![Photo of [DENKI OTAKU] with his test circuit and oscilloscope](https://hackaday.com/wp-content/uploads/2025/11/TO-247-4-banner.jpg?w=800)
Over on YouTube [DENKI OTAKU] runs us through how a 4-pin MOSFET works and what the extra Kelvin source pin does.
A typical MOSFET might come in a 3-pin TO-247 package, but there are 4-pin variants which include an extra pin for the Kelvin source, also known as source sense. These 4-pin packages are known as TO-247-4. The fourth pin provides an additional source for gate current return which can in turn lessen the effect of parasitic inductance on the gate-source when switching current, particularly at high speed.
In the video [DENKI OTAKU] uses his custom made testing board to investigate the performance characteristics of some 4-pin TO-247-4 MOSFETs versus their 3-pin TO-247 equivalents. Spoiler alert: the TO-247-4 MOSFETs have better performance characteristics. The video takes a close look at the results on the oscilloscope. The downside is that as the switching speed increases the ringing in the Vds waveform increases, too. If you’re switching to a 4-pin MOSFET from a 3-pin MOSFET in your design you will need to be aware of this Vds overshoot and make accommodations for it.
If you’d like to go deeper with MOSFET technology check out Introduction To MOSFET Switching Losses and MOSFETs — The Hidden Gate.
The company that originated the idea was in fact the components division of Motorola. And it wasn’t four pin devices, it was in fact five, the kelvin connection existed in back between those two, and a resistor was stuck between them, typically between 470 ohms to as much as 4.7M ohms depending on the environment.
Wait, people still use through hole components? 😬. You should see the performance benefit of TOLL fets.
Thermals. Heat dissipation with TO-247 is much better than TOLL. TOLL thermal pad is on the bottom so you have to take the heat out though the board which is much less effective than slapping a big heatsink on a TO-247
Scope envy here!
Test equipment is wholly the domain of Murphy.
The moment you buy a new piece of test equipment, you will soon encounter a problem which requires a superior piece of test equipment.
Just bought a 200MHz scope? Next day you’ll need a 500MHz scope to probe a board.
Just bought a 100MHz logic analyzer? Next day you’ll need to debug a 200MHz RAM bus or whatever.
It never ends.
There are always going to be folks cutting their teeth on someone else’s miscalculated purchase. When I buy gear, I try to buy name brand and gently used. I’m not making enough gig cash to justify dropping several thousand on super scopes, 10 digit meters and lab power supplies, IrDA GPU reflow systems, or mantis inspection binoscopes. I have what I have because it was affordable and solved a problem. I’m just buying what someone else realized wasn’t enough.
These days I start with the Aliexpress version which is usually ridiculously cheap and mostly works – often they turn out to be more than good enough for home tinkering.
I could not take my eyes of of it. It was so big! lol!
That PCB cutout that you can clamp an inductive probe around is pretty clever. Not seen something like that before.