Clearing The Fog That Surrounds Derating Curves

Aug. 19, 2002
A marketing guru once said: "A data sheet isn't only a technical document, but also a marketing tool." Following this mentality, dc-dc converter manufacturers creatively market their products. In the world of high-density dc-dc converters, especially...

A marketing guru once said: "A data sheet isn't only a technical document, but also a marketing tool." Following this mentality, dc-dc converter manufacturers creatively market their products. In the world of high-density dc-dc converters, especially open-frame converters, a commonly misunderstood term is derating.

Derating is the negative slope of the power-versus-temperature graph. Specifically, it shows that as the operating ambient temperature increases, the converter's maximum output power drops to ensure reliable system operation. Derating curves provide a quick way to estimate the maximum output power of a converter at a given temperature. The confusion starts with the definition of operating ambient temperature. Some manufacturers define derating over ambient temperature, while others use case temperature or ambient temperature with an additional airflow of 50 to 100 linear feet per minute (LFM).

The definition of ambient temperature is obscured by the fact that any high-density power device operating in free air convection (no moving air) generates its own ambient temperature by heating the surrounding air. The temperature differential between the environment's ambient and the converter's ambient could be 15°C to 30°C. Some vendors specify only thermal resistance, case-to-ambient, and the maximum operating case temperature. Then the designer must calculate derating based on the given data and the maximum power needs of the system.

Manufacturers use different versions of the definitions for derating mentioned above to judge the performance of their high-density converters. If the derating curve data is detrimental to their product's marketability, they may not get published. Or, the vendor could merely discuss the subject as a whole in a general application note. Other companies implement maximum-case temperature and conveniently forget to include ambient temperature. From their point of view, all published specs for derating are correct! Nevertheless, this shifts the burden of deciphering the derating methodology to the system designer.

The manufacturers' jargon and the promotional marketing info won't intimidate an experienced system designer. But a novice engineer can become frustrated and will likely have to spend many hours learning how to use the derating curves (when they're available). Only a few manufacturers with good reputations in the industry employ worst-case derating curves to ensure product reliability. Hardly any take on the extra burden of providing additional application notes and technical assistance to their customers.

Another factor complicating derating is the educational system. Few engineering schools offer power-supply courses. As young engineers exit the ideal "digital world" presented in school and dive into the real world, they sometimes find themselves unprepared to work with power supplies. System designers—experienced or not—must have familiarity with various disciplines, including electronics, thermodynamics, physics, and chemistry. But a system designer with a tight schedule may not have time to deal with thermal management of the system. Usually, the power converter is the last component considered in the project design. Many times the system designer will underestimate the power needs of the system, or won't consider the derating of the converter, resulting in a marginal or unreliable system over temperature.

After this problem has been identified, the system designer may try to solve it by increasing the airflow over the converter or installing a heatsink before considering a move to a higher-power-density or higher-efficiency converter. In the search of a fan or heatsink, the designer requires familiarity with the fan/heatsink manufacturer's jargon, such as unit conversion factors, available area, fin orientation, and interfacing materials.

Given the oversights and marketing limitations, how can derating curves be defined in a concise, standardized way that the power-supply community will accept? One method is to define derating curves versus temperatures that use airflow of a few LFM and call it ambient temperature. Yet, this will imply that the converter won't work without airflow. Another way is to use minimal airflow in the definition of the ambient temperature with the converter's ambient as the ambient temperature. But until all companies implement one standard for ambient temperature, some might use definitions to make their converters seem more thermally efficient.

As the prices of dc-dc converters plunge to the point of becoming commodities, I'm afraid that manufacturers will have to find more creative ways to present derating curve specifications, or not publish them at all. Unless some significant changes take place involving standardization, better training and education, and some ethical constraints in marketing, both we and our customers are doomed to live in this confusing world. Sadly, the lowest-priced manufacturer will survive and dictate price and specifications in the long run.

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