Online proceedings - EDA Publishing Association

24-26 September 2008, Rome, Italylarge diode covers the active area of the integrated circuit.Reverse biasing this large diode we get a “dull” but robustand non-destructive way for powering and sensing.Fig. 5 CMOS inverter, schematicWe carried out measurements on an Intel Celeron CPUmounted on a dummy board and placed in still air chamber(Fig. 6) [1].Fig. 8 Cumulative structure functiongenerated from the measurementFig. 6 CPU with rotating fan, mounted on a dummyFig. 7 Thermal voltage signal on an Intel CPUFig. 7 shows a raw transient at 25 W power using the substratediode method. We did not fit into the specification ofthe base tester defined in the previous section, we used apower booster for larger current. The electric transient endsat 30 μs which already allows deep insight into the chip regionof the measurement arrangement.We measured the transient at different fan speeds (S1, S2,S3 in descending order). Converting the measured transientinto structure functions ([4][5]) we got the result of Fig. 8.The real outcome of the measurement is not that simplethat at higher fan speed we get better cooling. Rather, that atS1 we step into the air flow very near to the point ofR th =0.9 K/W and C th =57 Ws/K which corresponds to thevolume of the heat sink. We cannot expect further improvementof the cooling by elevating the fan speed.In terms of the CONSIDERATIONS section this measurementwas carried out under “C-e-2-i" conditions.We have a more sensible way of tracing the temperaturechanges in a processor while operating in a real system. Thepower step on a CPU can be initiated by an interrupt whichstarts the training of a specific unit in the CPU or a by a suddenclock frequency change. Most processors have a diodefor temperature sensing. Other locations can be measuredwith a simple alteration of the motherboard. Some enable ordisable inputs are at steady low or high level during normaloperation. Instead of putting a fixed voltage at these pointswe can bias the input. For example, signals to be groundedcan be biased by –50 μA instead. Then we shall experienceapproximately –0.5V at the input, also meaning logical 0 forthe CPU. This voltage is already temperature dependent andcan be used for sensing.Case study 3 :Measurement of a RAM moduleWe have chosen a commercially available memory module,plugged in an IBM PC compatible motherboard. Themeasurement was done in a one cubic feet JEDEC standardstill-air chamber (Fig. 9).Fig. 10 shows the available signals on a RAM chip. Allinputs have protection diodes suitable for sensing. The outputshave a circuitry similar to Fig. 5. The inherent reversediodes between the drain electrodes and their respective substrateare of large surface and can be used for both powerdrive and temperature sense purposes.We wanted to trace the temperature change of each chipseparately. Most signals are bus-like (address etc.) but alloutputs of all chips go separately to the edge connector ofthe module. 8 outputs belonging to one chip were tied togetherfor exciting large surface on the chip.©EDA Publishing/THERMINIC 2008 108ISBN: 978-2-35500-008-9