Junction temperature

The cold-junction temperature can be fixed by immersing the cold junctions into some known thermal environment an ice bath or a properly maintained water triple-point cell. A temperature-controlled oven at a temperature above ambient may be used. 

As one can see by the typical values of the junction-to-air thermal resistance, it doesn t require much power to result in very high junction temperatures. If the designer can possible mount the power package on any metal surface to increase the radiating surface area, it will only improve the junction temperature. 

A.6.3 Determine the Junction Temperature of a Rectifier with a Known Lead Temperature... 

Electrons (emA sec) 8500 Holes (emA sec) 400 Breakdown Electric Field (V/cm) 4x10 Dielctric constant 12.8 Maximum Junction Temperature (°C) 300... 

Many years later in Singapore, we were using a specially formulated thermally conductive glue to fix the overtemperature sensing thermistor smack on to the very plastic body of the TO-220 power transistor. We had empirically ascertained that in this way, the junction temperature and the adjacent temperature as seen by the thermistor were less than 10°C apart, even during an abnormal event. So if, for example, we wanted to have the transistor turned off just before it hit 150°C, we simply needed to set the trip temperature (of the thermistor-based circuit) at about 140°C. In that way, we could also be sure that we wouldn t encounter nuisance tripping on a particularly hot day, when the temperature inside the enclosure would also be much higher. 

BL TEMPKRATURF- Thermocouples are the most commonly used temperature sensing devices. The two dissimilar wires produce a millivolt signal that varies with the hot-junction temperature. Iron-constantan thermocouples are commonly used over the 0 to 1300°F temperature range. 

These results are highly encouraging as they demonstrate the high power capability of the SiC RF-BJT devices along with acceptable power gain and efficiency. From thermal simulations, it is estimated that the junction temperature may reach a peak value of 171 C for pulse widths of 250 jus and 6% duty cycle without any external cooling. Since SiC junctions can easily sustain a temperature of 250 C, there is room to increase the pulse width even further. 

Figure 6.9 shows an ordinary thermowell-and-thermocouple assembly. The thermocouple junction consists of two wires of different metals. When this junction of the wires is heated, a small electric current, proportional to the junction temperature, is produced. Different metal wires make up the three most common junctions J, H, and K. It is not uncommon for a thermocouple, regardless of the type of junction, to generate too low a temperature signal. 

Thermocouples consist of two dissimilar electrical conductors which are joined to form a measuring junction, with the free ends of the wires constituting the reference junction. When a temperature difference exists between the measuring and reference junctions, an emf is produced between the free ends of the device. This emf, which is a function of the temperature difference, can be used to determine the temperature at the measuring junction if the reference junction temperature is known. A schematic of a typical thermocouple circuit is shown in Fig. 9.12. 

K. Pleschka and R. Gerstberger, eds., Integrative and cellular aspects of autonomic junctions temperature and osmoregulation, John Libbey Eurotext, Paris, 1994,... 

Figure 2.1 Generation of thermo-electromotive force in a closed electric circuit made of two different metals 7 and 2. T and T2 are the junction temperatures.

The anhydride hardened epoxies generally have better dielectric loss properties above the Tg than do the novolac epoxies. Consequently, a higher device Junction temperature can be tolerated because of less leakage at the plastic/chip interface and because the required heat dissipation can be obtained via the filler. This is, unfortunately, obtained at the expense of moisture resistance because the moisture resistance of the anhydride-hardened epoxies is not as good as the novolac epoxies. 

Higher thermal stability so that the higher Junction temperatures of today s and tomorrow s devices will not be limited by the thermal degradation of the plastic encapsulant. 

The major temperature drop occurs across the distance X2, indicating that this material provides the major thermal resistance. (In the case of heavily lagged, thin metal walls, the temperature drop and thermal resistance of the metal are so small that they can be ignored). Rearrangement of this equation and the elimination of the junction temperature gives Eq. (24). 

If a couple is calibrated with a cold-junction temperature of <5o C. and is used with a cold-junction temperature of the true temperature of the hot junction is obtained by adding to the observed temperature the value it a — to) K where K is a factor depending upon the particular couple employed and upon the temperature of the hot junction. The factor K varies from 1.5 to 0.3, but for rough work may be assumed 1.0 for base-metal couples and 0.5 for rare-metal couples. The following table gives the cold-junction factors for several different types of couple. 

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