Using Chip Resistors
Usage exceeding the rated power

Determining use based on average power

The rated power is defined as the maximum power that can be applied to the product that, if exceeded even for a moment, will not be covered under the product’s warranty.

As such it is necessary to determine whether the resistor can be used by considering the maximum power that will be applied to the product on a regular basis. However, judging usability based on the average power may lead to destruction in the worst case, so caution is required.

The following example illustrates how the maximum power can have a different effect on a product even with the same average power.

Test Conditions:
 Tested under two conditions with different pulse width and peak power, but the same pulse energy (Joule heat).
 Condition ①: Small peak power with long pulse width
 Condition ②: Large peak power with narrow pulse width

Test Conditions

In the above two waveforms, although the peak power x pulse width = Joule heat [J] (colored areas in the table) and average power per second are the same, there is a difference in the actual damage (resistance value change) to the chip element.

Resistance Change Rate

Resistance Change Rate

Given the same Joule heat, a larger peak power will cause more damage

As you can see, even with the same average power per second, damage to the product largely depends on the peak power, so when the peak power is large and pulse width short such as in Condition ② the amount of damage is significantly increased.
Consequently, it is dangerous to apply power to the product that greatly exceeds the rated power, even for a short time.

So when using the product under conditions where high power is momentarily applied, it is necessary to consider the actual power value of the product. ROHM can provide pulse limit power curves (reference) that show the withstand capability of the product when a pulse exceeding the rated power is applied. And since the actual value of this pulse limit will very depending on the product size and series, please verify before use. Contact ROHM for more information.

The next page provides a brief explanation regarding the destruction of thick-film chip resistors due to surges.

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