Why does TO-220 79xx voltage regulators conduct the input voltage to the heat sink?

I find it somewhat dangerous to connect a large heat sink on 79xx series voltage regulators. Because they transfer all the negative voltage to the heat sink. Why just they didn't make the 2nd pin of 79xx series GND, as it is for 78xx series? GND is safer in some sense compared to large negative voltage levels. Is there any reason for their pin configurations being different?

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... an then somebody came up with heat sink mounting/insulation kit. –  Nick Alexeev Jun 15 '12 at 6:43
Wild guess, because of the construction/layout of the die. I think the die is directly mounted on the base plate / heat sink. So whatever voltage the die caries, is transfered to the heatsink too. With GND on a 78xx being the lowest voltage and $V_{IN}$ being the lowest voltage on a 79xx regulator and probably the use of same kind of semiconductor (and doting), this is just the way it turned out. I agree it is sometimes annoying. –  jippie Jun 15 '12 at 7:19
@Nick Alexeev But your example is way overpriced. The two isolating parts cost 0.06€ here: reichelt.de/Waermeleit-paste-folien-scheiben/GLIMMER-TO-220/3//…; reichelt.de/Waermeleit-paste-folien-scheiben/IB-2/3//…; –  starblue Jun 15 '12 at 7:59

Almost every TO220 packaged IC that you can obtain will have the most negative circuit point on the centre terminal and tab. "Insulated tab" versions are often available in cases where an exposed negative tab is liable to be a major disadvantage in many applications.

The reason for the negative tab is that this point is at the potential of the IC substrate or die - it is the lowest potential layer in the silicon construction for power purposes. Small signal voltages lower than this may occur BUT they are dealt with specially to ensure that the substrate is not reverse biased . Connecting this ground power level directly to the tab makes most sense thermally. Insulating it from the tab has a thermal penalty and makes cooling more difficult.

In the 7805 positive regulator the most negative point = ground = tab

In the 7809 negative regulator the most negative point = -Vin = tab as it is a negative regulator.

Exception that demonstrates the rule:

An apparent exception to this rule (and an actual exception when the IC is in use) is the LM317 positive regulator which is a "floating design - the actual regulator is all above ground in normal use. In this case the most negative point when the LM317 is used as a voltage regulator is the "adjust" terminal BUT the tab is the output - which is 1.25 V more positive than the adj terminal when in use. A look at a block diagram f the IC shows that even this one obeys the "tab = substrate = most negative point" "rule". The digram below shows that tab = substrate = output BUT that the adjust terminal is then pulled to 1.25V below this level during operation. A minor node inside the IC is pulled to one Vbe below IC "ground" with the -1.25V appearing at the end of an internal resistor.

ground = tab

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More exceptions include most FETs and IGBTs, where the tab is typically tied to the drain/collector, the highest voltage point on the device. If you're not careful, you can have a high-voltage heat sink, which can cause clearance issues with traces around it. Not to mention high-frequency switching turning it into a big antenna! –  Stephen Collings Oct 10 '12 at 14:38