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Method of Obtaining Thermal Resistance

IP.com Disclosure Number: IPCOM000090799D
Original Publication Date: 1969-Jul-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 2 page(s) / 38K

Publishing Venue

IBM

Related People

Cade, PE: AUTHOR

Abstract

A method of obtaining the thermal resistance, junction-to-case theta jc, of a transistor 10 includes the use of the equation theta jc = (T1-T2)/›Ie(Vcb2-Vcb1)|. T1 is a first ambient temperature of the case of the transistor under test with Vcb1 its collector-to-base voltage. T2 is a second ambient temperature of the transistor case with Vcb2 its collector-to-base voltage. Ie is a constant emitter current flowing through the transistor at both ambient temperatures. For the equation to be valid, the junction temperature has to be constant regardless of the ambient case temperatures. For a constant current through any semiconductor junction, the voltage drop is a monotonic function of the temperature.

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Method of Obtaining Thermal Resistance

A method of obtaining the thermal resistance, junction-to-case theta jc, of a transistor 10 includes the use of the equation theta jc = (T1-T2)/>Ie(Vcb2-Vcb1)|. T1 is a first ambient temperature of the case of the transistor under test with Vcb1 its collector-to-base voltage. T2 is a second ambient temperature of the transistor case with Vcb2 its collector-to-base voltage. Ie is a constant emitter current flowing through the transistor at both ambient temperatures. For the equation to be valid, the junction temperature has to be constant regardless of the ambient case temperatures. For a constant current through any semiconductor junction, the voltage drop is a monotonic function of the temperature. Therefore, the emitter-to-base voltage Ve is a measure of the temperature of the emitter-base junction which is effectively that of the collector- base junction.

Constant current supply 11 is adjusted to provide the constant operating emitter current Ie for NPN transistor 10. A high input impedance unity gain buffer amplifier 12 senses Ve without loading the circuit. The output Ve of amplifier 12 is compared to a positive reference voltage from source 13 equal to the magnitude of the desired emitter voltage. The difference between the actual Ve and the magnitude of the desired emitter voltage is fed into an inverting high-gain amplifier 14. Its output is fed via isolation resistor 15 to power amplifier 16 which drives the colle...