Submitted 1 week ago by iliketurtiles@programming.dev to askelectronics@discuss.tchncs.de
Not sure if this is a dumb question but this has me quite puzzled.
The legs on TO220 packages are very small. How is it that there are e.g MOSFETS rated as being able to continuously conduct ~100A? e.g IRF1404Z
From what I understand such large currents need busbars on PCBs and these appear a lot larger than the legs on these components.
From the IRF1404Z data sheet:
Calculated continuous current based on maximum allowable junction temperature. Bond wire current limit is 120A. Note that current limitations arising from heating of the device leads may occur with some lead mounting arrangements.
You need to design the PCB so that the heat from the legs can be properly dissipated, or they won’t live up to the rated current.
Also, traces on a PCB are much thinner than those legs. A trace with the same cross-sectional area would be impractically wide.
for how many picoseconds?
Which attribute in the datasheet are you looking at?
For the IRF1404Z, under the absolute maximum ratings, continuous current is given as 140A. If it was for something like picoseconds, it doesn’t seem meaningful labeling it ‘continuous current’?
litchralee@sh.itjust.works 1 week ago
The datasheet for the IRF1404Z does indeed show that the TO-220 package has a limit of 120 A continuous at 25 C. It should be noted that the junction temperature is rated for up to 175 C, which might provide a lot of headroom for, but we’ll see.
The minimum dimensions for the drain and source leads are 0.36 mm by 1.14 mm. This gives us some 0.41 mm^2 cross sectional area. Assuming the leads are made of aluminum – I’m on mobile and can’t quickly check the composition for the generic TO-220 package – which has a resistance of about 60 Ohm per km, and with the lead being a maximum length of 14.73 mm, the resistance of either lead will be some 0.88 mOhm.
At 120 Amps, the resistance heating would be about 12.6 Watts. That’s quite hot for a short lead, and there’s two of them. But the kicker is that these aluminum leads are also thermally conductive, either into the package towards the junction, or away and into a generous PCB layer or to suitably-sized copper wires.
Either way, that will sink a fair amount of heat, although the thermal resistance for the package legs is not given in this datasheet. It may be defined for generic TO-220 packages though.
As a practical matter, to operate a MOSFET ar 120 A would likely require active cooling, and their test jig plus all reasonable implementations will have a fan. Moderate airflow over the leads will also wick temperature away, which might bring the leads down to a “hot but not fire-inducing” levels. But an EE or thermal engineer would need to sit down to do that simulation.
hendrik@palaver.p3x.de 1 week ago
Thanks for doing the maths. I would have also guessed it's due to the short distance these amps have to travel. And in practice, we'd likely be using just the thicker parts at the top of the legs and clip most of them off, so it'd be way less than the almost 15mm in your numbers. Still probably an issue with the thickness of the pcb traces, but that's something the designer has to worry about.