2N6766 FET Derating Curve

2N6766 Design Issues

2N6766 FET [TO-204AE package, Modified TO-3]. Hermetic MOSFET
IRF250: 200V Single N-Channel Hi-Rel MOSFET in a TO-204AE package
2N6766 Applications: switching power supplies, motor controls, inverters, choppers, audio amplifiers and high energy pulse circuits

2N6766 Derating Chart

2N6766 Maximum Drain Current vs Case Temperature

Use the chart above to derate the 2N6766 FET. The derating chart shows device's Drain current vs. case temperature, for both the 2N6766 and the 2N6766T1.
As an approximation use a linear derating factor of 1.2 W/⁰C beginning at 25⁰C.
Once the case temperature reaches 25⁰C the device must be derated up to a maximum case temperature of 150⁰C, insuring the junction temperature remains below the maximum permissible temperature.
The derating graph covers the 2N6766 with continuous current. The maximum Pulsed Drain Current [I_DM] is 120 amps, but is not accounted for in the graph.

2N6766 Maximum DC Ratings
Voltage Gate to Source V_GS = 20 volts dc
Voltage Drain to Source V_DS = 200 volts dc
Voltage Drain to Gate V_DG = 200 volts dc
Junction Temperature T_J = -55 to +150^oC
Storage Temperature T_STG = -55 to +150^oC

MIL-PRF-19500/543; Semiconductor Device, Transistor, Field Effect, N-Channel, Silicon, Repetitive Avalanche Types 2N6764, 2N6764T1, 2N6766, 2N6766T1, 2N6768, 2N6768T1, 2N6770, and 2N6770T1, JAN, JANTX, JANTXV, JANS, JANHC, and JANKC [TO-204AE; formerly the JEDEC TO-3 package designation]

FET Derating Guidelines, Transistor Derating Guidelines

TO-204 Flange mount. Is a type of package which provides a method of attaching a surface of the semiconductor device, the flange in this case, to a heat dissipater [a component that acts as a heat sink] to achieve thermal management of the case temperature. The Drain is electrically connected to the case [for this device]. The Junction to Case Thermal Resistance for the TO-3 case is 0.83 ⁰C/W.

Temperature considerations and Electronic Part Derating with increasing current.

The page detailing the TO-204 package covers the physical component package in more detail, but only deals with the aspects of the package and not the semiconductor embedded within the package.