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# Talk more about chip temperature

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Industry news
Source:
2019/01/15 12:18
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[Abstract]:
Even after years of fear in the IC world, many people expect to encounter the following problems with the temperature of chips:
(1) high and low temperature test. The surface temperature of the chip e
Even after years of fear in the IC world, many people expect to encounter the following problems with the temperature of chips:
(1) high and low temperature test. The surface temperature of the chip exceeds the operating temperature required in the specification of the chip. Can it be used again?
(2) consult the FAE of the manufacturer. What does the surface temperature, air temperature and junction temperature of the chip mean?
(3) often PK with the testing department, the r&d department believes that this temperature is not a problem, and the testing department thinks that there is a problem, but neither of them has any basis.
(4) the specification often gives the maximum power at room temperature. How to convert it into the maximum power at high temperature?
Here's what you can learn:
(1) given the actual allowable operating conditions (ambient temperature and power) of the chip, it can be calculated whether the chip exceeds the junction temperature requirement.
(2) given the current maximum temperature requirement, what is the maximum power that can be achieved? Or whether the actual power is exceeded.
(3) what is the maximum power at the specified temperature for power devices with or without radiators.
Terms related to chip temperature
The temperature associated with chips is complicated because there are so many technical terms, let's take a look at these terms.
Temperature in four areas of the chip: core, packaging surface, air perimeter, PCB board:
(1) TJ (Die Junction Temp) : silicon core temperature of the chip, which is the temperature of the core inside the chip. As can be seen from the abbreviation, this is a dead temperature, which cannot be crossed by the designer.
(2) Ta (Ambient Air Temp) : Air temperature around the chip. This is generally used as the calculation parameter for small power devices with small radiators. Figure 1: schematic diagram of Tj and Ta

(3) Tc (Package Case Temp) : surface temperature of chip Package. High power devices with radiators usually take this as a technical parameter.
(4) Ambient board Temp: the surface temperature of PCB installed on the chip. Figure 2: Tc and Tb schematic diagram

The most important concept: thermal resistance
When the heat inside the object in the form of heat conduction, the resistance is called the thermal resistance of thermal conductivity, heat resistance in the heat flow path, reflect the heat transfer between the medium or medium size, show that the size 1 W heat caused by the temperature rise, the unit for ℃ / W.
Three important thermal resistances:
(1) thermal resistance Rja: the total thermal resistance from junction of chip to ambient air,
(2) thermal resistance Rjc: the thermal resistance between the heat junction of the chip and the package enclosure, which is the most commonly used and useful thermal resistance.
(3) thermal resistance Rjb: chip
The thermal resistance between the junction and the PCB.
As can be clearly seen from the thermal resistance # model below, the heating path of the chip core is: core - encapsulated surface - PCB - air. Figure 3: model of thermal resistance

Calculation formula of thermal resistance
After knowing so many terms and theories above, our most important purpose is: under the power condition of our current design, whether the current measured chip packaging temperature meets the requirement of maximum junction temperature. Readers must read this sentence carefully, which has three meanings:
(1) current design power: because our design is usually not under the maximum power, but with a reduction. For example, a NMOS maximum can be 3A, and the actual maximum power is 1A. It makes sense to calculate the temperature of power consumption according to the current 1A.
(2) chip packaging temperature Tc: since there are many methods to test chip temperature, Tj testing of core temperature is basically impossible. The easiest and simplest way is to directly test the temperature of chip packaging surface or air, and the path to calculate thermal resistance is also the simplest.
(3) meet the maximum junction temperature requirement: all of our calculation purposes, is the power of the current, the current surface temperature, the conversion into chips inside the core temperature of Tj, general manufacturers can give Tj, if not to Tj, we can according to the silicon tube estimate maximum junction temperature 150 ℃.
Important basic formulas for calculating thermal resistance:
Tc (Max) = tj-p *Rjc; Among them: -- -- -- -- -- -- -- -- -- (1)
Tc (Max) : maximum temperature allowed on the chip packaging surface;
Tj: junction temperature, the maximum temperature allowed by the chip core;
P: actual power consumption of the chip;
Rjc: the core of a chip - encapsulates the thermal resistance of the surface.
The basic calculation formula is provided in the formula wc, which is based on the premise that the radiator is large enough and in good contact.
Tcmax = Tj - P * (Rjc + + Rsa Rcs). -- -- -- -- -- -- -- -- - (2)
Tc (Max) : maximum temperature allowed on the chip packaging surface;
Tj: junction temperature, the maximum temperature allowed by the chip core;
P: actual power consumption of the chip;
Rjc: core of a chip - encapsulates the thermal resistance of the surface.
Rcs: represents the thermal resistance of the shell, the radiator
Rsa: represents the thermal resistance of the radiator
Examples of calculating thermal resistance
To make it easier for readers to understand, let's take 2N5551 as an example. Figure 4:2N5551 thermal resistance parameters

There are several key parameters in the specification:
25 degrees (Tc) when the power is 1.5 W (P), power derating is 12 mw / ℃, Rjc is 83.3 ℃ / W, RJA is 200 ℃ / W, junction temperature is 150 ℃, and behind these parameters calculation is used.
Hypothesis: the actual use power triode is 1.2 W, the actual test to the shell temperature was 60 ℃, excessive temperature, please?
According to the formula Tc (Max) = tj-p *Rjc, Tj=60+1.2*83.3=159.96, the core temperature of the chip exceeds the junction temperature of the chip, and the design is unreasonable. We can also from another parameter of manufacturer, power derating is 12 mw / ℃, 60 ℃ the derating is: 60-25) x 0.012 = 0.012 W, 1.5 W to 0.42 W = 0.42 W, the largest power requirement chip can't more than 1.08 W, 1.2 W is overweight.
Calculation and classification of thermal resistance
The above basic calculation methods of thermal resistance are popular. In practical application, there are two kinds of calculation methods approved by the industry: separating the semiconductor into large power and small power.
Calculation of thermal resistance of high-power semiconductor:
Power rating of power device generally refers to the power with the radiator, radiator is large enough and good heat dissipation, can think the thermal resistance between the surface to the environment is 0, so ideal shell temperature is equal to the ambient temperature. Power devices with special process, so its maximum allowable junction temperature can reach 175 degrees. However, for the sake of safety, we can calculate uniformly at 150 degrees (the specifications are subject to manufacturer's data). Applicable formula: Tc = tj-p *Rjc. When designing, the maximum value of Tj is 150, and Rjc is known.
Calculation of thermal resistance of low-power semiconductor:
For example, small transistor, IC, is generally used without radiator. So you have to think about the thermal resistance between the shell and the air. Rja, the thermal resistance between the junction and the environment, is given in the general manufacturer's specification (Rja=Rjc+Rca). The formula for low power semiconductor devices is Tc = tj-p *Rja. Rja: the thermal resistance between the junction and the environment. Usually manufacturers of low-power semiconductor devices will give this parameter in the specification. The Rja manufacturer of 2N5551 gives a value of 200 degrees /W. Given that the maximum junction temperature is 150 degrees, the allowable power consumption when the shell temperature is 25 degrees can be substituted into Tc = tj-p *Rja to get 25= 150-p *200 and P=0.625W. In fact, the specification is 0.625W. Since 2N5551 will not be used with radiators, the power of 2N5551 is 0.625W instead of 1.5w. It is also important to note that the power rating and Rja data of the sot-23 encapsulated transistors are measured when welded to the specified pad (which has a certain cooling function).
V. summary of the application of thermal resistance
The application of the calculation of thermal resistance can be generally summarized into the following scenarios:
(1) the junction temperature Tj, the thermal resistance Rja and the actual operating power are known, and the actual surface temperature of the chip is determined to be over the standard
According to the formula Tc = tj-p *Rja, the maximum allowable shell temperature under the current power condition can be calculated.
(2) assumes that the factory did not give a junction temperature Tj, give only the normal temperature 25 ℃ maximum power, thermal resistance Rjc
Junction temperature Tj generally in accordance with the silicon tube junction temperature 150 ℃, can use formula Rjc = (Tj - Tc)/P, such as 2 n5551 triode, 25 ℃ maximum power of 1.5 W, the Rjc = (150-25) / 1.5 = 1.5 ℃ / W
(3) relation between environmental derating coefficient and thermal resistance
Derating factor is actually the reciprocal of the corresponding thermal resistance, such as 2 n5551 in Tc = 25 ℃ when the rated power is 0.625 W, Rjc = 83.3 ℃ / W, its shell temperature drop coefficient Kc forehead = 1/83.3 = 0.012 W / ℃
(4) the relationship between environmental derating coefficient and design derating;
The environmental derating coefficient mentioned above is the coefficient calculated by temperature rise. The actual design derating amount shall be used with the design specifications of each company, and the derating amount shall be 0.5-0.8.
(5) how to calculate the maximum allowable power of high-power devices without radiators
High-power devices without radiators can only use the calculation formula of low-power devices: Tc = tj-p *Rja. The deformation is: P=(tj-ta)/Rja. For example: BU406 rated power for 60 w (Tc = 25 ℃). Rja is 70 ℃ / W, the biggest Tj junction temperature is 150 ℃. 25 ℃ when the maximum allowed power for: P = (150-25) / 70 = 1.78 W, visible high-power devices not fin, the same temperature, the maximum allowed power directly from the reduced 60 W to 1.78 W.
(6) relation between maximum storage temperature and junction temperature
Maximum allows storage temperature, the power P of course is 0, so the formula into Tcmax = Tjmax * Rjc 0, namely Tcmax = Tjmax, namely the storage temperature is equal to the junction temperature, but the actual specification book, generally, for example, consider the long-term storage of reliability, the storage temperature has a great deal of derating, knot WenKeDa 110 ℃, for example, require the storage temperature to 80 ℃.
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