Parasitic Capacitance between TO220 Tab and Ground

Semiconductor fab limitations. Some are offered with an isolated tab, but this is acheived at the cost of a physical isolation layer.

There are patented 'well' technologies that build the mosfet in an isolated tub to ~ ground the substrate. Power Integrations use this in their 'Top' switches. They are not sold as discrete devices.

You can increase the thickness of the isolator or try screening the part from a quieter source-related node.

The distance criteria argues for aluminium or berylium oxide.

If you don't have to ground it, it makes things simpler. It likely does have a return, either directly or through a capacitor, to a quieter source-related node in the circuit, for screening purposes.

RL

They make ceramic insulators. Used in R.F. for those type of problems, alone with handling the heat of course! :)

An example here but not exactly what I was thinking of by may work.

Thickness = 1.78mm

Assuming pure AL2O3 (sandpaper :P)

Al2O3 dielectric constant = 9.1 (air ~1,rubber~7)

A=TO220 area say 16mmx9.6mm A~154mm^2

C=er*eo*A/d C=9.1*8.854xE-12*((154mm^2/1.78mm)/1000) C~7pF

If I switch from fullpack case to metal tab mosfet+ceramic insulator, I'll get a case to heat sink capacitance of 7pF.. Or if I use the fullpak case, I'll get a case to heat sink capacitance of C1C2/C1+C2 = 4.4pF All's well if the mosfet temp is ok.

Interesting..

The best ratio of thermal conductivity to dielectric constant (ignoring diamond) is BeO. AlN is close.

The best insulator is no insulator.

John

That sounds like it! Thanks.

I was googling for more info and tripped across EMC for Product Designers by Tim Williams.. Page 309 (google books) has a bit on problems of high drain dv/dt and heat sinks.

Is that the same Tim Williams on here? Gee...Just how many book writers are on here.. (Winfield Hill, Don Lancaster...)

A vacuum.

I understand I've also written books on filters and etc., but I personally have no memory of doing so. :^)

Yes, different Tim... it's a fairly common name, and I've met or seen a fair number of Tims and Williamses in electronics. Even worked with a Jim Williams once (unrelated to the more famous one). ;-)

As for EMC... why does it have to be grounded? Just has to? Is 12pF really THAT much? You could put a heat spreader style washer between transistor and heatsink, as if shielding it. Double the insulators, more capacitance between "source referred connection" and GND, but it gets rid of your 12pF directly at least.

Tim

I'm attaching the mosfet to an earth grounded chassis to dodge adding a heat sink. The chassis is earth grounded for electrical code.

I might try that. :)

Last time I dealt with this, rather than bolting the MOSFETs to the case, as Tim W. suggests.. I put an internally connected heat spreader that was connected to circuit ground and then an insulator to the (earthed) case. Quite large currents circulate between the heat spreader and circuit ground when the switching frequency is high so you have to think out the layout. Otherwise you're building a kind of transmitter...

Best regards, Spehro Pefhany