How to calculation Junction Temperature (From approximately Temperature)

Junction temperature (or channel temperature) deserve to be calculated from the ambient using the adhering to equation.

You are watching: What is tj, the temperature of the junction point?


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*Rth(j-a): thermal resistance that "junction-to-ambient" varies depending upon the form of circuit board. Because that reference, provided below is the Table of heat Resistance by Package"based on the usage of bromheads.tvs traditional circuit board.

*Rth(j-a) worth differs for each part number, but the values will be close if the parcel is the same.

**If the current intake is not secure and transforming time come time, climate the averaged values of usage current shall it is in assigned in the calculation formula to gain the approximate worth (See the "Usable or not Judgement Method".

Below is an example of the correlation between current consumption and the junction temperature once Rth (j-a) is 250 deg./W , the approximately temperature is 25 deg.


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The junction temperature rises in proportion to the existing consumption. The proportionality consistent for this is Rth(j-a).

Since Rth(j-a) is 250 deg./W, the junction temperature rises by 25 deg. Every each increment the 0.1W of current consumption. This means, the junction temperature becomes 150 deg. Once the current intake is 0.5W, and the graph in this case says that the current past 0.5W deserve to not be applied to TR.


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When the same present is applied, the junction temperature likewise rises together the approximately temperature walk up. This diminishes the applicable present subsequently. Best current intake is affected by heat resistance and also the ambient temperature as well.

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The preferably current intake decreases v the above ratio.


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The Derating curve over illustrates the ratio of existing attenuation by percentage, which have the right to be used to all packages. Because that example, in instance of the MPT3 parcel (SOT89), the best applicable power is 0.5W in ~ 25 deg. And the applicable existing is reducing in ~ a rate of 0.8%/deg. This method that the value goes down to 0.4W. That is 80% indigenous 100% the its initial worth (20% down), and then under to 0.2W i m sorry is 40% of its initial worth (60% down).


Transient heat Resistance

In the instances above, we discussed cases in i m sorry the current is used to a an equipment continuously. Next, us will discuss a instance in i beg your pardon the temperature rises by momentary existing application.


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The graph above shows the thermal resistance at short-term time (Transient heat Resistance), plot the pulse width on the X-axis and also Rth(j-a) is ~ above the Y-axis.

This graph shows us that the junction temperature rises as the existing application time persists, getting to a plateau state (called heat saturation) after 200 seconds.


We can use the calculation formula above to attain the junction temperature once the current is used momentarily together a single pulse.


Calculation an approach of Junction Temperature (From instance Temperature)

The junction temperature have the right to be calculated from the temperature that the case, as watched below. An instance would it is in to change Rth(j-a) in the formula through Rth(j-c).


* case temperature is measure up by the radiation thermometer, together the best temperature top top the surface ar of the package, where the marking is put..

* Please keep in mind that the case temperature differs considerably depending on the measure up method/point.

** The worth is taken into consideration an approximation once the applied current is not constant, moving from time come time through assigning the averaged consumption figure.

Since the Rth(j-c) value varies relying on the species of circuit plank and additionally on the warm dissipation conditions (which incorporate soldering state), the formula over potentially wouldn"t apply to her calculation since the the measured worths on RHOMs circuit board might not be the exact same as the measured values on her circuit board. Because that instance, the instance temperature might be lower in comparison even though the applied current is the same, when the circuit plank has an excellent heat dissipation characteristics.

An illustration below shows the Rth(j-c) becomes reduced as the collector land sample on the circuit plank gets smaller. (Collector soil area / thickness / products plus circuit plank material, size circuit broad will additionally bring different measurement outcomes on Rth(j-c).


The Rth(j-c) value can differ relying on the nature & conditions of the circuit board. It is daunting to select the right ar for measuring the situation temperature precisely. Since of this, that is not recommended to approximate the junction temperature native the situation temperature.


Junction-to-Case thermal Resistance Rth(j-c) - Details

In principle, Junction-to-Case heat resistance Rth(j-c) is an table of contents basically used for TO220 packaged (through hole) gadgets by soldering it come the heatsink. In this case, because Case-to-Heatsink is the heat-radiation path, the is possible to exactly calculate the junction temperature by measuring the case temperature at the suggest in the center of such path. In particular, if a heatsink with ideal heat dissipation power is offered (e.g. Unlimited heatsink), the warmth dissipation capacity is thought about to it is in limitless. That is taken because that granted that "Case temperature" = "Ambient temperature" , and Case temperature = 25 deg. (Tc = 25 deg.) is offered in the calculation formula.

(Thermal resistance of infinite heat-sink : Rth(c-a) = ; climate Rth(j-a) = Rth(j-c))


For surface-mount devices, the heat-radiation route is greatly the component of a circuit plank that"s just beneath the device; which can make it quite daunting to measure up the situation temperature since of the location. Even if the temperature on noting side of an equipment is measured, its section of heat-dissipation in the entire heat-dissipation is quite small. Therefore, the is not perfect to use this temperature in ~ the formula to calculate the junction temperature.


Nevertheless, because there are many requests from our customers about Rth(j-c) value for SMT devices, bromheads.tv sometimes offers the Rth(j-c) worth on the problems that the temperature is measured from the noting side of the device being placed on the abovementioned standard circuit board. Due to the fact that of this, the Rth(j-c) value need to be thought about as a reference.

If the machine is placed on the circuit plank in a different way from ours, the part of heat-dissipation in the entire heat radiation will be different, make it hard to figure out the junction temperature accurately.


Thermal Resistance of typical Packages (Reference Data)

The worths in adhering to data are not the guaranteed worths nor best / minimum values. Please treat these just as reference data.*

* Data listed here came from the results of measuring a certain production lot.

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* Rth ( j-a ) varies depending on the circuit board, the heat-dissipation problems involving soldering methods and the technique of temperature measurement.

PackageVMT3EMT3EMT5EMT6TUMT3
Circuit Board
*
*
*
*
*
FR4 dimensions(unit:mm)20×12×0.820×15×0.820×15×0.820×15×0.820×12×0.8
Rth(j-a)/Rth(ch-a)833℃ / W833℃ / W1042℃ / W1042℃ / W313℃ / W
NoteWhen steering 1-dieWhen steering 1-die
PackageTUMT6UMT3UMT5UMT6SMT3
Circuit Board
*
*
*
*
*
FR4 dimensions(unit:mm)15×20×0.820×12×0.820×15×0.815×20×0.820×12×0.8
Rth(j-a)/Rth(ch-a)313℃ / W625℃ / W1042℃ / W1042℃ / W625℃ / W
NoteWhen control 1-dieWhen steering 1-dieWhen steering 1-die
PackageSMT5SMT6TSMT3TSMT5TSMT6
Circuit Board
*
*
*
*
*
FR4 dimensions(unit:mm)20×15×0.820×15×0.830×15×0.820×15×0.820×15×0.8
Rth(j-a)/Rth(ch-a)625℃ / W625℃ / W250℃ / W250℃ / W250℃ / W
NoteWhen control 1-dieWhen driving 1-dieWhen driving 1-dieWhen driving 1-die
PackageSOP8MPT3CPT3SST3
*
Circuit Board
*
*
*
*
FR4 dimensions(unit:mm)20×20×0.812×20×0.812×30×0.820×12×0.8
Rth(j-a)/Rth(ch-a)160℃ / W250℃ / W125℃ / W625℃ / W
NoteWhen control 1-die