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SPECIFYING THE DC-DC CONVERTER

Posted by Dave Davis / June 03, 2012 / NEWS

In our last article we focused on the distinction between the AC transformer and the DC to DC power supply, frequently referred to as the DC to DC converter. Here, we explain the importance of specifying them to suit environment and/or application. Defined environments coupled with desired electrical specifications make up the comprehensive specification of a given DC-DC converter.

The two key factors confronting the manufacturer of a DC DC converter are:
1) where th DC Converter will be used and
2) what will be the DC Converter load; i.e. what is it intended to power.

First let us examine the aspects of where (location). There is the saying in the business of real estate, that what determines the value of a property is “location, location and location.” Similarly there are 3 primary environmental/mechanical challenges that need to be addressed upon which the physical attributes of a DC-DC converter depend. They are ambient temperature, size, and ruggedness. The cost of any DC-DC converter is proportionate to the difficulty in meeting required temperature, size, and ruggedness demands. We illustrate these environmental differences with examples of 2 units of identical electrical ratings:

a) The first is used in a communications setup to operate a 12V radio using power supplied by a 36V battery bank. (48V to 12V, DC to DC power supply). The application calls for the converter to be mounted on an open bare wall, located in a climate-controlled room. In such an environment, because the temperature is controlled, there is no expectation of operation at thermal extremes. The unit is wall mounted where there is plenty of room, therefore volume limitations are not a consideration. The stationary nature of the application minimizes concerns that the hardware with which the DC DC converter is constructed would loosen due to effects of vibration.

b) Take the same 12V radio and install its converter under the driver’s seat in the cab of a forklift truck in Minnesota. In this location, one would encounter conditions of a significantly more oppressive environment than in example (a). The temperature could be expected to fluctuate from –30F in winter to +90F in summer. The volume of the mounting location is restrictive, and the vibration of the engine and the shock of travel could most certainly have adverse affects on the unit’s ability to remain intact.

In the case of (b) environmental conditions dictate that the DC-DC converter would need to be smaller, more rugged in construction and more compact than its counterpart in example (a). And this reality necessitates the implementation of appropriate design measures by manufacturers of DC to DC converters who compete in the realm of heavy equipment installations. We will investigate these techniques in a later article.

The what factor is the electrical specification. Here too there are 3 main parameters requiring definition. The first is the voltage available for the input of the DC-DC converter. The second is the voltage it is required to deliver. Third, is its maximum output current capacity. This information is fundamental in determining whether a step up DC to DC converter or a step down DC-DC converter is required. Output current capacity will be the specification to determine physical size. Current capacity considered in conjunction with the converter’s output voltage is indicative of its power capacity or maximum power rating. Power capacity is calculated by multiplying maximum output current by output voltage.

There are secondary electrical specifications that relate to how constant the DC-DC converter’s output remains under varying conditions. Line regulation is a measure of how well the converter maintains its output voltage while its input voltage fluctuates over its entire intended input range. Load regulation is a measure of how well it maintains its output voltage as its load current fluctuates over its entire output current range. Most state of the art converters have tight regulation specifications down to as low as 0.1%. What this means is that the DC-DC converter output will change by as little as 0.1% (12 millivolts in the case of a 12V output) as the output current can be varied from 0 to its full capacity.

Tertiary specifications refer to a DC-DC converter’s safety and protection mechanisms that come into play in situations of overload, abuse or failure. It is important that any one of these situations do not jeopardize the integrity of the load which the DC-DC converter is intended to energize. Frequently the load represents a higher cost element than the converter itself.

But paramount is the safety of individuals. It is for this reason that often times an isolated DC-DC converter is specified. An isolated DC-DC converter is such that should an individual come in physical contact with one of its input lines, they would be protected from simultaneously being connected to the output lines. This is especially desireable in cases where either the input or output sides is a source of high voltage.

Now that we have reviewed some of the specifications of DC-DC converters, the reader has a notion that the DC-DC manufacturing industry has myriads of models to offer. The combinations of specifications are limitless when taking the above environmental and electrical specifications into account. In the next article we will begin to hone in on certain applications focusing on their uniqueness and emphasizing how their characteristics impact in leading to their most desirable DC-DC converter specifications.

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