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Registered Member #9039
Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117
Ash Small wrote ...
I don't have any 'scope shots of a GDT waveform, but I have a couple of flyback 'scope shots with spikes and ringing.
The first one shows a spike and some ringing,
The second shows the worst ringing I've ever had, the square wave is the output from a signal generator and is inverted, because I used a darlington transister in between the signal generator and 2N3055, with some other protection for the sig. gen.
It's 'probably' the 'fairly tall spikes' that are causing your problems.
I'm hoping to get back to working on my new flyback driver soon, I've had a lot on lately, but I'm catching up with most of the other stuff I need to get on top of first.
The first image is exactly what I was seeing in my circuit, on the output of the TC4422A, when powering the FETs with 80V. So it must be that with 100V, that spike is just too much for the TC4422A to handle.
Chances are that I will have the same problem with the TO-220 version and I am going to need that diode pair on the TC4422 output to protect it.
Perhaps I should also put a snubber on the FETs, just in case.
Registered Member #9039
Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117
I think I have figured out what is killing the the TC4422.
With the drive circuitry in close proximity to the globe, there is massive RF interference going through it.
I relised that when I brought my scope probe close to the running circuit to check for spikes coming in from the GDT.
So the voltage spikes are the result of the RF not any kick back from the GDT.
The TC4422 just can't cope with this where as the darlington transistors seem to be far more robust in response to the RF.
I out to try and shield the circuitry from the RF with some Al tape perhaps.
Could those who have more experience with dealing with this sort of thing provide me with some further advice here.
You can see how I have made the thing with the globe sitting on a pedestal and the circuitry beneath it.
If I was to line the lower surface of the MDF sheet on which the globe sits with Al tape and then enclose the flyback transformer (sitting adjacent to the circuitry) in an Al tape lines PVC tube or something then is that likely to reduce the RF induced signals bouncing back and fourth through my circuitry?
Short of enclosing all the drive circuitry in a metal box faraday cage, would lning the lower
Registered Member #4074
Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335
It sounds like shielding and ground planes could help with your interference issues. Instead of trying to shield the flyback and globe itself, I'd put the sensitive drive circuitry in a grounded metal enclosure (or at least glue a ground plane under the circuit board). Shielding HF HV circuits can get messy, with insufficient insulation corona becomes a problem and you'll probably need to enclose a rather large area, in comparison to the small driver circuit.
I'd also suggest buying copper tape instead of aluminium, since you can get cheap tape meant for deterring snails and slugs (should be able to find it at any Bunnings or similar). The copper tape is infinitely better: For shielding, your metal structure HAS to be solidly grounded, otherwise the metal will act as an antenna and may make everything even worse (ex: if you shielded the flyback but didn't connect it to earth then the shielding will radiate a ton of noise and will be floating at a very high voltage, even high enough for corona and sparking due to capacitive coupling). You can't solder aluminium and the adhesive is usually insulating, which means its difficult to get a solid connection to the earth wire or between individual pieces of tape. With copper you simply solder all the individual pieces together, which makes life so much easier.
Neatening and shielding your GDT layout and connections would also be very beneficial at this point. Of course, making a new transformer with a tri-filar winding evenly spaced around a small ferrite toroid would be ideal, but there's still lots of room for improvement with your current version. Using the shortest possible connections between your circuits and the GDT is best (always use twisted pair wiring, unless the GDT is mounted directly on the circuit board) and there are several options available for shielding. Your could rewind the transformer with shielded cat5 cable, or you could scavenge the shielding braid from a length of coax and thread it over the transformer's connections. Like seen on some audio transformers, you could wrap the entire transformer in copper tape (around the outside of the core of course, don't want any shorted turns!).
A combination of the grounded metal enclosure for the driver, some shielding on the GDT (along with its shortened and shielded connections) and perhaps a ground plane on your MOSFET bridge should help a lot.
Extra decoupling caps on the DC rails and a line filter on the 230V input is usually a good idea with circuits like these, although it won't solve your interferance issues. The 230V filter will just ensure your circuit isn't injecting a ton of noise onto the mains conductors.
Registered Member #9039
Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117
GrantX wrote ...
It sounds like shielding and ground planes could help with your interference issues. Instead of trying to shield the flyback and globe itself, I'd put the sensitive drive circuitry in a grounded metal enclosure (or at least glue a ground plane under the circuit board). Shielding HF HV circuits can get messy, with insufficient insulation corona becomes a problem and you'll probably need to enclose a rather large area, in comparison to the small driver circuit.
I'd also suggest buying copper tape instead of aluminium, since you can get cheap tape meant for deterring snails and slugs (should be able to find it at any Bunnings or similar). The copper tape is infinitely better: For shielding, your metal structure HAS to be solidly grounded, otherwise the metal will act as an antenna and may make everything even worse (ex: if you shielded the flyback but didn't connect it to earth then the shielding will radiate a ton of noise and will be floating at a very high voltage, even high enough for corona and sparking due to capacitive coupling). You can't solder aluminium and the adhesive is usually insulating, which means its difficult to get a solid connection to the earth wire or between individual pieces of tape. With copper you simply solder all the individual pieces together, which makes life so much easier.
Neatening and shielding your GDT layout and connections would also be very beneficial at this point. Of course, making a new transformer with a tri-filar winding evenly spaced around a small ferrite toroid would be ideal, but there's still lots of room for improvement with your current version. Using the shortest possible connections between your circuits and the GDT is best (always use twisted pair wiring, unless the GDT is mounted directly on the circuit board) and there are several options available for shielding. Your could rewind the transformer with shielded cat5 cable, or you could scavenge the shielding braid from a length of coax and thread it over the transformer's connections. Like seen on some audio transformers, you could wrap the entire transformer in copper tape (around the outside of the core of course, don't want any shorted turns!).
A combination of the grounded metal enclosure for the driver, some shielding on the GDT (along with its shortened and shielded connections) and perhaps a ground plane on your MOSFET bridge should help a lot.
Extra decoupling caps on the DC rails and a line filter on the 230V input is usually a good idea with circuits like these, although it won't solve your interferance issues. The 230V filter will just ensure your circuit isn't injecting a ton of noise onto the mains conductors.
From a practicality point of view, shielding the flyback itself would be the easiest thing to do. If I connected my PVC pipe shielding to the mains earth then would that not do exactly the same thing as shielding all the circuitry.
Apart from the RF coming from the globe itself which doesn't seem to as strong in the vicinity of the circuitry. The strongest RF around the circuitry is coming from the flyback it appears.
See the problem is I have used matrix board so there is a limit to how short I can make the solder traces. And worse I have 4 individual matrix boards with connecting wires between them which will all be acting as antennas as you have pointed out.
Which is why shielding the flyback is the easier option if iyou think I can get away with it.
I also have a wire soldered to the return of the flyback and that is wrapped around a strut and connected via a terminal block to the wire going to mains earth. So all that would need to be inside my flyback shield as well....but easily done.
There will still be a small amount of RF interference from the globe itself but, by the looks of what I saw on the scope, I can ignore that. This is not a precision circuit after all.
This RF will be less of a problem with my tesla coil because most of the drive circuitry will be in a box some distance from the coil itself. The only part that will be near the coils will be the GDT and FETs so puting that in a grounded metal box will not be too much of a problem. I will need to sheild the long connecting wires though. Any suggestions on how that can quickly and simply be acheived other than wrapping all the wires in copper or Al tape?
This is good, with my free scope I am a bit further along the road to knowledge and experience. With my previous attempts with this HV stuff I was totally unaware of the the significance of RF interference, which is rather abstract until you see it on a scope.
Registered Member #9039
Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117
dexter wrote ...
the Tc 4422 is isolated from the power section by the gate drive transformer so as long as the isolation holds should work fine regardless if 80v or 100V on the bridge i thing there is something wrong somewhere else in the circuit...
In theory yes, but the GDT and all my connecting wires are apparently acting as an antennas for the RF, which is generating some rather impressive continuous voltage spikes in my circuitry. At one stage, after I connected the GND of my scope probe, to the circuit near my TC4422 there was some just visible arcing going on.
I am actually impressed that my darlington totem pole can take all this.
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Limitation of TC4422 in Steve Ward's flyback driver schematic
