The allowable range of drain current is not constant with respect to temperature, and depends on the ambient temperature (package temperature) and heat dissipation condition (thermal resistance).
The thermal resistance of the package and the maximum channel temperature listed in the datasheet are largely related to the allowable range for each product. In addition, there may be limitations due to the current capability of the package and the safe operation area.
The drain current ID・IDP listed in the datasheet are specified under ideal heat dissipation conditions of 25°C ambient. ID indicates the maximum rating of the forward-drain current when DC is applied. IDP is rated for the forward drainage current at the specified pulse width.
These currents are set so that the channel-temperature does not exceed Tch (max) when MOSFET is operating in the on-region (linear region) under specified conditions.
PRloss of power dissipation due to the drain-to-source on-resistance, RDS(ON) is the major factor causing MOSFET to rise in the on-region.
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.
The following documents have relevant information: Use this as a reference.