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LAN_403

Avalanche

Fri Apr 21 2006, 06:39PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

There's nothing like having some free time to work on projects for once! I don't get much spare time to work on projects, but thought I'd get a few things ready for the Derby (UK) Teslathon which I plan to attend in a few weeks time.

Anyway, I'm working on a Flyback driver for a mini SGTC. A few questions/observations/thoughts. Firstly, I decided against using the ZVS circuit, mainly because I don't fully understand it, and probably wouldn't stand a chance at de-bugging if it didn't work! So I went for a halfbridge, which I constructed from IRFP460s, and some metallised polyester caps (probably not ideal but never mind). I was going to build a 555 based oscillator to drive the GDT, but I was in a lazy mood so I built up another one of my PLL boards instead (Steve C's circuit), for a quick way to get a squarewave. First thought - has anyone actually made a PLL flyback driver? If you could get the feedback sorted, I reckon it would be a pretty neat way to drive a flyback, driving it at resonance whatever load you put on the flyback. ZVS performance without the voltage limitation, and without the huge resistors! The feedback could possibly be obtained from some of the un-used pins on the flyback as well, or by winding a few tens of turns onto the core. Just thoughts anyway, I haven't even got first light yet.

Secondly, I noticed a few odd things about my flyback, it came from a 17" monitor I gutted last night. The first strange thing is that there is no continuity between the two 'ground' pins on the bottom of the flyback. I have the pinout for my flyback (see attachment) as it was printed on the PCB, so I took a picture before desoldering. Another thing - I cannot measure a resistance between the HV output and any of the pins on the flyback. Could this be because it contains an internal voltage multiplier?

One more thing. I think I have found the primary, the 'VCP' and 'B+' pins have a resistance of .5R. I assume the only reason people wind their own primaries is to get a higher output?

Anyway, some pics :

ragnar

Fri Apr 21 2006, 11:09PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

There is no voltage limitation with the ZVS. Just use a high primary voltage with a reasonable series inductor and you'll get 'normal' purplish arcs starting at 4".

A smart bloke like you would be able to fiddle around for about 5 minutes and 'get the feel' for it ^^

Most flybacks don't have continuity between the ultor (HV output) and ground... mainly for the reason the voltage drop across the rectifying diodes / multiplier can be in the tens of volts.

Avalanche

Fri Apr 21 2006, 11:20PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

I'll have to try the ZVS then I suppose

I've got my halfbridge working, with 10mm arcs from a 12V lead acid battery (7T primary). I'm not sure if that's good or not in terms of flyback drive-age

I just found the pin it most liked to arc to, that must be ground.

I think I'm driving it at resonance (16Khz-ish), yet the MOSFETs are getting very hot

I guess I just need to work on it some more.

I know the general rule of thumb is 1kv/mm for voltage, but is there any easy way of guesstimating the current?

Hazmatt_(The Underdog)

Fri Apr 21 2006, 11:49PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

To check for gnd you need about 40Vac or so into the Hot lead from the flyback, then you can poke around and measure the difference on the pins and find the windings that way.

You said there are 2 gnd's? I didn't see 2, but I could be wrong. You sure got a nice TV though, they spelled everything out for you, wish I had that kind of luck once in a while.

Marko

Sat Apr 22 2006, 01:55AM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I usually grab the hot lead and sweep with it around base of flyback when driver is powered on,
and GND is the pin where it starts arcing.
I do this at low power so I dont shock myself. or roast something, arc the primary internally, etc...

One more thing. I think I have found the primary, the 'VCP' and 'B+' pins have a resistance of .5R. I assume the only reason people wind their own primaries is to get a higher output?

I used one of flyback's internal primaries with my flyback-converter circuit and got far better preformance than with externally wound primaries.
COupling is better and number of turns obivously suited my driver.

This is actually a bit tricky, ground pin uses to spark to the primaries, transistor, focus pin (especially if pot is flipped in wrong direction) but fortunately circuit has enough Magic resistance to withstand it.

I would actually need to put flyback completly under oil to get rid of that.

Desmogod

Sat Apr 22 2006, 03:37AM

Registered Member #139 Joined: Sat Feb 11 2006, 11:01AM
Location: Perth, Western Australia
Posts: 358

My avatar is of my miniSGTC that I built about 6 months ago.
(It's on the old board somewhere.)
That used a basic ZVS @ 50V, a beer bottle cap, and a basic copper spark gap. It worked very well on a RSG until it burnt out the motor.
The coil form was 11x45mm, wound with 0.125mm wire. The output was really good for such a small setup. Go the ZVS, If you can build a PLL and get it running, you can knock up a ZVS.

Dr. Dark Current

Sat Apr 22 2006, 11:07AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

It has two gnds not connected together because one is gnd for sceondary and the other is for screen dividers and a HV filtering cap. I have a flyback that must have connected three pins to ground otherwise it arcs everywhere on its base.

Connect a HV resistor (few 100k ohm) to the hot lead if you dont want to damage your HV cap. (you know you have one when the output is more like buzzing/sparking with a low rep. rate (few Hz) as opposed to continuous arc without the cap).

J.M.

Dr. Shark

Sat Apr 22 2006, 12:40PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Jmartis wrote ...

Connect a HV resistor (few 100k ohm) to the hot lead if you dont want to damage your HV cap. (you know you have one when the output is more like buzzing/sparking with a low rep. rate (few Hz) as opposed to continuous arc without the cap).

J.M.

Very good point! I have noticed this problem, but I never though about curing it with a resistor.

=Jmartis wrote ...

I think I'm driving it at resonance (16Khz-ish), yet the MOSFETs are getting very hot I guess I just need to work on it some more.

As long as there is not capacitor in series with the primary, there is no such thing as a resonant frequency.
I would also suggest to go with the Marzzilli Flyback driver, there is practically nothing to go wrong, since it is such a simple circuit. It is also a great advantage that you dont need two separate power supplys for logic and power.

Dr. Dark Current

Sat Apr 22 2006, 12:45PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

joe wrote ...

As long as there is not capacitor in series with the primary, there is no such thing as a resonant frequency.

I cant agree there. When I connect a flyback to my halfbridge (untuned), it outputs hot arcs starting at ~5-10mm (10T prim at 60V) but when I tune it the HV output (on AC flyback) starts throwing streamers in air and it easily makes 4cm sparks. That "in tune" region is very small and usually higher in freq. (on my bigger AC flyback it is ~50khz)

J.M.

Hazmatt_(The Underdog)

Sat Apr 22 2006, 07:43PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

No resonance?!?!?!?!?!? What the #$%

The core most certinaly resonates, and the coil does as well, which is exactly why you get maximum output in a narrow frequency band!

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