Grounded casing?
Firnagzen, Tue Jun 19 2007, 08:48AMHi, I've got a question, so I thought I'd better ask:
I'm planning to build Steve Ward's mini SSTC, and since the circuit is nice and compact, I decided to make a box out of perforated aluminum sheet (Imagine a sheet of aluminum which looks like sterotypical cheese ie. full of holes) and using the entire box as a heatsink for the MOSFETs and housing for the circuitry, then mounting the coil on top. And then a thought came to me, why not ground the casing as well, since it's metal.
Now, here's the question: Would RF grounding the case be bad? Because I've got this little voice at the back of my head nagging me that I'm going to kill the MOSFETs that way. (Ps. I'm using a counterpoise, if it matters.) Or should I ground the casing to the earth wire of the house system, or not at all?
Thanks.
Re: Grounded casing?
colin heath, Tue Jun 19 2007, 06:46PM
Hi There,
It all depends if you FETs are isolated from their metal casing or if collector is tied to it. If isolated all should be fine. if not then it must float and the 2 fets must be on seperate heatsinks. although you could use sil pads and ignore all i have just said
Cheers
Colin
I hate it when my answers become questions about the answer
colin heath, Tue Jun 19 2007, 06:46PM
Hi There,
It all depends if you FETs are isolated from their metal casing or if collector is tied to it. If isolated all should be fine. if not then it must float and the 2 fets must be on seperate heatsinks. although you could use sil pads and ignore all i have just said
Cheers
Colin
I hate it when my answers become questions about the answer
Re: Grounded casing?
Firnagzen, Wed Jun 20 2007, 04:18AM
Well, they'll be IRFP460's in a TO247, so that's not entirely isolated.
But as I understand it, the entire casing, should it be hooked to the RF ground, will be floating at a semi-high voltage, correct? Won't that be able to arc over even a silicon pad?
Firnagzen, Wed Jun 20 2007, 04:18AM
Well, they'll be IRFP460's in a TO247, so that's not entirely isolated.
But as I understand it, the entire casing, should it be hooked to the RF ground, will be floating at a semi-high voltage, correct? Won't that be able to arc over even a silicon pad?
Re: Grounded casing?
CT2, Wed Jun 20 2007, 08:06PM
If it's ground it shouldn't be at any potential. But I guess if you wan't to be totally safe just ground it to your house wire, that'll make things easy. Like colin said don't forget to insulate the FETs with a silicone pad. Oh and I'm not totally sure on this one but I think if you happen to get a primary secondary flash over, your FETs will be toast.
CT2, Wed Jun 20 2007, 08:06PM
If it's ground it shouldn't be at any potential. But I guess if you wan't to be totally safe just ground it to your house wire, that'll make things easy. Like colin said don't forget to insulate the FETs with a silicone pad. Oh and I'm not totally sure on this one but I think if you happen to get a primary secondary flash over, your FETs will be toast.
Re: Grounded casing?
Firnagzen, Thu Jun 21 2007, 05:19AM
Well ok, thanks for your help!
Firnagzen, Thu Jun 21 2007, 05:19AM
Well ok, thanks for your help!
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