I assume this is an off line switcher. If so, the FET is at mains potential
and the required creepage and clearance distances is the same as the
hot-to-ground creepage requirement(4 mm or is it 3 mm? for creepage).
I would not use the chassis as a heatsink myself. Even dirt cheap PC power
supplies don't do this. But if you must, is a heatsink pad defined as single
insulated? You might need two pads to constitute double insulation. Another
route is to use a full-pack FET (already encased in an insulating material)
along with a pad. But do we define creepage under the assumption that one
insulation layer has a defect? If so then the full-pack and pad will not do
if we assume that the pad has a defect.
The FET could be held with a clamp of some sort. This FET-insulation-chassis
arrangement will have quite a bit of thermal resistance> You can calculate
this and might find that you are better off thermally and cost-wise with a
small heatsink attached directly to the FET and floating at mains potential.
To reduce the E-field noise being thrown around, by the heatsink connected
to the FET Drain, you can use a full-pack FET and connect the heatsink to
the floating circuit common. In this case you will not need a pad and I
wouldn't bother with heatsink paste. A package such as a TO220 or TO218,
when mounted on a flat heatsink, will work just fine without that messy
paste. I also wouldn't worry about the junction temp if it is 100 C and even
125 C is acceptable. I see engineers spending too much time and money trying
to keep junction temperatures luke warm. I can get into a whole page on
reliability in the real world (and how to calculate it and how some methods
are bogus) but will spare you at this time.
Dave Cuthbert
Micron Technology
From: Peter L. Tarver [mailto:[email protected]]
Sent: Friday, January 17, 2003 8:26 AM
To: EMC-PSTC (E-mail)
Subject: RE: Is this a case of basic insulation?
Vic -
Your assumption that at least Basic insulation is required
is correct. However, creepage distances are based on rms
voltages, not peak.
The below assumes that the voltages you mentioned are
present on the heatsink of the FET.
Assuming that a true rms meter will indicate an rms voltage
between 250 Vrms and 300 Vrms, the minimum require creepage
>from Table 2L is 3.2 mm.
Based on the peak voltage you mentioned, the minimum
required clearance distance from Table 2H is 2.0 mm, plus
the added distance from Table 2J of 0.2, gives a total
minimum clearance distance of 2.2 mm.
Without knowing the particulars of the FET and its heatsink
in any great detail, an insulating pad is an appropriate and
common means of complying with the standard. Unless you can
demonstrate that the pad will not compress to less than 3.2
mm thickness in the application, you'll probably need a
shouldered washer or something similar to add additional
creepage between the screw shank and the FET's heatsink.
Polymeric screws are available, but I can't speak regarding
their use in an elevated temperature.
Regards,
Peter L. Tarver, PE
Product Safety Manager
Sanmina-SCI Homologation Services
San Jose, CA
[email protected]
> -----Original Message-----
> From: Gibling, Vic
> Sent: Friday, January 17, 2003 12:00 AM
>
>
> We drive a FET based H-bridge from
> rectified 230VAC mains; thus
> 320Vpk across the bridge. We have assumed the
> insulation between the
> rectified power return and chassis (Class 1
> product) is basic, thus
> requiring 4mm creepage (IEC950). The proposed
> FETs need to be attached to a
> heat sink and we would like to use the chassis
> for this purpose.
>
> Our problem is that the FETs have around
> 2mm creepage between their
> exposed heatsink surface and the fixing screw,
> insufficient to meet the
> basic insulation creepage distance.
>
> Is our interpretation regarding basic
> insulation correct/reasonable?
>
> If we use an insulative thermal pad between
> the FET and chassis,
> does the compression of the pad exclude the air
> path thus offering
> sufficient protection? I am aware of such pads
> offering 4.5kV breakdown.
>
> Thank you
>
> Vic Gibling
> Compliance Engineer
>
> e2v technologies Ltd
> Waterhouse Lane
> Chelmsford
> ESSEX CM1 2QU
>
> Telephone: +44 (0) 01245 493493
> Direct Line: +44 (0) 01245 453352
> Facsimile: +44 (0) 01245 453410
>
> E-mail: [email protected]
> Internet: www.e2vtechnologies.com
>
>
>
> -------------------------------------------
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