2N4238 Transistor Derating Curves

2N4238 Temperature-Power Derating Curve [2N4237, 2N4239]
NPN Low Power Transistor. Package, TO-39 [no other package styles].
2N3438 Applications; General Purpose Amplifiers [GP Transistor Amps]

How to Derate a 2N4238 Transistor, graphically
2N4238 Temperature-Power Derating Curve

Use the Graph to determine 2N4238 power dissipation based on operational ambient temperature surrounding the package. Attaching a heat sink to the metal can will allow the 2N4238 to operate at a higher temperature with out the need to reduce power consumption, or at a minimum reduce the junction temperature of the device. The graph provides maximum DC power dissipation in still air at the ambient temperature indicated. The 2N4238 is one of those transistors that has a thermal runaway curve that is almost on top of the maximum junction temperature line.
Additional Transistor Derating Curves for other NPN devices or PNP BJTs.
Temperature derating is a standard design practice for electrical engineers.
Derating a transistor insures that the junction temperature never exceeds its maximum rating, and that the device is operated below it maximum rating.

Refer to MIL-PRF-19500/581B; Silicon NPN Semiconductor Device, Transistor, Amplifier Types: 2N4237, 2N4238, and 2N4239 [links to the left].
Qualified to JAN, JANTX, and JANTXV programs.
The package is a through-hole TO-39 package, shown to the left.

2N4238 Maximum Ratings: at 250C
Collector Emitter Voltage = 60 volts dc [Vceo]
Collector Base Voltage = 80 volts dc [Vcbo]
Emitter Base Voltage = 6 volts dc [Vebo]
Power Dissipation = 1 Watt, 250C [Ta, ambient temperature]
Power Dissipation = 6.0 Watt, 250C [Tc, case temperature]
Operating Junction Temperature = -65 to +2000C [Tj]
Storage Temperature = -65 to +2000C [Tstg]

Complementary Device; 2N4235 PNP Transistor

2N4238 Derating Curve Notes:
The top curve is thermal runaway loci and cannot be used as a derate design curve since it exceeds the maximum ratings for this part [dashed line]. Operating under this curve using these mounting conditions assures the device will not have a thermal runaway. This is the true inverse of the worst case thermal resistance value extrapolated out to the thermal runaway point.

The curve to the left represents the maximum possible operating curve without exceeding the permissible 200 degrees C junction temperature. The best design uses the curve that terminates at 100C. However; pick the desired junction temperature [under 200C] draw a line parallel to one of the curse to determine the operating region of the device [2N4238]. Take note that the thermal runaway line is only a few degrees different than the maximum junction temperature. So operating the 2N4238 at or near the maximum operating point runs the risk that the device could go into thermal runaway. Although a transistor should never be operated near its maximum junction temperature, caution should be used if the requirement comes up in a design.

Board Layout Hint; Mount the TO-39 close to the surface of the printed wiring board, so the 2N4238 terminals are as short as possible [leading to the most efficient heat transfer through the component leads]. Route traces to a copper island for improved heat dissipation if possible, keeping the board traces as short as possible. Don't place the 2N4238 next to another heat generating component [the curve doesn't account for that].

Department of Defense Specifications
MIL-PRF-19500 - Semiconductor Devices, General Specification.
This specification may no longer be in service, or actively up-dated.

Department of Defense Standards
MIL-STD-750 - Test Methods for Semiconductor Devices.
This DOD standard may no longer be in service, or actively up-dated.

TO-39 Transistor Case
TO-39 metal Can Transistor

Pins; Lead 1 Emitter, 2 Base, 3 Collector
Lead 3 is connected to case
TO-39 Dimensional Data

Related Transistors
2N4237 Derating
2N4239 Derating
NPN Transistor Derating

Component Vendors
Transistor Manufacturers
Heat Sink Manufacturers
Fan Manufacturers

How to Derate Components;
Guideline to Derating Devices

How to derate a transistor.
How to derate a 2N4238.
Why derate a transistor.

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Modified 1/21/12
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