Discussion on the concepts of chip heat dissipation and heat generation
This article mainly discusses the concepts of chip heat dissipation/heating, thermal resistance, temperature rise, and thermal design.
Chip heating and loss
The power loss of the chip, on the one hand, refers to the difference between the effective input power and output power, which is called dissipated power. This part of the loss will be converted into heat release. Heat generation is not a good thing, and it will reduce the reliability of components and equipment. It will seriously damage the chip.
Dissipation power, there will be this parameter in the SPEC of some chips, which refers to the maximum allowable power dissipation, power dissipation and heat are corresponding, the greater the allowable power dissipation, the corresponding junction temperature is also will be bigger.
On the other hand, chip power consumption refers to the amount of energy consumed by electrical equipment per unit time, and the unit is W, such as an air conditioner of 2000W and so on.
Thermal Resistance and Temperature Rise
We all know a saying: Snow does not cool and snow turns cold. This is a physical process. Snowfall is a process of desublimation and exotherm, and melting snow is a process of melting and absorbing heat. The temperature rise of the chip is relative to the ambient temperature (25°C), so the concept of thermal resistance has to be mentioned.
Thermal resistance refers to the ratio between the temperature difference at both ends of the object and the power of the heat source when heat is transmitted on the object, and the unit is °C/W or K/W. As shown in the figure below, when a chip is soldered on a PCB, there are three main heat dissipation paths for the chip, corresponding to three thermal resistances.
1. The thermal resistance from the inside of the chip to the shell and pins - the chip is fixed and cannot be changed.
2. The thermal resistance from the chip pins to the PCB board - determined by good soldering and PCB board.
3. The thermal resistance from the chip case to the air - determined by the heat sink and the peripheral space of the chip. Semiconductor chip thermal resistance parameters
Ta is the ambient temperature, Tc is the case surface temperature, and Tj is the junction temperature. Θja: Thermal resistance between junction temperature (Tj) and ambient temperature (Ta). Θjc: Thermal resistance between junction temperature (Tj) and case surface temperature (Tc). Θca: Thermal resistance between case surface temperature (Tc) and ambient temperature (Ta).
The calculation formula of thermal resistance is: Θja = (Tj-Ta)/Pd → Tj=Ta+Θja*Pd where Θja*Pd is the temperature rise, which can also be called the calorific value.
1. Under the condition of constant thermal resistance, the smaller the power consumption Pd is, the lower the temperature will be.
2. In the case of a certain power consumption, the smaller the thermal resistance, the better, and the smaller the thermal resistance, the better the heat dissipation.
Junction temperature calculation errors
Many people use this formula to calculate the junction temperature: Tj=Ta+Θja*Pd, which is stated in TI's documentation, but it is not accurate.
The general meaning is that Θja is a multi-variable function, which cannot reflect the real situation of the chip soldered on the PCB, and has a strong correlation with the design of the PCB and the size of the Chip/Pad. As these factors change, the value of Θja will also change. There is a big difference between chip manufacturers testing Θja and our actual usage, so it is used to calculate the junction temperature, and the error will be large.
The thermal resistance Θja has a strong correlation with these parameters
At the same time, using the formula Tj=Tc+Θjc*Pd to measure the temperature Tc of the chip shell with an infrared camera, and then calculating Tj is not very accurate. The Θja and Θjc given by the manufacturer may be more for us to evaluate the thermal performance of the chip and compare it with other chips.
In the parameters of some chips, there will be ΨJT and ΨJB. These two parameters are not real thermal resistance. The method used by chip manufacturers to test ΨJT and ΨJB is very close to the application environment of the actual device, so it can be used to estimate the junction temperature. It is also adopted by the industry, and it can be seen that these two parameters are smaller than Θja and Θjc, so under the same power consumption, the junction temperature calculated by Θja is higher than the actual temperature .
ΨJT refers to Junction to Top of Package, the parameter from junction to package shell, the calculation formula is Tj=Tc+ΨJT*Pd, Tc is the chip shell temperature. ΨJB, refers to Junction to Board, junction to PCB board parameters, the calculation formula is: Tj=Tb+ΨJB*Pd, Tb is the temperature of the PCB board.
ΨJT and ΨJB can be used to calculate the junction temperature
Thermal design
The thermal design is the same as the EMC problem, it is best to solve it in the early stage, otherwise the later rectification will be very troublesome. In the early stage of the design, the structure, PCB stacking, layout, decoration, etc. are considered, and the heat dissipation materials are considered in the later stage.
