Hi. Trying to make sense of this.
How can mosfets such as this one BUK9M10-30EX - Nexperia USA Inc.
be so small yet can handle some 10, 20, or even (in this case) 54 AMPS CONTINUOUS! These packages as well as the leads are absolutely TINY!
Moreover, there is not enough space to make traces thick/wide enough to allow that kind of current.
Heck, even if you add up all of the SOURCE or DRAIN pads, the PCB trace width (assuming typical 2oz or 1oz thickness) will not even come close to being able to handle that amount. In this case if you make a trace as wide as all of the pads you MAYBE will be able to safely get 2-3A max.
I am not question the capability of the MOSFET itself regarding heat dissipation. Rather the cumulative trace width of the SOURCE. Or even the entire DRAIN is not wide enough. If there would be no MOSFET, just a plain PCB the traces of this thickness itself would be underrated for this type of current.
A trace width calculator will tell you that for even 10A @ 2oz you would need at least 0.142" of trace width. THAT IS WIDER THAN THE ENTIRE MOSFET!
Your worrying about the PCB design not the MOSFET ?
When you design the PCB for this chip you would make sure the wires connected to the DRAIN and SOURCE were very close to the device leads, perhaps within 5mm.
There are 3 SOURCE leads, lets say they a #24AWG and lets say they have a total length of 4 mm.
This would be the same as ~24AWG at 1mm (0.00328084 feet).
24AWG copper is 25 ohms per 1000feet and we have 0.00328084 feet . . .
You can do the current carrying capacity of the 3 source leads.
90% likely you would be using these in a circuit < 10 amps anyway.
It is quite common that a spec does not necessarily reflect a realistic usage situation and some maximum ratings are intended as a guide only.
Take AMS1117, a simple linear voltage regulator. The spec says max power 1Amp. The 3.3 version says max voltage 15volts.
Does that mean you draw 1 Amp through it with a 15v input?
Possibly, if you have a big enough heat sink to stop package melting while dissipating 12 watts.
Well they do reflect reality but you have to read all of them to get the full picture. Just like in the news papers if you just read the headline you will get a distorted view of the story. Conditions can be contrived to maximise any one parameter, when you dig int to the full story you will see that this parameter is rarely going to be the limiting factor, there are other factors that come into play that define the limits of any one design. So the headlines are true but just like politicians they lie by using the truth.
You are assuming a 2 oz PCB I don't think a 50 amp design would be designed at 2 oz.
Getting back to the apparently unrealistic specifications. In general you will find the specifications provide a maximum allowable value. In this case, ignoring the PCB, the limit will be getting rid of the heat, not the 50A max current.
As stated above, the specifications of nearly all semiconductor based devices "headline" specifications at the very limit of capability. Now since all semiconductor companies do the same thing, it makes a rough comparison relatively easy.
To be honest I raised my eyebrow at the spec's of this part. I finally considered the max currents to be useful for transient situations that are much longer than the thermal time constants of the device. Perhaps a motor startup current under load.
Power MOSFETs are vertical current flow, so that the whole width of the chip is available to carry current.
Silicon when doped is capable of low resistivities, for instance doped at 10^20 atoms per cubic cm, silicon has a resistivity of about 10^-5 ohm-m, only 60 times that of pure copper.
And then the current is flowing though a chip that's perhaps 3mm x 3mm and 100µm thick, so theoretically you could have a resistance as low as 100µohms in that chip if made of
heavily doped silicon - its not hard to see that a resistance of a few milliohms in a real device
is entirely plausible.
And of course the bond-wires have to carry large currents too, but they just use lots of
thick bondwires for high currents, each heavily heatsinked (by virtue of being short and welded to the heat-tab).
The device in question has an on-resistance of 8mohm, so 54A dissipates about 23W, which is only
possibly with appropriate heatsinking - the datasheet is only talking about the raw device,
not the combination of device and PCB. For most uses of the device you'd not be expecting
anything remotely like 54A (5A might be a typical value if you want to use the device without a
large copper block heatsink!).
Any then the other issue to consider is that manufacturers want their device to look as good as
possible. No good engineer would be taken in by these values - in practice this device is not
usable at the max continuous rating, its simply far too expensive to get the heat out of the chip
when its this small, you'd choose a much better package if you needed to lose 23W continuously.
In short just ignore current specs for a MOSFET, the on-resistance is what matters in all practical uses!