Thermal resistance refers to when there isquantity of heatThe ratio between the temperature difference at both ends of the object and the power of the heat source when uploading the object.The unit is Kelvin per watt (K/W) or Celsius per watt (℃/W).
When heat flows through the interface between two solids in contact, the interface itself shows obvious thermal resistance to the heat flow, which is calledContact thermal resistance。[1]
When there is heat transmitted on the object, the ratio between the temperature difference at both ends of the object and the power of the heat source, in Kelvin per watt (K/W) or Celsius per watt (℃/W), that is
In the above formula,
Is the temperature at one end of the object
Is the temperature at the other end of the object and
Is the power of the heat source.
Thermal resistance diagram
When the heat inside the objectheat conductionThe resistance encountered is calledheat conductionThermal resistance.For the flat plate with the same cross-sectional area where the heat flow passes, the thermal resistance of heat conduction is L/(k * A).Where L is the thickness of the plate, A is the cross-sectional area of the plate perpendicular to the direction of heat flow, and k is theThermal conductivity。
stayConvection heat transferIn the process, the thermal resistance between the solid wall and the fluid is called convective heat transfer resistance, 1/(hA).Where h isConvective heat transfer coefficient, A is the heat exchange area.Two objects with different temperaturesRadiation heat transferThe thermal resistance of is called radiation thermal resistance.If both objects are black bodies (see black bodies and gray bodies), and the absorption of heat by the gas between the two objects is ignored, the radiation thermal resistance is 1/(AoneF1-2）Or 1/(AtwoF2-1）。Where A1 and A2 are the surface areas radiated by two objects, F1-2And F2-1byRadiation angle coefficient。
When heat flows through the interface between two solids in contact, the interface itself shows obvious thermal resistance to the heat flow, which is calledContact thermal resistance。The main reason for the contact thermal resistance is that the actual area of direct contact between any two objects that appear to be in good contact is only a part of the interface (see figure), and the rest are gaps.Heat dependentheat conductionandthermal radiationAnd their heat transfer capacity is far less than that of ordinary solid materials.When the contact thermal resistance makes the heat flow flow through the interface, the temperature T along the heat flow direction suddenly drops, which is a phenomenon that needs to be avoided as far as possible in engineering applications.The measures to reduce the contact thermal resistance are: ① increase the pressure on the contact surface of two objects to deform the protruding part on the object interface, thus reducing the gap and increasing the contact surface. ②There is a high level of paint on the interface between two objectsheat conductionThe ability of gelatinous objects - heat-conducting grease.
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The resistance encountered by the heat on the heat flow path reflects the heat transfer capacity of the medium or between the mediums, indicating that the resistance caused by 1W heattemperature riseSize in ℃/W or K/W.The temperature rise on the heat transfer path can be obtained by multiplying the thermal power consumption by the thermal resistance.A simple analogy can be used to explain the meaning of thermal resistance. Heat exchange is equivalent to current, temperature difference is equivalent to voltage, and thermal resistance is equivalent to resistance.
Thermal resistance Rja:Total thermal resistance, multiplied by its calorific value, the device temperature rise is obtained.
Thermal resistance Rjc: the thermal resistance between the heat source junction of the chip and the packaging shell, which is multiplied by the heat generation to obtain the temperature difference between the junction and the shell.
Thermal resistance Rjb: the thermal resistance between the junction of the chip and the PCB board, multiplied by the heat dissipation through the single board heat conduction to obtain the temperature difference between the junction and the single board.
Thermal resistance calculation
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Thermal resistance formula
In general, in the formula of thermal resistance, the formula Tcmax=Tj - P * Rjc is valid only if the radiator is large enough and the contact is good enough. Otherwise, it should be written as Tcmax=Tj - P * (Rjc+Rcs+Rsa)Rjc represents the thermal resistance from the chip interior to the shell, Rcs represents the thermal resistance from the shell to the heat sink, and Rsa represents the thermal resistance of the heat sink. When there is no heat sink, Tcmax=Tj - P * (Rjc+Rca).Rca represents the thermal resistance from housing to air. The general service conditions are approximated by the formula Tc=Tj - P * Rjc.The manufacturer's specifications will generally give parameters such as Rjc and P.Generally, P is the power consumption at 25 ℃. When the temperature is greater than 25 ℃, there will be a derating indicator.
example
For example, the power at 25 ℃ (Tc) is 1.5W (P) and Rjc is 83.3 ℃/W according to the 2N5551 specification of the first and third stage tubes.The substitution formula is: 25=Tj-1.5 * 83.3, from which Tj can be deduced to be 150 degrees.The maximum temperature of the chip is generally constant.So Tc=150 Ptc * 83.3, where Ptc represents the power consumption when the temperature is Tc. Assuming the power consumption of the tube is 1W, then Tc=150-1 * 83.3=66.7 degrees.Note that the power of this tube at 25 ℃ (Tc) is 1.5W. If the shell temperature is higher than 25 ℃, the power will be derated for use. The derating given in the specification is 12mW/℃ (0.012W/℃).We can use the formula to verify this conclusion. If the temperature is Tc, then the power derating is 0.012 * (Tc-25).Then the maximum total power consumption is 1.5-0.012 * (Tc-25).Substitute the conditions at this time into the formula to get: Tc=150 - (1.5-0.012 * (Tc-25)) × 83.3, and the formula is validIn general, Tj cannot be measured. Ttj can be estimated by measuring Tc. The formula is Tj=Tc+P * Rjc.
Taking 2N5551 as an example, assuming that the actual power is 1.2W and the measured shell temperature is 60 degrees, then Tj=60+1.2 * 83.3=159.96 has exceeded theMaximum junction temperature150 degrees!According to the principle of derating 0.012W/degree, the derating at 60 degrees is (60-25) × 0.012=0.42W, 1.5-0.42=1.08W. That is to say, the power must be less than 1.08W when the shell temperature is 60 degrees, otherwise the maximum junction temperature will be exceeded. Assuming that the specification does not give the value of Rjc, it can be calculated as follows: Rjc=(Tj Tc)/P, if Tj data is not given,Then the maximum Tj of general silicon tube is 150 to 175 degrees. Take 2N5551 as an example.It is known that the power at 25 degrees is 1.5W, assuming Tj is 150, then substitute the above formula: Rjc=(150-25)/1.5=83.3 If Tj is 175 degrees, then Rjc=(175-25)/1.5=96.6, so the Rjc of this device is between 83.3 and 96.6. If the manufacturer does not give the power at 25 degrees, then you can add a certain amount of power to make its shell temperature reach the maximum allowable shell temperature,Then put the data into: Rjc=(Tjmax Tcmax)/P It is best to give Tj. When there is no Tj, the Tj of silicon tube is generally 150 degrees.
Supplementary notes
I would like to make some additional remarks.
Semiconductor devices can be divided into high-power devices and low-power devices.
1. The rated power of high-power devices generally refers to the power with a radiator. When the radiator is large enough and has good heat dissipation, the thermal resistance between its surface and the environment can be considered as 0. Therefore, in an ideal state, the shell temperature is equal to the ambient temperature. Because of the special process used for power devices, the maximum allowable junction temperature can reach 175 degrees.But for the sake of safety, it can be calculated as 150 degrees. The applicable formula: Tc=Tj - P * Rjc. When designing, the maximum value of Tj is 150, and Rjc is known. Assuming that the ambient temperature is also determined, according to the shell temperature that is equal to the ambient temperature, then the allowable P will be determined accordingly
2. SmallPower semiconductor device, such asSmall transistor, IC, generally without radiator.Therefore, the thermal resistance between the device housing and the air should be considered.Generally, the manufacturer's specifications will give Rja, that is, the thermal resistance between the junction and the environment(Rja=Rjc+Rca)。Taking the three-level tube 2N5551 as an example, its maximum power of 1.5W was obtained when its shell temperature was 25 degrees. Assuming that the ambient temperature was just 25 degrees, 1.5W power was consumed, and the shell temperature was also 25 degrees, the only possibility was that it could get enough heat dissipation!But generally, TO-92 sealed triodes like 2N5551 cannot be used with a heat sink.So at this time, the formula used for small power semiconductor devices is: Tc=Tj - P * Rja.Rja: thermal resistance between junction and environment. Generally, manufacturers of low-power semiconductor devices will give this parameter in their specifications.The Rja manufacturer of 2N5551 gave a value of 200 degrees/W.It is known that the maximum junction temperature is 150 degrees, so when the shell temperature is 25 degrees, the allowable power consumption can be substituted into Tc=Tj - P * Rja to get 25=150-P * 200, and P=0.625W.In fact, it is 0.625W in the specification. Because the 2N5551 will not be used with a radiator, we usually say that the power of the 2N5551 is 0.625W instead of 1.5W!It should also be noted that the rated power and Rja data of SOT-23 packaged transistors are measured when they are welded to the specified bonding pad (with certain heat dissipation function).
3. In addition, let me tell you a trick. In fact, the maximum allowable storage temperature in the general specification is also the maximum allowable junction temperature.The maximum allowable operating temperature is actually the maximum allowable shell temperature. At the maximum allowable storage temperature, the power P is of course 0, so the formula becomes Tcmax=Tjmax - 0 * Rjc, that is, Tcmax=Tjmax.Isn't it amazing!The maximum allowable operating temperature is 70 ℃ for general civil (commercial) grade and 80 ℃ for industrial grade. Common products use civil grade devices, and industrial grade devices are generally much more expensive.The calculation of the thermal path is based on the principle that the temperature difference between any two points is equal to the power of the device times the thermal resistance between the two points from the inside of the chip. This is a bit like Ohm's law.The voltage drop between any two points is equal to the current times the resistance between these two points.However, it should be noted that in the process of heat transmission, any medium and any medium have thermal resistance. Of course, when the thermal resistance is small, it can be ignored. For example, when the radiator area is large enough, it is close to the ambient temperature, and then it can be considered that the thermal resistance is 0. If the heat of the device itself causes the ambient temperature to rise, it means that its heat sink (if it has heat sink)Or the thermal resistance between the housing and the environment is relatively large!At this time, the simplest method is to directly use Tc=Tj - P * Rjc to calculate. Tc is the shell temperature and Rjc is the thermal resistance between the shells. If you change Tc to the surface temperature of the heat sink (if there is a heat sink), then the thermal resistance in the formula must also be the thermal resistance between the shells plus the heat sink itself between the shell and the heat sink!In addition, if your temperature point is based on the environment, think about the other heat paths involved.For example, the thermal resistance between the heat sink and the air inside the test chamber, and the thermal resistance between the air inside the chamber and the air outside the chamber are difficult to calculate.
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The resistance of microorganisms to heat is called heat resistance, which refers to the lethal time of microorganisms under certain conditions (mainly temperature).Relative thermal resistance refers to the ratio of the lethal time of a microorganism under a specific condition to that of another microorganism under the same condition.[2]
