[introduction] transient state hot impedance measures parts of an apparatus to be brought to bear on with Yu Heng the expression when pulse power, it decided parts of an apparatus is in low the expressional kind that takes sky comparing and load of low frequency pulse to fall, because this is very important.

IC is sealed of a lot of hot index, for example θ_JA With Ψ_JT. These parameter make the knot lukewarm estimation below stable state becomes very simple. The relevant and basic theory that the article will discuss behavior of hot transient state and hot impedance.

Heat up parameter overview

The heat that holds chip to enclose is characteristic by parameter θ_JA, Ψ_JT With Ψ_JB Token. θ_JA It is a knot to environmental thermal resistance (it is an unit with ° C/W) , it is systematic level parameter, depending on greatly is subordination sex, if install the PCB design of this parts of an apparatus and layout. Among them, circuit board is regarded as to solder to bring on-line radiator to parts of an apparatus. Conduct heat to natural convection and character, the quantity of heat of 90% above by circuit board send out, is not from enclose the surface to send out. θ_JA Can adopt formula (1) will calculate:

Among them, T_J For the knot lukewarm (° C) , T_A For environmental temperature (° C) , P_D Those who be parts of an apparatus medicinal powder quantity of heat (W) .

Ψ_JT It is token T_J With enclose the characteristic parameter that temperature changes between coping temperature (it is an unit with ° C/W) . The quantity of heat that because be flowed to from chip,encloses coping is sealed, so Ψ_JT Not be true knot to coping thermal resistance, but the general power that circuit design personnel often assumes it is parts of an apparatus. Although this hypothesis is invalid, but Ψ_JT Still be an useful parameter, because its are characteristic very as similar as the applied environment that IC encloses. For example, a thinner apparel have lesser Ψ_JT Value.

But want to notice, Ψ_JT What can harden according to circuit compose sheds a condition compatibly is different and differ somewhat. Use formula (2) but estimation Ψ_JT:

And had Ψ_JB , the system designs staff with respect to what can get according to measuring circuit board temperature is calculated the knot of parts of an apparatus is lukewarm. Ψ_JB Index should be close to θ_JB, because of PCB already dissipation quantity of heat of major parts of an apparatus. T_J computation is formulary (3) as follows:

Among them, T_PCB It is adjacent enclose naked solder dish the circuit of place board temperature (° C) . Graph 1 what explaining is a knot to environmental thermal resistance.

Graph 1: The knot comes environmental thermal resistance

The resistor that heats up plane through reducing PCB to come loose can realize inferior θ_JA . In order to conduct to basically conduct heat method (this means convection refrigeration law to suffer be restricted) in application, the area checks the power source plane of PCB θ_BA the effect is most distinct.

Heat is characteristic

In the application such as electric machinery driver, width of high power pulse is restricted in a few or less than of hundreds of millisecond, this means design personnel to must value the influence of thermal capacitance. If thermal capacitance is enough big, it is OK the knot lukewarm control is in the rating limits of parts of an apparatus, although be put in tall dissipation peak value,also be such. Accordingly, measurable radiating tube esp. physics can raise the function of parts of an apparatus and dependability.

Caloric delivers have 3 kinds of kind: Conduct, convection and radiate.

Conduct

Conducting is a kind conduct heat importantly means, because final quantity of heat is,send out through exterior area. Through conducting, ability of quantity of heat distributes needs surface. Follow Fu Lixie law through conducting the heat that have to deliver, this law points out, through the thermal current of material the cross section of rate and material is accumulated and the difference in temperature of two end becomes material direct ratio; Contrary, the ply of thermal current and material becomes inverse ratio. Some material (for example copper) compare other data (for example FR4) heat conduction is faster. Express the 1 coefficient of thermal conductivity that showed different stuff (K) . These common material have apparently different coefficient of thermal conductivity.

Express 1: The conductivity of different material

Convection

Convection is from material quantity of heat the surface delivers the method in air. Temperature rise is the result that power dissipation creates, it and watch area and heat deliver coefficient (H) becomes inverse ratio. H is wind speed and between circuit board and ambient air the function of difference in temperature.

Radiation

Heat radiation includes to deliver quantity of heat through electromagnetic wave. Rate of its thermal current and watch area become direct ratio, with radiation component (for example circuit board, component) of temperature just become direct ratio 4 times.

Through conducting the semiconductor in undertaking the most comfortable Yu Gaogong leads application heating up delivering. The standard of the hot property that encloses as IC is described, θ_JA In the action in pulse application not quite, still meet those who cause redundant or high cost come loose even hot design.

But through combining thermal resistance and thermal capacitance, can undertake building a model to the complete heat impedance of parts of an apparatus.

Thermal capacitance (C_TH) it is the index that measures package to accumulate heat energy power, it is similar to capacitance to gather the kind of charge. To giving structural element, C_TH Depend on specific heat (C) , bulk (V) and density (D) . Its computation is formulary (4) is as follows (it is an unit with J/ ° C) :

The hot behavior of a specific application (include active parts of an apparatus or appliance, enclose, PCB and external environment) in electric region but analogy is unit of a string of RC, every unit has constant of time of a feature (τ ) . This invariable and usable formula (5) computation:

Graph the 2 electric models that simplify through revealed every unit how to affect the transient state thermal resistance that encloses parts of an apparatus to fight.

Graph 2: Simplifying equivalent heats up circuit

Pulse power is operated

Bear when power parts of an apparatus when pulse is laden, it can support dissipation of taller peak value power. Power seals apparel to have certain thermal capacity, although parts of an apparatus consumes overmuch power,this is meant, also won’t achieve critical T instantly_J. To intermittent operation, the limitation of power dissipation may be lengthened. Prolate time depends on the duration that operates cycle (also call pulse duration) the frequency that happens with the operation (also call take empty factor) .

If the graph is shown 3 times, parts of an apparatus once electrify, chip can begin to warm up instantly.

Graph 3: Chip warms up / cooling: Odd pulse

If power lasts dissipation, criterion generation of quantity of heat and abreaction between can achieve a balance, stabilize T thereby_J. Among them partial heat energy is stored by the thermal capacitance of parts of an apparatus. Stable condition criterion by reach its to heat up the thermal resistance decision related the environment with transistor.

Stop dissipation when power, parts of an apparatus can cool gradually, the rule that warm up and cools is identical (see a picture 3) . But, if power dissipation stops before transistor temperature is stable, criterion T_J the value that peak value general reachs under the place of continuous power dissipation of same standard (see a picture 3) .

If the 2nd pulse and the first pulse are identical, criterion parts of an apparatus is when end of the 2nd pulse, temperature of its peak value is met the peak value temperature when the first pulse ends prep above. Pulse repeats ceaselessly, reach a new stable value till temperature (see a picture 4) . Below these stable conditions, temperature of parts of an apparatus can issue wave motion on average.

Graph 4: Chip warms up / cooling: Repeat pulse

If the knot after a series of pulse is lukewarm exorbitant (for example T_J >125 ° C) , criterion the electric function of parts of an apparatus and life expectancy may drop. This kind of circumstance may happen in have low in taking the high power pulse that compares for nothing, although its are average power the dc rating under parts of an apparatus also is such.

Graph 5 showed a shorter only power pulse.

Graph 5: Shorter only power pulse

Increase as pulse duration, T_JA stable cost is approximated when pulse ends (see a picture 6) .

Graph 6: Longer only power pulse

Thermal resistance is fought (Z_TH(JA)) mirrorred in a limited time the temperature rise that power pulse brings. This parameter offerred a kind of simple method to come the written guarantee that estimation parts of an apparatus issues in condition of dissipation of transient state power is lukewarm.

Transient state hot impedance tends the thermal resistance that is equal to permanent power dissipation, can adopt formula (6) undertakes estimation:

Graph 7: Z of transient state impedance_TH(JA) With the relation of time

Follow a repetition rate decrescent, the knot cools completely between pulse gradually, accordingly every pulse can be handled alone.

Enclose to power, transient state fuel factor can be in about 0.1 to disappear inside 100 seconds. This when long depend on chip size, enclose type and dimension. In addition, PCB folds layer and layout to be affected to its very big also.

PCB is equivalent to a radiator, enclose for IC offerred the way in delivering quantity of heat effectively circuit board and environment of its photograph adjacent. Accordingly, the biggest turn the metallic trace area that encloses power source and ground connection to bring a foot to be in, can raise heat effectively to deliver.

T_A With P_D Not big to enclosed hot property influence. Inside this time, the effect that the power pulse with too long duration produces and successive load are similar.

Epilogue

Written guarantee lukewarm meeting affects the working life of parameter of a lot of jobs and parts of an apparatus. Designing the challenge with high power the biggest circuit is the demand that decides whether a parts of an apparatus can support relevant applying.

Thermal resistance of effective transient state suffers a variety of element effects, the air current all round the hot quality of parts of an apparatus of photogenic adjacent of the fever that includes parts of an apparatus or appliance of adjacent of Fu copper area and layout, photograph, PCB and parts of an apparatus. Want accurate estimation temperature rise, best method is hot impedance of the token in using circuit directly.

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