Oddities in high powered component outlines

[KX36]

Hi all, I'm Matt, I'm an amateur, and I have a few informal (probably kinda newbie) questions which I thought this might be a good place to ask.

1) How come there are specifications for minimal clearances for high voltages, but then high voltage components often have less of a gap between their leads than this, and especially when you look at the clearance between the pads on a PCB, they might seem to be far too close? E.g. There are high voltage (600V+) transistors in TO-220 packages, but the gap between the pins is pretty tight and that between pads if one were to use standard TO-220 packages in software is even moreso?

2) How come there are high power transistors such as IGBTs rated for e.g. Vce=1200V, continuous current=20A, yet they're in plastic packages that don't lend themselves to heatsinking?

[HLA-27b]

Interesting questions. I'm curious too.

To speculate a bit, considering there is 1.1mm or more gap between the legs of a TO-220, in theory it should be ok up to 1000V. Also maybe they assume conformal coating, potting or immersion in oil?

[Neilm]

Firstly, it depends on the specs you have seen for high voltage clearance. If they are for mains circuits then you have more than the mains voltage to consider - there are also transients. Even on ordinary mains there is provision for a couple of kV transients. The transients can be caused by several things, heavy machinery switching, lightening strikes and the like.The extra clearance in the standards are to ensure that there is no breakdown when these transients occur.

The transistors or FETs in TO-220 would be used after whatever protection is used so there would only be the mains voltage (or rectified mains) to deal with.

On your second question, the ones I have seen had a point on the back where a heatsink could be attached, but the rest of it was plastic. Do you have any particular examples you had a query over?

Neil

[KX36]

Thanks for the replies.

The clearances I was referring to is the simplified IPC table as written in www.alternatezone.com/electronics/files/PCBDesignTutorialRevA.pdf , which says 2.5mm clearance for 500V on an external surface of a PCB used at <3050m altitude. I haven't looked in depth at the original IPC specifications.

The IGBT probably wasn't the best example for me to choose because I've not had my hands on one, but I'm sure I have seen "TO220CP" plastic packages that don't have exposed metal to touch a heatsink, and I remember reading this is done for electrical insulation from the heatsink or whatever it's bolted to, yet I see specs on the datasheet for these that are identical in voltage and current ratings to the same model with the more standard TO220 package. Sometimes these ratings are very high, far higher than I would think the thermal resistance of plastic casing would be able to dissipate in heat.

Here's a surface mount package D2PAK IGBT rated at 11A, 1200V http://uk.rs-online.com/web/p/igbt/6503482/
and a TO-247AC package one rated at 20A, 1200V http://uk.rs-online.com/web/p/igbt/5411837/
If these are entirely enclosed in plastic, I can't see how they could keep up with those ratings.

[Neilm]

Neither of the devices you have shown are TO-220 packs, one is surface mount the other is a TO-247 which is bigger than TO-220, although it is a similar shape.

The surface mount part would use the PCB as a heatsink. A designer would use copper pour on the PCB to act as the heatsink.

The through hole part uses an isolated tab. If you find a component that is available in both isolated and non isolated you would find that the isolated version has a lower power rating.they can be bolted to a heatsink, but the thermal transfer is not as good.

Yours

Neil

[Zero999]

If you read the real standards you'll discover that there are different classes of insulation, depending on the safety requirements.

Functional insulation: only required to for basic functionality. The insulation only needs to be strong enough to withstand normal operational voltages and transients. The clearance between the component leads fits into this category.

Basic insulation: failure may result in an electrical shock, if the earth connection is poor. A fair safety margin is built-in to the calculation to ensure that failure is unlikely. This class of insulation is required to separate the live parts (i.e. the mains) and metal parts of the equipment which are connected to the protective earth/ground conductor, for example a soldering mains powered iron tip.

Supplementary insulation: an additional layer of insulation to basic insulation basic insulation so that no single point failure can result in an electrical shock. Basic insulation, plus supplementary insulation is equal to double insulation. Supplementary insulation should have the same level of robustness as basic insulation.

Reinforced insulation: failure may result in an electrical shock regardless of whether the earth connection is good or not. An enormous safety margin is built-in to the calculation to ensure there's no chance of failure during operation. This class of insulation is required to separate live parts of the equipment and the user, for example the insulation between the mains and secondary side of a 230V to 12V wall wart. Reinforced insulation offers the same level of protection as basic insulation plus supplementary insulation.

Do a bit of Googling and you should be the creapage and clearance distances required to fulfil the requirements for all of the above. You'll also find that the distances vary depending on the humidity/pollution class, altitude and the maximum transient voltage.