It is good practice to ensure some design margin for the electronic circuit in terms of absolute maximum ratings.

What is the proper way of selecting such a margin for Total Power Dissipation?

Let's assume 20% as a safety margin for any reason and SOT363 NPN Transistor as an example:

On the chart below there is a common power derating curve for BC847 NPN Transistor in SOT363 Package - [Blue].

I calculated the -20% Power alternative [Green] it looks good however the closer we get to the Absolute Maximum Temperature Rating [150C] the smaller the margin becomes. This is why I wonder if it is a proper way of doing this?

As an alternative, I added another line which is basically -10C of the base the value [Green].

Is there a way to define something "in-between"?

enter image description here

  • \$\begingroup\$ I'm not sure that curve is accurate without knowing the PCB thickness, weight, and footprint it's mounted on. \$\endgroup\$
    – DKNguyen
    Feb 28 '19 at 21:28
  • \$\begingroup\$ As well as any margin calculations people advise - note that graphs are usually "typical ratings unless otherwise specified. Look at tabled specs to get some idea of how much typical and max and min values compare. \$\endgroup\$
    – Russell McMahon
    Mar 1 '19 at 1:16
  • \$\begingroup\$ For the Thermal characteristics, the Maximum thermal resistance values are defined, so it should not be worse than what I presented here. \$\endgroup\$
    – KJA
    Mar 1 '19 at 6:18

If you have never done MTBF calculations before there is a general failure rate acceleration factor that often doubles for every 10'C rise (Arhennius Law).

My criteria for a commercial design when I managed this function, was 85'C for junction temperature at maximum ambient of 40'C normally operated at room temp.

  • package thermal resistance, Rjs = 230'C/W from junction to solder point
  • FR4 Rsa= 338'C/W from solder point ambient with free moving air, while sealed air is worse and adding 1 cm^2 exposed copper to collectors reduces by 20'C/W per cm² area of collector copper ²

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