Industrial Strength Design - Part I

Chemical plant at nightWhen you think of engineering for industrial applications, my first thought goes to environmental conditions of applications such as steel mills.  There are motor controllers working right next to (or attached to) giant electric furnaces and smelters, huge overhead cranes and massive electric fields.  It’s just another day at the office, huh? You may think it’s fairly straight forward to design for this environment until you realize that your electronics have passive cooling – no fan.  Fans fail and if the circuit is part of a system that needs to be shut down to service, the financial impact to the company could be enormous. 

So, here are some simple rules to get you in the ball park.  First, heat is not your friend – even in Minnesota in the middle of winter.  Consider how semiconductors are fabricated.  There are processes (not unlike metallurgy) to anneal crystal defects or diffuse impurities… all are accomplished by the application of heat.  So as a component heats up, this process continues.  There are other creeping problems aided by elevated temperatures and large current which can cause metal interconnects within components to migrate and short.  Designers of industrial control systems often de-rate operating junction temperatures based on their models – mostly from years of experience. 

These systems stay in operation for over 20 years … working every day without failure until replaced or retired.  So designers must take into consideration how to keep the temperature of the die well below the maximum operating point.  To do this a thermal model is used.  It can be extremely complicated using computers to calculate the temperature rise and heat flow.  Or it can be simple to see if the circuit has a chance of working at all.  To calculate the temperature rise of the die, the thermal flow can be modeled pretty much the same way we model current flow. 

The thermal “impedance” or resistance to heat flow is given in data sheets relative to Point A and Point B.  For instance ӨJC (pronounced “theta” sub “J” “C”) is the junction (die) to case thermal impedance.  It is given in degrees centigrade per watt (°C/W).  It means that for every watt of power dissipated by the device, the junction temperature will rise so many degrees above the case temperature.  So if the ӨJC is 3°C/W and the device dissipates 10 watts, then the junction temperature will be 30 degrees C higher than the case temperature.  There is also a junction to ambient version called ӨJA which is the thermal impedance to the ambient air surrounding the part (assuming no heat sink).  Any device dissipating more than 100mW will probably need a method to carry the heat away.

The worst case problem with this scenario is having large thermal impedances with high power dissipation.  This happens often in small linear regulators such as the LM340 in a SOT-223.  Even with unlimited copper, the best ӨJA will be no lower than 50°C/W which is a limitation of the package.  So if the ambient air in the box is 85°C, the input voltage is 12V, the output voltage is 5V (7V drop) with a load current of 100mA (700mW power dissipation), then the temperature of the die will be 120°C.  You can see that package selection is EXTREMELY important.  Take the same part and put it in a DPAK (32°C/W) and the die temperature (same conditions) drops to a bit over 107°C.

So next time you’re designing for industrial strength, take out the hand calculator for your sanity check to make sure your devices won’t end up in thermal shutdown… or worse!  Next time I’ll cover some more on industrial strength design ideas for keeping your circuits alive in harsh environments… Till next time!

edit: Industrial Strength Design: Part II now available.

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