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I am designing a discrete MOSFET-driver stage based on BJTs in simple push-pull configurations and wondering, which of the values under absolute maximum ratings I have to derate at higher ambient temperatures. The device in consideration is the BC807W which only has to conduct during turn-off of the MOSFET.

The datasheet gives me a max value for I_{CM}=-1A. Can I drive this peak current only at 25°C of ambient temperatures or also at higher temperatures as long as the maximum junction temperature is not exceeded?

The background of this question is that I head a similar issue with diodes some time ago. The datasheet would give me a maximum surge-current which has to be linearly derated above 25°C (according to the very competent FAE). This current must not be exceeded, even if a nice margin of the junction temperature remained. The point there was, that not only the heat destroys the diode, but also the current density.

So, to bring this to an end: is it the same with the peak collector current of a BJT?

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    \$\begingroup\$ That value of \$I_{CM}\$ is in the table of absolute maximum ratings (table.6 - limiting values). Those are values from which you must stay absolutely clear, so don't think of going nowhere near 1A, even at nominal 25°C. Designing against absolute maximum ratings is bad design practice. \$\endgroup\$ – Lorenzo Donati -- Codidact.org Aug 19 '15 at 21:28
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    \$\begingroup\$ If you foresee 1A peaks (or something near that) probably you should select a tougher part if you want a reliable design. \$\endgroup\$ – Lorenzo Donati -- Codidact.org Aug 19 '15 at 21:30
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Take a look at this explanation of Absolute maximum ratings as defined by the IEC.

source: http://www.electronics-cooling.com/2004/02/the-temperature-ratings-of-electronic-parts/

The International Electrotechnical Commission (IEC) [5] defines absolute maximum ratings as “limiting values of operating and environmental conditions applicable to any electronic device of a specific type as defined by its published data, which should not be exceeded under the worst possible conditions. These values are chosen by the device manufacturer to provide acceptable serviceability of the device, taking no responsibility for equipment variations, and the effects of changes in operating conditions due to variations in the characteristics of the device under consideration and all other electronic devices in the equipment. The equipment manufacturer should design so that, initially and throughout life, no absolute-maximum value for the intended service is exceeded with any device under the worst probable operating conditions with respect to supply voltage variation, equipment component variation, equipment control adjustment, load variations, signal variation, environmental conditions, and variation in characteristics of the device under consideration and of all other electronic devices in the equipment.” In other words, the part manufacturers select the AMR values, and the companies, which integrate electronic parts into products and systems, are responsible for assuring that the AMR conditions are not exceeded.

so based on this standard the it is up to the part manufacturer to specify their absolute maximum ratings such that they work "under the worst probable operating conditions". Of course whether or not a specific manufacturer actually does so is another story.

Again as was mentioned by others absolute maximums are the point where the part can be assumed to break so it is best design practice to stay well away from that limit under all conditions.

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  • \$\begingroup\$ I think I have to clarify that I don't want to design the circuit in a way that I go up to 1A. But suppose I want to go up to 800mA. Is this ok even at higher temperatures? \$\endgroup\$ – christoph Aug 20 '15 at 11:36
  • \$\begingroup\$ Based on the standard definition of Absolute Maximum being accurate under the worst probable operating conditions it should be. \$\endgroup\$ – Cole LaCount Aug 21 '15 at 12:44
  • \$\begingroup\$ To clarify, it should be fine based on the letter of the definition of "Absolute Maximum" but the best way to be sure is to contact the manufacturer of the part to find out if they actually tested it at those limits. \$\endgroup\$ – Cole LaCount Aug 21 '15 at 15:48

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