(3) Calculation of junction temperature
A junction temperature can be calculated by the following equation after measuring the temperature of the
Tj = Tc + Rjc x P
Tj : Junction Temperature
Tc : IC case(Tab) Temperature
Rjc : Thermal resistance of between junction and IC case (Tab) (
P : Total IC power consumption
Measuring method of Tc
The thermo-couple is set in the tab of IC (heat sink) and temperature Tc of the IC case is measured.
The temperature of Tc has the time dependency, please measure the temperature after it is saturated.
How much to derate a unit from the maximum rating is an important issue to consider a reliable design.
Items to be considered in the stage of system design include the derating of voltage, current, power, load,
and electric stresses, along with the derating of temperature, humidity and other environmental conditions
and vibration, impact and other mechanical stresses.
Table 5-7 specifies the standard examples of derating to be considered a reliable designing. To consider
these derating items in the stage of equipment design is desirable from the point of reliability securement. If
any item is difficult to control within the standard, another means will be necessary, such as selecting a
device having higher maximum ratings. Please consult our sales representative in advance.
Table 5-7. Typical derating design standards
Junction temperature Tj
VS power supply voltage
Typical derating standards (example)
ECN30204, ECN30206, ECN30207
ECN30601, ECN30603, ECN30604