SMPS Question: Volt-uS derating for inductors?

http://powerelectronics.com/mag/power_guide_selecting_inductors/ said:

Core Loss and Temperature
Allowable losses for an inductor are limited by the maximum permissible temperature. Thus, for most off-the-shelf parts, this limit is a surface temperature of 125°C — although it can be higher. The rms current rating is typically the dc current that leads to a 40°C temperature rise, theoretically allowing operation in an 85°C ambient temperature. However, in most applications, some ripple current resulting from core loss exists. Under such conditions, Irms would need to be de-rated to keep the temperature rise down to 40°C. Also, the specified 40°C rise is normally achieved with no restrictions in airflow due to natural convection, which in most applications is not the case.

The two main issues with this area of inductor selection are calculating the core loss and the required de-rating of Irms to keep the temperature rise down to an acceptable level. Different inductor manufactures have different ways of expressing core loss — some give no details at all, while others provide the information required to calculate dissipation. However, one of the more practical approaches comes from the Coiltronics® catalog, which shows the maximum allowable percentage loss from Irms against applied volt-seconds at various frequencies. Since the volt-seconds product is proportional to core loss, it can be easily determined by using these curves.

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think you just take your maximum pulse on-time and multiply it by the PWM ON voltage to get Volt-Seconds.

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