Component leads and current.

[davelectronic]

Its always puzzled me how some component leads can carry the current the of that device. One that gets me are transistors, be it TO22 or TO3P TO3 etc. I read maximum ratings and see for a few Amps maybe low tens of Amps it looks up to the job.

But when you talk of transistor collector currents above, say 15 Amps, and as much as 50 Amps i wander why the component leads are not thicker or heavier duty.
Same on many other components. Just puzzled me. Two devices looking the same, one can handle 15 Amps, the other in the same package can handle 50 Amps. Yet the component leads are the same gauge for both. Same with other components in high current senarios.

Your input cables are massive, your output cables are equally as heavy duty, but the gauge of your high current component leads look whimpy in comparison. Anyone know why manufacturers don't make there device leads more heavy duty ? The devices still work supplying the currents needed with out bother, but they seem flimsy in comparison to the wiring needed to carry these heavy currents..

[ataradov]

It is all about resistance.  Component leads are very short, so their resistance is negligible in most cases.

Furthermore, those leads go to super tiny bonding wires connecting them to the die.

[SeanB]

Most components also spec the current capability at 25C case temperature, so to actually use them at that current rating you need active cooling, as most will be limited by thermal effects before they actually reach this current.

The thing that also helps is the leads generally are very short, and are well connected via soldering to both a large surface of copper trace to dissipate heat, plus they are connected to a heatsink on the device side. I have seen some TO3 power transistors (BUX20, BDY25) which have very thick copper leads that are capable of handling high current. TO220 devices generally are only going to handle 10A on a continuous basis.

[mash107]

IPC 2152 is a good reference for cross-section copper area vs current vs temperature rise of said copper piece

You typically want to limit copper temp rise to less than 25degC, or even lower if you cannot afford either the heat rise or increase in resistance (http://hyperphysics.phy-astr.gsu.edu/hbase/electric/restmp.html)

[davelectronic]

Thanks for thoughts replys, it must be lead length and modest tempreture levels i guess. It was just an observation really.

[David Hess]

Higher current transistors in TO-3 type packages *do* use thicker leads.