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Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
Hey all!
I wanted to build a small and compact 24VDC SSTC for my mother (a very late mothers day gift), but a modified SSTC5 with a halfbridge of 75V 209A MOSFETs are not the way, it can only do 1cm spark if drawn, no brekaout on its own.
Its a small 550 - 600 kHz coil.
Just as I wanted to change the FETs for some 500V and rebuild it to be a wallplugged SSTC instead I took the power off the bridge but did not turn off the driver section, then I went to my flatmates room and talked abit.
To my BIG slash HUGE surprise the flourescent tube was still blinking when I came back to my room, 4-5 minutes after it was turned off.
I am now wondering where the energy comes from and have come up with the following
- Picking up some radio signal - Energy somehow floats through GDT and leaks into the bridge, since it only works with drivers on
Here is a short video showing the unplugged bridge, the only plugged in cable is the thin black one to the drivers.
EDIT!!!!: to my surprise, after writing this thread, I looked over my shoulder to the worktable and saw the flourescent tube was lit, no longer flickering, so I measured bus voltage again, 17VDC and slowing climbing....
I dont know what to think anymore.
2ND EDIT: Could it be leaking GDT voltage that is seen as AC and rectified through the bridge to charge up 17VDC over the bridge cap?
Registered Member #289
Joined: Mon Mar 06 2006, 10:45AM
Location: Conroe, TX
Posts: 154
Yes, the power is supplied by your gate driver. There is capacitance between the gate and the drain of a MOSFET, and hence RF current flow between the two. On a system as small as yours this has a significant effect. On larger systems it is less noticeable.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
hvguy wrote ...
Yes, the power is supplied by your gate driver. There is capacitance between the gate and the drain of a MOSFET, and hence RF current flow between the two. On a system as small as yours this has a significant effect. On larger systems it is less noticeable.
That explains why I have not seen it before on my larger coils, on a sidenote the drivers selfdestructed, tube had gone out and one of the driver ICs were hot enough to melt the fat off of my finger and leave me with a hurting burn :(
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Gate-drive power will definitely capacitively couple through MOSFETs to some extent when they are un-powered. This is because device capacitances are much higher when there is no drain-source voltage across the device. The so called "Miller capacitance" (or Cdg) in MOSFETs is well known to couple RF between ports of solid-state RF amplifiers in both directions when no power is applied, albeit quite lossily though.
The action of the MOSFET's intrinsic body diodes may then be to rectify this power and charge the DC bus reservoir capacitor. That is probably what is happening, especially if the lamp stops flashing when you turn off the gate drive circuitry.
However, I'm impressed with how externsively and brightly the lamp appears to be illuminated considering there is no B+ applied to the bridge! Maybe the resonator is indeed picking up the signal from a nearby AM broadcast station. Waving a scope probe near the top of the secondary should easily settle the discussion. If you see an amplitude modulated RF envelope then it's a local MW broadcast station that is ringing up the coil.
Have you tried shorting the two DC bus rails together? Does this stop the lamp flashing?
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SSTC picking up energy from unknown source?
