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# Digital Thermal Resistance Test Circuit

IP.com Disclosure Number: IPCOM000079237D
Original Publication Date: 1973-Jun-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 3 page(s) / 32K

IBM

## Abstract

This test method and circuit will perform the thermal resistance measurement on power rectifier and transistors, using digital techniques. The readout is a direct digital readout in degree C/Watt.

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Digital Thermal Resistance Test Circuit

This test method and circuit will perform the thermal resistance measurement on power rectifier and transistors, using digital techniques. The readout is a direct digital readout in degree C/Watt.

Thermal resistance is the temperature rise per unit of power dissipation of a junction above the case under conditions of equilibrium.

Theta (j-c) or Rt(j-c), normally just called Rt, can also be defined by: Rt = T(j) - T(c) over delta P = delta T(j) - delta T(c) over delta P. Where T(j) = Junction temperature in degrees C. T(c) = Case temperature in degrees C. degrees P = Change in power dissipation. Rt = degrees C/Watt. Rt can be rewritten as follows for rectifiers: Where SLOPE = V(AC3) - V(AC4) over T(C4) - T(C3) = delta V(AC) over delta T(J) = approximate -2mV/ degrees C. V(AC1) = Voltage measured a long time after the heat cycle (1-20 sec.) and at a sense current (I(AS) approx. 100mA). heat cycle. V(AC2) = Voltage measured at a sense current (I(AS)) and at a fixed temperature (T(C3)). V(AC4) = Same as V(AC3) except measure at T(C4). T(C1) = Case temperature measured at the same time as V(AC1). T(C2) = Case temperature measured at the same time as V(AC2). I(A) = Heating current. V(AC) = Heating voltage (anode-cathode).

As shown in the figure, the above parametera are measured by the analog section 10 of the tester and the digital data is stored in a memory 20, until all data is collected; at this time the calculator section 30 performs the six calculations to arrive at the Rt reading in degrees C/Watt.

It is only practical to include all these terms in a test system, as the one described below, Which uses a digital calculator rather than an analog calculator, or where some parameters such as slope, I(...