使用模拟组件的热计算工具

　　Component Thermal Classification

　　For the purposes of this discussion， electronic components will be classified into three categories with regard to their power dissipation and required thermal resistance. These are not rigid categories and are a function of several parameters such as package size， construction， materials， die size， and power dissipation profile. These categories can serve as a general guideline when considering the level of thermal analysis needed for a component during system design.

　　（a） The first category is referred to as “low power” components. These are generally components such as passives， logic devices， or other components that do not dissipate high levels of power during operation. Low power components generally do not require any thermal analysis and can be designed in to almost any system without concern of exceeding the maximum operating characteristics. There is no rigid definition of this category， but generally， if the temperature rise in an appropriate JEDEC thermal test environment， as calculated by the test coupon Theta-JA （qJA） multiplied by the power （qJA x Power） is less than 10°C， it could be considered low power. Depending on the component construction and application environment， the tolerance for low power could be as low as 100mW or less， or could reach as high as 500mW or even 1W （for example in systems with forced airflow）。 Ultimately， if there is any uncertainty， a component should be considered in the next category.

　　（b） The second category is referred to as “medium power” components. These are generally components which are dissipating enough power that their maximum operating temperatures could be exceeded， if care is not taken with good system and PCB design. These components have generally been designed to operate safely， provided that they are able to dissipate heat through a specific thermal path. For example， many medium power components are designed with a direct thermal path such as an exposed pad， which connects to a PCB pad with vias into a spreading plane. Medium power components require some thermal consideration during system design， and verification with measurements on the assembled system is essential. Because their heat dissipation paths are often carefully considered by the component supplier， medium power parts can often be analyzed during the system design phase using calculators or simplified modeling approaches， and then confirmed with measurements during the prototype phase. For these reasons， this document focuses particularly on methods to design for medium power devices， and particularly those using exposed pads to connect to the PCB thermal path. There is no rigid definition of this category， but generally components which would have a temperature rise of at least 10°C above the ambient temperature could be considered as medium power or possibly even in the next category. This temperature rise can be estimated by multiplying the appropriate JEDEC qJA by the power （qJA x Power）， but this calculation should only be considered as preliminary and as an initial threshold for more thorough analysis. Some examples of components which are considered “medium power” would be those which have a high temperature rise above the ambient or PCB temperature， components which will operate in an environment with high ambient temperatures， or components which is in close proximity with other parts that contribute to the temperature rise of the PCB and overall system.