The term “high usable power” is widely used by the manufacturers of DC-DC converters to describe their product’s performance.  Many Engineers though tend to focus on the datasheet’s rated current and assume that by selecting a more expensive, higher power converter, it will give them more derating and improved reliability.

To determine which converter is best suited for your application there are a number of steps that have to be taken in addition to determining the input and output voltage.

Looking at the industry standard quarter brick, it is obvious from the construction, even with an integral baseplate and some heat conduction to the card it is mounted on, the converter will need some airflow, a heatsink, or both.  Cold plate is not usually available in most applications.

Fortunately most manufacturers do a very good job of providing a host of curves and their specified test methods. Below are the curves for TDK-Lambda’s iQG 500W DC-DC converter against various airflow rates and the test method.

The test method simulates vertically mounted cards in telecom and datacom cabinets.

How does this relate to our original subject regarding usable power? Below is a simplified pair of curves for 2m/s airflow. The blue line is for an 800W 12V 67A DC-DC converter, and the green line for our 500W 12V 42A converter. Even though the 800W model is 1.6 times more powerful at low ambient temperatures, in the yellow area at higher ambients the ratio drops to 1.35 times at 70oC and 1.24 times at 85oC. Typically customers operate DC-DC converters in the 65 to 80oC range.

Although the 800W converter has more available power, the 500W unit has more usable power (as a % of its rated power), demonstrated by a much less steep derating curve. It can be seen that at higher ambient temperatures, it would be more cost effective to use the 500W converter.