Forget Power Device Current Ratings, Calculate Application Losses

Feb 1, 2010 12:00 PM
Havanur, Sanjay, Alpha & Omega Semiconductor - Principal Applications Engineer

System designers are often charged with selecting the most suitable power device from a wide array of products, available with very similar ratings from different manufacturers. While a detailed parameter-by-parameter comparison is technically the best way to make the selection, it is not the most practical approach.

Designers resort to making their first cut based on three or four simple parameters. Among these are package, voltage and current ratings, R_DS(ON), etc. Here, we will take a look at current rating. For this illustration, we will focus on MOSFETs in low- and medium-power packages, but the considerations can be applied to other technologies as well.

RATED CURRENT: SEVERAL VERSIONS OF THE TRUTH

The definition of voltage rating is well accepted; it is measurable with a high degree of consistency under conditions that are not far removed from the real world. It is reasonable to expect that the device can be subjected to its rated voltage continuously without causing failure. The situation is different for current ratings. There is no such thing as a measured value of rated current; it is always arrived at by indirect calculation.

As a result, several versions of rated current can exist. The most common is specified at case temperature, T_C = 25°C, and another at T_C = 100°C. SMT packages are also specified at ambient temperatures, such as 25°C and 70°C. We will focus on T_C-based definitions for leaded packages; the conclusions can be easily extrapolated to other packages.

The usable current of any device is mainly limited by the heat it generates within the die and the maximum permissible junction temperature, T_J(max), for the silicon. Continuous dc current is assumed, so rated current follows:

The inequality has its reasons. With low-voltage MOSFETs heading towards sub-mΩ values of R_DS, at T_C = 25°C the formula can yield hundreds of amperes as rated current in a tiny DFN package. In these cases, the limiting factor is not the silicon, but the mechanical construction of the package — such as the bond wires and pin size.

Even with the equality sign, this may not indicate the device's usable current. Most quick references give the value for T_C = 25°C, which either requires a massive heat sink to maintain, or is theoretically impossible when the ambient is in excess of 25°C. Even if you look at the T_C = 100°C definition, it assumes continuous dc current and zero switching losses, excluding the majority of today's applications. Most companies also set strict derating standards and limit junction temperature to 125°C, even if the manufacturer claims a T_J(max) of 175 °C. Usable current rating is then only 57.7% of the specification.

While ignoring absolute value, many users are tempted to use current rating as a comparative benchmark for different devices. If R_DS(ON) values are close enough, it reduces to a comparison of package thermal resistance. Technically, this is a meaningless exercise, as a number of variations are possible within the definition. Different manufacturers assume different tolerances on the thermal resistance for the same package; the number can vary from 15% to 50%. Some do not even provide a maximum value and use the typical value to boost the current rating.

If that is not enough, many manufacturers use a creative version of steady-state and limit it to 10 s. Power is applied and junction temperature is measured after only 10 s to calculate R_θJC. Not all data sheets are clear about exactly which definition of steady-state is being used. As a result, for the same R_DS(ON) and case temperature, rated current can vary as much as 20% among essentially identical products. Given a specific application and operating current, the assumption that selecting a device touting a higher rated current somehow leads to better performance or reliability is misguided, to say the least.

Want to use this article? Click here for options!
© 2007 Penton Media, Inc.