Thermal design of electronic products

The effective power output of electronic products is far less than the input power required for circuit operation. Most excess power is converted into heat and dissipated. Currently, most electronic products are seeking to reduce size and increase component density, which leads to heat concentration. Therefore, it is necessary to adopt reasonable thermal design measures to effectively dissipate heat and make the product work within the specified temperature range. Thermal design technology refers to the design technology that uses heat transfer conditions to control the temperature of all components in electronic products through cooling measures, so that they can stably work at the specified maximum temperature under product operating conditions.
1、 Purpose of thermal design of electronic products
Electronic products will generate different degrees of heat energy when they work, especially some components with large power consumption, such as transformers, high-power transistors, power electronic devices, large-scale integrated circuits, and resistors with large power loss. In fact, they are a heat source, which will raise the temperature of products. When the temperature changes, almost all materials will expand or shrink. This expansion or contraction will cause the problems of fitting between parts, sealing and internal stress. The local stress concentration caused by uneven temperature is harmful. The metal structure will produce stress under the heating or cooling cycle, which will lead to the destruction of metal due to fatigue. In addition, for electronic products, components have a certain operating temperature range. If the temperature limit is exceeded, the working state of electronic products will be changed, the service life will be shortened, and even damaged, leading to the unstable and reliable operation of electronic products.https://store.stoneitech.com/
The main purpose of thermal design for electronic products is to reduce the operating temperature of the product, control the temperature of all components in the electronic product, make it work normally under the working environment temperature that does not exceed the specified maximum allowable temperature, avoid failures caused by high temperatures, and improve product reliability.
2、 Introduction to Cooling System of Electronic Products
The three basic modes of heat transfer are conduction, convection, and radiation. The corresponding heat dissipation methods include conduction, convection, and radiation.
A typical heat dissipation system is described as follows:
(1) Natural cooling system
The natural heat dissipation system refers to the natural emission of heat generated by electronic products into the surrounding air through conduction, convection, and radiation (the ambient temperature is slightly higher), and then through air conditioning and other equipment to reduce the ambient temperature to achieve the purpose of heat dissipation. The design principle of this type of heat dissipation system is to minimize heat transfer resistance, increase the convection pipes and heat exchange area within the product, and increase the heat dissipation area of the product surface. Natural cooling is the simplest and most economical cooling method, but it has a small amount of heat dissipation and is generally used for products with low heat flux.
(2) Forced air cooling system
Forced air cooling heat dissipation system refers to a heat dissipation method that uses equipment such as fans to blow cold air to the surface or interior of a product, thereby quickly removing heat. Forced air cooling is divided into ventilation cooling and blowing cooling according to the working mode of the fan. When the heat source of the equipment is evenly distributed, exhaust cooling is used, while uneven heat sources are cooled by blowing. According to the heat output of the product, there are special requirements for the selected fans and their layout (such as flow, pressure, noise, etc.).
According to the direction of air flowing through the heating element, forced air cooling can also be divided into horizontal ventilation cooling, longitudinal ventilation cooling, and longitudinal ventilation cooling. Horizontal ventilation cooling refers to the flow of cold air through static pressure air ducts to components or radiators that require heat dissipation. After heat exchange, hot air is discharged from the other side of the equipment. Longitudinal ventilation cooling is used for parallel mounted printed board assemblies. After air exchange, the hot air is discharged from the other side of the product. Longitudinal ventilation cooling is commonly used for vertically mounted printed board assemblies, where air enters the product from the bottom and hot air is discharged from the top.
When designing a cooling system, it is necessary to arrange heating elements reasonably. Place components with low heat and temperature resistance upstream of the airflow, and then arrange them according to their heat resistance from low to high. This cooling method is mostly used for equipment with a small heat output.
Both natural cooling and forced air cooling are direct cooling methods. Compared to indirect cooling methods, these cooling methods have simple structure, simple design, and low cost, and are mostly used for products with high heat flux density.
(3) Air cooled cooling plate heat dissipation system
The cooling medium of an air-cooled heat sink cooling system is air, and the system is generally composed of a cover plate, fins, bottom plate, and packaging. The fin is the main part of the cold plate and the basic part of the diffusion surface. The fin material is generally aluminum or copper. In airborne products, due to strict quality requirements, fins are generally made of aluminum. There are many different structural parameters. Fin shape, rib spacing, rib height, and rib thickness can be selected based on the cooling working environment (temperature, air pressure, humidity, pollution, etc.).
Air cooled heat dissipating plate is a common indirect cooling method with simple structure and easy implementation. Its disadvantages are limited heat dissipation capacity, large structural dimensions, and far lower heat dissipation efficiency than liquid cooled panels. It is commonly used in small airborne electronic equipment.
(4) Liquid cooled heat dissipation plate heat dissipation system
The cooling medium of the liquid cooled heat sink cooling system is liquid. The heat dissipation material is usually aluminum or copper with good thermal conductivity. Aluminum is commonly used in airborne electronics. The size of the cooling plate and the shape of the liquid channel can be determined based on the size of the space, the amount of heat dissipation, and the pressure of the cooling source. According to the working ambient temperature of the cold plate, select the appropriate coolant (such as ethanol)’s freezing point, boiling point, specific potential, thermal conductivity, heat of vaporization, and dynamic viscosity.
Liquid-cooled cooling plate is a common cooling method with high heat flux (up to 45 × 103W/m2), high heat dissipation efficiency, uniform heat load, small temperature gradient and compact structure. Compared with other heat exchangers of the same volume, it is light in weight and large in heat exchange area, suitable for heat dissipation of high-power components, and widely used in airborne electronic products.
When designing the cold plate, it is necessary to consider many factors such as pump pressure, coolant flow, coolant temperature rise, surface temperature of the cold plate, secondary cooling of the coolant and so on, so as to reasonably formulate the structure plan.
3、 Basic problems and requirements of thermal design of electronic products
For the thermal design of electronic products, there are several issues that need to be clarified in advance.
(1) Thermal characteristics of electronic products (including heating elements)
The basic basis for thermal design is the thermal characteristics of components (also known as thermal boundary conditions), including the heating power of the components (or products), the heat dissipation area of the heating components (or products), the maximum allowable operating temperature, and the temperature environment of the heating components or thermal sensitive components (or products). These data parameters are generally given in the component data manual (provided by the manufacturer), whereby the designer can determine the cooling scheme and cooling medium flow rate. When these data are insufficient, in principle, thermal design cannot be accurately performed. Designers need to determine various parameters through measurement and testing to ensure the accuracy of the design.
(2) Ambient temperature of components (or products)
The principle of heat transfer is that heat is always transferred from a high-temperature object to a low-temperature object, and the speed of heat transfer is related to the temperature difference and the transfer method (or medium). Under the same transmission mode, the greater the temperature difference, the faster the heat transfer. It can be seen that the final temperature of electronic products (including heating elements) is not only related to the thermal characteristics of the elements, but also closely related to the ambient temperature. Therefore, before conducting thermal design, it is necessary to accurately understand the temperature of the working environment of electronic products (or components).
In practical work, the inlet and outlet temperatures (temperature differences) of the coolant in a cooling system are usually determined based on the operating ambient temperature of the component (or product) and the maximum allowable temperature of the component (or product), and are used as reference data for preliminary thermal design estimates.
The principle of thermal design is to limit the temperature of the product to a certain maximum and minimum range, and minimize the temperature difference between various points in the electronic product. Specific requirements include:
① Ensure that the system has good heat dissipation function.
The cooling method is determined according to the heat loss value, use and temperature rise of the product. In the thermal circuit, the thermal resistance (resistance in the heat transfer process) between the heating surface of the element and the connector should be as small as possible; Different heat dissipation measures shall be adopted between different components to ensure that all components in electronic products can work normally in the specified thermal environment.
② Ensure the reliability of the cooling system.
No matter how the environment changes, the cooling system must be able to complete the specified functions in a repeated and predetermined manner. Within the specified service life, the failure rate of cooling system shall be lower than that of components.
③ The cooling system should have good adaptability.
The cooling system shall be designed with heat dissipation allowance. Because some products need to enhance their heat dissipation capacity due to changes in some factors, such as heat dissipation or increased fluid flow resistance, after working for a period of time. If there is no margin, it needs to be redesigned, which can cause unnecessary trouble and increase costs.
④ The cooling system design should have good economy.
To design a good heat dissipation system, it is necessary to comprehensively consider all factors, so that it can meet both the heat dissipation requirements and the electrical performance indicators. At the same time, the cooling cost used is minimal, the structure is compact, and the work is reliable.
⑤ The cooling system should have good maintainability.
The design of the cooling system should be as simple as possible, using as few common components as possible to facilitate maintenance and replacement.
4、 Thermal Design Principles for Electronic Products
The basic principles of thermal design are manufacturability, maintainability, and economy. That is, to meet the use requirements of the product with the simplest design and the lowest cost.
Based on experience, thermal design should generally follow the following design principles:
① When the heat flow density exceeds 0.08W/cm2 and the bulk power density exceeds 0.18W/cm3, forced air cooling, forced liquid cooling, evaporative cooling, heat pipe, or other cooling methods should be used.
② If the space between electronic components is conducive to air flow or a radiator can be installed, a forced air cooling system is recommended.
③ Liquid cooled cooling systems are recommended for components or equipment that must operate in a high temperature environment and have a small temperature gradient with the cooled surface or a high volume power consumption density.
④ Generally, all exposed components (including chassis) are designed to operate at ambient temperatures below 35 ℃, with temperatures not exceeding 60 ℃, and panels and controllers not exceeding 43 ℃.
⑤ Thermal design should be conducted simultaneously with electrical design, structural design, reliability design, etc. If there are contradictions, balance analysis and compromise resolution should be conducted. However, electrical performance shall not be damaged, and reliability requirements shall be met to minimize the life cycle cost of the equipment.
⑥ When designing a cooling system, factors such as economy, volume, and quality must be considered. Try to use basic heat dissipation methods such as conduction, radiation, and convection to avoid additional heat dissipation devices.
⑦ When designing a cooling system, maintainability must be considered. Heat exchangers, coolants, and piping should be selected from the perspective of the entire system. At the same time, the coolant should not corrode the heat exchanger and piping.
⑧ Temperature monitoring devices should be installed around temperature sensitive components or equipment to automatically alarm or power off when the ambient temperature exceeds the allowable operating temperature range of the components to protect equipment safety (electronic equipment that requires long-term continuous operation is not applicable).
⑨ The allowable error in thermal design is large, and numerical analysis and calculation should be conducted for the cooling system at the initial stage of design.