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Registered Member #97
Joined: Thu Feb 09 2006, 05:40PM
Location:
Posts: 61
Seriously what is killing my mosfets? Here is the situation. I have successfully built Steve Wards Mini-SSTC and it has been working beautifully, until recently. The problem started out that the coil would "skip." By "skip" I mean that it would stop for a longer time than it should have with the interrupter, for a noticeable time, and sometimes it wouldn't start right away. Then I started powering the half-bridge with a small 12 volt 450mA walwart because it is perfect for a lightbulb acting as a plasma globe. The coil wouldn't skip at this low power level, but then suddenly it died. I thought that it was the 74HC14, but that wouldn't explain what was killing the mosfets. I changed mosfets and tried it again at the low power level and it worked, but then I turned it off and let it sit for a little while and the when I came back the FETS were dead.
So any ideas on what the problem is? Is it the low power? Could a faulty 74hc14 chip hurt anything? The worse I can see it doing is going high, but that shouldn't affect the FETS.
Registered Member #139
Joined: Sat Feb 11 2006, 11:01AM
Location: Perth, Western Australia
Posts: 358
I know what it's like to remove ones hair over an SSTC! Do you have a scope to see whats happening at all? What's the output like? is it a good output? or way too little? It could be that your primary is reversed? Are you using a 555 as a start-up oscilator through a 100K pot? If you are, this could be a cause of the problem, maybe the aerial isn't close enough, and it still running out of tune from the osc.
Registered Member #97
Joined: Thu Feb 09 2006, 05:40PM
Location:
Posts: 61
I have a scope. The gate wave forms are a little on the sinusoidal side, but just barely. The sparks are tiny when run off the walwart. I am not using a 555 for start up. Should I? I doubt that would help me from killing my FETS. I am really still confused.
Edit:
I checked the mosfets that were "dead" and they came up fine. So I put them back in and checked everything. To check everything I have a 555 output attached directly to the antenna and then scope everything out. The wave forms looked fine, the interupter was working, but I couldn't get any output from the half-bridge. Then I scoped out the waveforms directly across the mosfet and one of them was blank. So, it looks like all of my trouble is most likely a faulty connection somewhere around that mosfet, but I can't find it. Oh well at least I know the problem and that makes it 10^6 times easier to fix.
Edit #2:
Definetly a bad connection. I am using the connections pictured. I don't like them so anybody got any suggestions on better connectors?
Registered Member #139
Joined: Sat Feb 11 2006, 11:01AM
Location: Perth, Western Australia
Posts: 358
I use ceramic versions of those connectors for the power etc on the bridge. If you are sure the physical driver board is running ok, then I would definately have a look at the connections on the bridge. Have you tried reversing the polarity of your secondary?
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
I have found there is 2 ways to make a reliable connection... 1. solder it 2. use a sharp contact that digs into the leads and makes real physical contact. For screw terminals you need like those 'star washers' that have a buch of shap edges arround them, and for wire connections you need to have terminals that really dig in--like when you pull out the fet the leads are all distorted looking and there is a shiney spot where the screw was. Also, don't even think of having >1 wire in teh connector. If you must have multiple wires, solder them in one big blob and then put that in the connector.
Really those things you posted are the bast I have found for fets, for wires I have been using these beefy gold plated speaker terminals (there is like a 2mm screw with a square captive washer with spikes on the sides to really grab that wire) tightned to about as thight as I can get them.
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What is killing my mosfets?
