[sdiy] Question about a FET I stumbled across in a datasheet

Tom Corbitt tom.corbitt at gmail.com
Fri May 8 20:47:35 CEST 2020


So my daughters lamp failed the other day and I opened it up to discover it
was much more complicated than I imagined it to be. Inside. along with a
Holtek 8 bit micro was the awesomely named VIPER12A; a google search shows
it to be the brains of a switching power supply circuit

https://www.st.com/resource/en/datasheet/viper12a-e.pdf

My question relates to the 4th connection shown in the block diagram on
page one (and later in the technical explanation) Just what the heck is the
bottom element shown on the FET? (the lowest element under what I'd call
the traditional source elements?) Is it the body? Looking at wikipedia I
see that the source can have a source connected to a P type island (which I
guess is connected to the substrate?)
[image: image.png]
What makes it odder to me is in figure 5 they show a constant current
source "Is" which they say is proportional to the current flow across the
fet (Id)

Is this a special feature made/incorporated into this specific FET or is
this something that you can do with proper biasing of the body?

Thanks in advance, just always trying to expand my understanding of the
various bodies electric.

Cheers,
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