Heatsinks

Overall efficiency This will determine how much heat will be generated within the product and whether any heatsinking will be needed in the physical design. 

Finalize the physical design. This would include physical packaging within the product, heatsink design, and the PCB design. 

The linear power supply finds a very strong niehe within applieations where its ineffieieney is not important. These inelude wall-powered, ground-base equipment where foreed air eooling is not a problem and also those applieations in whieh the instrument is so sensitive to eleetrieal noise that it requires an eleetrieally quiet power supply—these produets might inelude audio and video amplifiers, RF reeeivers, and so forth. Linear regulators are also popular as loeal, board-level regulators. Here only a few watts are needed by the board, so the few watts of loss ean be aeeommodated by a simple heatsink. If dielee-trie isolation is desired from an ae input power souree it is provided by an ae transformer or bulk power supply. 

In general, the linear regulator is quite useful for those power supply applieations requiring less than 10 W of output power. Above 10 W, the heatsink required beeomes so large and expensive that a switehing power supply beeomes more attraetive. 

If the system eannot handle the heat dissipated by this loss at its maximum speeified ambient operating temperature, then another design approaeh should be taken. This loss determines how large a heatsink the linear regulator must have on the pass unit. 

A small elip-on style heatsink is required to bring the junetion temperature down to below its maximum ratings. 

Design required required heatsink and heat removal considerations... 

This node(s) presents a different problem. Its AC voltage can be easily capac-itively coupled into any adjacent traces on different metal layers, as well as radiate EMI. Unfortunately, it is generally the trace that must also act as a heatsink for both the power switch and the rectifiers, especially in surface mount power supplies. Electrically, the trace wants to be as small as possible, but thermally, it wants to be large. There is one good compromise in the surface mount designs, and that is to make the top PCB island identical to the bottom PCB island and connect them with numerous vias (or thru-hole connections). This can be seen in Figure 3-62. 

Figure 3-62 A good method for enhancing the heatsinking capabilities of a PCB island and minimizing capacitive coupling to other PCB traces.

Note These dissipated heats are within the range of free-standing, screw-on heatsinks—call for samples of thermalloy heatsinks. 

Thermal resistanee from the junetion to the air Thermal resistanee from the junetion to the ease Thermal resistanee from the ease to the heatsink Thermal resistanee from the heatsink to the air... 

All of the semieonduetor ease-related parameters are published by the semieonduetor manufaeturers. The sink-to-air parameter is published by the heatsink manufaeturers, if one buys a heatsink. If one makes his or her own, it is easy to measure these resistanees from any model. 

The nodes in die model are the respeetive surfaees of bodies along the path of flow of the heat. These ean be transistor eases, heatsink surfaees, the semi-eonduetor die, ete. The ealeulated temperatures of these surfaees ean aetually be measured using a temperature probe at their respeetive surfaees. If the power dissipation is not known but all the thermal resistanees are known, one ean extrapolate baekwards within the model and determine the power being dissipated within the die by simply measuring the temperature differenee aeross one of the thermal boundaries. 

Sinee the heatsink performs the vast majority of the heat radiation, it is assumed that all the power flows through all the other thermal elements. 

Power packages not mounted on a suitable heatsink can expect to dissipate less than five percent of the maximum specified power capability of the package. So 100 W devices will only dissipate 1 to 2W when they are free standing. This also includes using the PC board copper plating as a heatsink. Thus, great discretion should be used when cost is the most important issue. 

There are some heatsinks for radially-leaded diodes that solder to one of the leads. These are also available from the transistor heatsink manufaeturers. In this situation the thermal equation (see Figure A-6b) beeomes... 

These heatsinks will help a marginal heat situation. The alternative is to use a reetifier in a power transistor paekage sueh as a TO-220, TO-218, ete., and plaee it on a heatsink or to investigate a different teehnology of diode that exhibits a lower forward voltage drop sueh as a Sehottky. 

The use of surfaee mount parts is widespread. Surfaee mount parts ean rid themselves of heat only through their leads whieh are soldered to a printed eireuit board. The thiekness and surfaee area of the eopper island beeome the heatsink system. The thermal resistanees in surfaee mount deviees are mueh higher, therefore their designs have mueh less margin and room for error. Table A-3 has nominal values for thermal resistanees of eommon surfaee mount paekages. Please refer to the individual part data sheet for the exaet value. 

In surfaee mount printed eireuit board applieations, more than one issue usually must be eonsidered. Heatsinking must be eonsidered, along with signal and EMI/RFI eonsiderations. The traee that must dissipate the greatest heat within a switehing power supply is also the node that has the largest dv/dts whieh eouple very easily to the surrounding traees. 

A.6.1 Determine the Smallest Heatsink (or Maximum Allowed Thermal Resistance) for the Application... 

This approach is useful for determining the smallest possible heatsink that an application can use before the thermal limit of a power device is exceeded. This is an example of a consumer market approach to designing a heatsink system. 

Rearranging Equation A.l and solving for the thermal resistanee of the heatsink,... 

The PC board-mounted heatsink ehoiees are Thermalloy part numbers 7021B through 7025B for low-eost sheet-metal type heatsinks. 

A.6.2 Determine the Maximum Power That Can Be Dissipated by a Three-terminal Regulator at the Maximum Specified Ambient Temperature without a Heatsink... 

A 3-terminal regulator s overeurrent proteetion is totally dependent upon the heatsinking system. When the die reaehes approximately 165°C, the regulator shuts down. This example demonstrates the nonheatsink eapabilities of a qA7805. 

The new board was missing a huge heatsink. How can the heatsink disappear ... 

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