Forced Air May Be Needed
High-power equipment that has to operate continuously may require the use of forced-air cooling. As a general guideline, it's recommended that device-cooling accessories, such as heatsinks, limit the junction temperature rise of semiconductor devices to within about 125°C. The lower the junction temperature, the better the conditions are for device reliability.
A word of caution: overspecifying heatsink requirements will increase cost without significantly reducing temperature. Similarly, beyond a certain limit, increasing fan speed and, consequently, air speed in a forced-convection cooling system won't greatly improve the cooling rate.
The aim of good thermal design is to achieve optimum cooling at minimum cost. The following steps will help you attain this goal:
- Good thermal design begins with good board design. By selecting appropriate components and placing them properly on the board, you will avoid thermally induced failures in heat-sensitive components. Heat-generating components, like power devices and power resistors, should be mounted away from heat-sensitive components, such as electrolytic capacitors.
- Provide adequate heatsinking for power-dissipating components and baffles, if necessary, for better air circulation.
- Because hot air is less dense and rises, cool air should be passed from the bottom so it gets heated and rises. Mount cooling fans at the bottom of cabinets and provide holes near the top to ensure a chimney effect.
- Keep air inlets and outlets away from each other to prevent hot air from getting sucked up into the chassis through the cooling fan.
- Any dust filters used in the system cabinet should be kept clean to ensure proper air passage.
- The key to better heat disposal is employing a heatsink with a large surface area and lower thermal resistance to the ambient.
- Cooling accessories like heat-sinks and fans should be specified during initial design. They can't be added later as an afterthought. Such retrofits will have space constraints and limited impact on achieving better air circulation.
- Derate your device's thermal specifications, depending on your application and the degree of reliability to be achieved. An 80% derating factor is a good guideline for the junction temperature in °C and the power-dissipation rating in watts.
- Use a thermal simulation package, if it's available, to estimate the thermal profile of your board. Conduct a thermographic study of your prototype under actual operation to ex-pose any thermal problems. In a thermographic study, thermal imaging equipment, like an infrared system, creates a graphic thermal-profile picture of the heat distributions throughout a powered-up system, such as a pc board.
When all is said and done, you must achieve the right tradeoff between cooling requirements and the economics involved in your design, bearing the reliability factor in mind.
Currently there's a growing trend toward developing miniature electronic systems-on-a-chip in shrinking packages operating in an environment of high levels of thermal stress. Obviously, accomplishing high operational reliability will be a challenge for electronic designers in the new millennium.
Forced Air May Be Needed
High-power equipment that has to operate continuously may require the use of forced-air cooling. As a general guideline, it's recommended that device-cooling accessories, such as heatsinks, limit the junction temperature rise of semiconductor devices to within about 125°C. The lower the junction temperature, the better the conditions are for device reliability.
A word of caution: overspecifying heatsink requirements will increase cost without significantly reducing temperature. Similarly, beyond a certain limit, increasing fan speed and, consequently, air speed in a forced-convection cooling system won't greatly improve the cooling rate.
The aim of good thermal design is to achieve optimum cooling at minimum cost. The following steps will help you attain this goal:
- Good thermal design begins with good board design. By selecting appropriate components and placing them properly on the board, you will avoid thermally induced failures in heat-sensitive components. Heat-generating components, like power devices and power resistors, should be mounted away from heat-sensitive components, such as electrolytic capacitors.
- Provide adequate heatsinking for power-dissipating components and baffles, if necessary, for better air circulation.
- Because hot air is less dense and rises, cool air should be passed from the bottom so it gets heated and rises. Mount cooling fans at the bottom of cabinets and provide holes near the top to ensure a chimney effect.
- Keep air inlets and outlets away from each other to prevent hot air from getting sucked up into the chassis through the cooling fan.
- Any dust filters used in the system cabinet should be kept clean to ensure proper air passage.
- The key to better heat disposal is employing a heatsink with a large surface area and lower thermal resistance to the ambient.
- Cooling accessories like heat-sinks and fans should be specified during initial design. They can't be added later as an afterthought. Such retrofits will have space constraints and limited impact on achieving better air circulation.
- Derate your device's thermal specifications, depending on your application and the degree of reliability to be achieved. An 80% derating factor is a good guideline for the junction temperature in °C and the power-dissipation rating in watts.
- Use a thermal simulation package, if it's available, to estimate the thermal profile of your board. Conduct a thermographic study of your prototype under actual operation to ex-pose any thermal problems. In a thermographic study, thermal imaging equipment, like an infrared system, creates a graphic thermal-profile picture of the heat distributions throughout a powered-up system, such as a pc board.
When all is said and done, you must achieve the right tradeoff between cooling requirements and the economics involved in your design, bearing the reliability factor in mind.
Currently there's a growing trend toward developing miniature electronic systems-on-a-chip in shrinking packages operating in an environment of high levels of thermal stress. Obviously, accomplishing high operational reliability will be a challenge for electronic designers in the new millennium.