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Low output and mosfets burn

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brandon3055

Wed Jul 18 2012, 10:45PM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

The cap on the primery is for dc blocking only unless you wer trying to build a drsstc try removing some and increasing your prymery terns to about 20 also the gdt looks like it has a lot if leakage try removing the outer cover and winding it tighter and as far as I know to Manny terns are less of a problem then two few so try increasing it to 16
@mads barnkob irfp460's have good insulation around the mounting hole

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Wed Jul 18 2012, 10:46PM

Registered Member #5171 Joined: Tue Jun 05 2012, 11:32AM
Location:
Posts: 67

1)
so i should remove all the caps and let only two of them in parallel just like in the scheme of Mads?
2)
should i remove the CAT5 cable's external sleeve?
should i also increase the number of windings around the GDT core?

EDIT
I was thinking, since the pop of the fets accurred after i moved the antenna, could be a tuning problem? I'm not an expert, but with wrong frequency you have no energy transfer on the secondary, that'd be like a short or not?
That would explain the mosfets blowing up and the high amps drain right?

zzz_julian_zzz

Thu Jul 19 2012, 12:13AM

Registered Member #3964 Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332

Follow the scheme of mads, just add 1 non polar/ mylar or poly propelene cap of 10 uf across your Voltage rail (VDC) => your design wont work properly in higher voltage without this. This is due to your large inductance bridge. Mads, layout is much much more neater than this, all fets and supply are very close to each other, so maybe, he did not need the snub caps. (this is for transient voltage snubbing)

but your problem right now is getting decent feedbacking, try making the driver circuit board to a more neat PCb, sometimes, Breadboard have bugs on it! so i dont trust it specially on high RF applications. as i have said earlier on this thread try swaping the primary wire from fets. remove the capacitor parallel, you dont need that to make it work, also, check your resistor and capacitor in series with 74hc14..

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Thu Jul 19 2012, 12:16AM

Registered Member #5171 Joined: Tue Jun 05 2012, 11:32AM
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Posts: 67

Thanks for the help.
The driver part is not made on a breaboard, but on a project holed board (everything soldered).

So, what you mean is remove the capacitors in series with the primary?

I didn't exactely understand the middle part, about the "snubbling capacitor"
Could you explain better?

Thanks.

brandon3055

Thu Jul 19 2012, 12:51AM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

I think he means use caps in series as shown in schematic and ad a 10uf cap to the hvdc input (as close to the fets as possible) and I would still recommend increasing the number of primary turns atleast until you get it working (more prymery terns means lower current draw)

Edit can you pleas post a link to the schematic your using

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Thu Jul 19 2012, 01:00AM

Registered Member #5171 Joined: Tue Jun 05 2012, 11:32AM
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So, the snubbling cap is a capacitor that goes in parallel with the HV DC in?

driver schematic

bridge schematic

brandon3055

Thu Jul 19 2012, 03:54AM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

From what I understand yes
Edit the gdt in that schematic is 15:15:15:15:15 not 10:10:10:10:10

Edit I think the way it works is the large filter cap removes large fluctuations in the power supply and the snubber removes smaller ones

zzz_julian_zzz

Thu Jul 19 2012, 01:20PM

Registered Member #3964 Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332

just connect the 10 uf paralleling the HVDC supply ( that's it ) snub caps. these capacitor will snub or cut any transient voltages that runs through your bridge due to excessive inductances (those inductance are consist of all the wirings on your bridge). every millimeter added on your layout or connecting wires will affect the transient voltage which will kill your fets. put the caps nearest as possible to the fets.

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Thu Jul 19 2012, 01:21PM

Registered Member #5171 Joined: Tue Jun 05 2012, 11:32AM
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Posts: 67

So, what i have to do is to put a10 uF capacitor in parallel with the DC?
just after the smoothing cap?

zzz_julian_zzz

Thu Jul 19 2012, 01:27PM

Registered Member #3964 Joined: Thu Jun 23 2011, 03:23AM
Location: Valenzuela City
Posts: 332

yes thats it!

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