes, surges, dips and drop-outs may be present. Converters with wide input ranges, such as 4:1, give more flexibility to cope with these transient conditions. Note that efficiency will normally vary strongly with input voltage, so examine the product data sheet to ensure that the nominal falls close to the optimum value. For example, two converters, one with nine to 36V input and another with 18 to 72V input would both cover a nominal of 24V. The first would cope with supply dips better, the second copes with surges and high voltage transients better. Their efficiencies at 24V input however, are likely to be quite different.
Even a tiny 1/16th brick converter can deliver 66W 3. Output voltage Here are ten important considerations when choosing a DC/DC
converter. Some are closely inter-related and the relative importance of each will depend on the application.
1. Output power
It is good practice to operate any electronic component at lower than itsmaximumratings.While converters will be qualified by themanufacturer for continuous operation at full load, theremay be limits to the ambient temperature allowed with relative performance indicated by a derating curve. Also, lifetime and reliabilitymay be affected when running a converter close to its
24 |March/April 2012
The user’s systemdictates the output voltage required. Some converters, especially at high power, have remote sense and trimming options to adjust the output voltage for drops in external wiring. Ensure that the converter power rating is adequate under these conditions. For example, a 100Wrated, 5.0V output converter loaded to 20A at 5.0V would be asked to provide 110Wif the output were adjusted up 10 per cent.
The output voltage will have a regulation figure. Consider
whether a low value is really necessary. If the input is fixed, say froma systembus, and the load is relatively insensitive to voltage variations, an unregulated type of convertermay be adequate, at much lower cost.
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Murata's standard SIP7 package demonstrates how power density has evolved
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