Question about 78xx AND 79xx regulators in TO-220 cases

I'm curious about a little something. Why do positive voltage regulators
(like 7815) have their case connected to GND and negative ones (7915) have
it connected to Input voltage (Vin)? Seems to me it would be more logical
and practical if both had it grounded. Can someone enlighten me on the
reason for this? TIA

 

The silicon chip is soldered to the can for best heat transfer, and in
most linear IC processes the silicon substrate has to be the most
negative potential to allow using back-biased junctions for isolation.

There are a few exceptions, like LM317s I think.

John

 

Typically the substrate of the IC is eutectically, bonded to the case
or heat sink, in case of a TO-220. The substrate, in a typical NPN
process must the be the most negative point on the device. In the 78
series, gnd is the most negative, in the normal configuration, and
V-in in the 79 series.
Dan