SBOS070D October   1997  – December 2019

PRODUCTION DATA.

1. Features
2. Applications
3. Description
4. Revision History
5. Pin Configuration and Functions
6. Specifications
7. Detailed Description
1. 7.1 Overview
2. 7.2 Functional Block Diagram
3. 7.3 Feature Description
4. 7.4 Device Functional Modes
8. Application and Implementation
1. 8.1 Application Information
2. 8.2 Typical Applications
1. 8.2.1 Basic Circuit Connections
1. 8.2.1.1 Design Requirements
2. 8.2.1.2 Detailed Design Procedure
3. 8.2.1.3 Application Curve
2. 8.2.2 Monitoring Single- and Dual-Supplies
3. 8.2.3 Programmable Power Supply
3. 8.3 System Examples
9. Power Supply Recommendations
10. 10Layout
1. 10.1 Layout Guidelines
2. 10.2 Layout Example
11. 11Device and Documentation Support
12. 12Mechanical, Packaging, and Orderable Information

• KC|7
• KTW|7
• KVT|7

#### 10.1.5.1 Heat Sink Selection Example

A TO-220 package is dissipating 5 W. The maximum expected ambient temperature is 40°C. Find the proper heat sink to keep the junction temperature less than 125°C (150°C minus 25°C safety margin).

Combining Equation 2 and Equation 3 gives:

Equation 3. TJ = TA + PD(RθJC + RθCH + RθHA)

TJ, TA, and PD are given. RθJC is provided in the specification table, 2.5°C/W (DC). RθCH can be obtained from the heat sink manufacturer. Its value depends on heat sink size, area, and material used. Semiconductor package type, mounting screw torque, insulating material used (if any), and thermal joint compound used (if any) also affect RθCH. A typical RθCH for a TO-220 mounted package is 1°C/W. Now we can solve for RθHA:

Equation 4.

To maintain junction temperature less than 125°C, the heat sink selected must have a RθHA less than 14°C/W. In other words, the heat sink temperature rise above ambient must be less than 67.5°C (13.5°C/W × 5 W). For example, at 5-W Thermalloy model number 6030B has a heat sink temperature rise of 66°C more than ambient (RθHA = 66°C / 5 W = 13.2°C / W), which is less than the 67.5°C required in this example. Figure 44 shows power dissipation versus ambient temperature for a TO-220 package with a 6030B heat sink.

Another variable to consider is natural convection versus forced convection air flow. Forced-air cooling by a small fan can lower RθJCA (RθCH + RθHA) dramatically. Heat sink manufactures provide thermal data for both of these cases. For additional information on determining heat sink requirements, consult Application Bulletin SBOA021.

As mentioned earlier, once a heat sink has been selected, the complete design should be tested under worst-case load and signal conditions to maintain proper thermal protection.