EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: max_torque on February 11, 2018, 09:19:59 pm
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Say i have a nominal 60v transistor (Vce max) how close can it be run, in a practical circuit, to that limit, and what happens if that threshold is exceeded?
(ie typically, how much safety factor is built into a typical transtor datasheet?)
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It can be run to 60 V without problems (unless spikes etc. can be present).
What you have to watch is SOA and RBSOA, which can be tricky to analyze.
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It depends on where the number showed up on the datasheet. If it was under Maximum Ratings then that is the maximum number before magic smoke leaks out. If the number had a margin, they would have jacked up the spec.
For the 2N2222a datasheet, the Maximum Rating for Vce is 50 VDC. It is also listed as the minimum breakdown voltage but there is no maximum so all they will guarantee is 50VDC.
I wouldn't expect any slack on the Maximum Rating. If I designed above the rating and it worked, great! But if it came apart, it's my fault. Maximum is kind of self-defining...
http://web.mit.edu/6.101/www/reference/2N2222A.pdf (http://web.mit.edu/6.101/www/reference/2N2222A.pdf)
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Transistor Vceo is tested and sometimes graded to make different part numbers. In practice it is the guaranteed minimum value and it is not uncommon to find transistors which actually have a maximum Vceo considerably higher than the datasheet specification.
For low voltage and power applications, I usually derate the peak operating Vceo to 2/3rds or less of the datasheet maximum. Sometimes transistor Vceo degrades with age if its quality is poor.
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Most quality brands will have a Vceo greater than claimed. Sometimes quite a lot greater. This is usually only a problem area with substandard parts.
As mentioned, much more of a problem is assessing power handling and SOA, especially since some manufacturers are prone to exaggerate here. For example, stating a Ptot which relies on the case being at 25C but then admitting that the minimum thermal resistance even with direct mounting makes that impossible.