The allowable range of drain current is not constant to temperature.
It depends on the ambient temperature (package temperature) and heat dissipation conditions (thermal resistance).
For an allowable range of each of our product, the thermal resistance of the package and the maximum rated channel temperature shown in the data sheet are greatly related.
The drain current ID / IDP described in the data sheet is specified under ideal heat dissipation conditions with an ambient temperature of 25°C. ID is the maximum rating of the forward drain current when DC is applied, and IDP is the maximum rating of the forward drain current that can be applied with the specified pulse width.
ID and IDP are specified so that the channel temperature does not exceed Tch(max) when the MOSFET is operating in the on region (linear region) under specified conditions.
The main cause of the temperature rise of the MOSFET in the on-region is the power loss PRloss caused by the on-resistance RDS(ON) between drain and source.
As an example, find the forward drain current ID when DC current is applied.
The loss PRloss due to the on-resistance RDS(ON) is given by the following equation.
PRloss = RDS(ON) x ID2
Assuming that the thermal resistance of the product is Rth(ch-c), the temperature rise ΔT due to the heat generated by the loss is given by the following equation.
ΔT = PRloss ｘ Rth(ch-c)
The channel temperature is obtained by adding ΔT to the case temperature TC during MOSFET operation, so the following equation must be satisfied.
Tc+ ΔT < Tch(max)
Solving the above formula for the ID gives: IDP can be considered in the same way.
Tch(max) : Maximum channel temperature
TC : Case temperature (25°C)
Rth(ch-c) : Steady-state thermal resistance
rth(ch-c)(t) : Transient thermal resistance
RDS(ON)(max) : Maximum drain-source on-state resistance at the maximum channel temperature
The following documents also contain related information:
In the actual design, please design with a margin for the calculated value.
Reliability will change significantly depending on the degree of derating. For information on derating, please refer to the following document.
Document title “Concepts of Derating”