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Grounding flybacks more likely to cause them to fail?

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Kiwihvguy

Sun Sept 03 2017, 12:18AM

Registered Member #3395 Joined: Thu Nov 04 2010, 08:42AM
Location: Christchurch, New Zealand
Posts: 193

Hi all,

As much as I dislike participating in a trend, I still tinker with my ZVS flyback driver, mostly because it's such a simple circuit that yields great power density, thus big arcs which we all like.

In my recent break, I resurrected my existing ZVS after troubleshooting concluded it's current flyback was fried (the fried one was this: http://www.donberg.ie/catalogue/line_output_transformers/hr_7927.html). After replacing the flyback transformer with another Eldor unknown part number, it worked again much to my pleasure. My existing setups grounds the low side of the HV winding, located at the base, to mains earth, to increase the safety of the setup by eliminating floating voltages and static charge buildup. Note that this current flyback and the following 2 were connected to the exact same ZVS driver, same capacitance values, only the physical location differed.

However, after about 15 minutes total operating time on 36VDC input from some SLAs, the output voltage suddenly dropped to 6kV from it's usual 30kV output at 36VDC input and the arcs were disappointingly smaller, which I concluded that either the HV windings or internal rectification failed (though I thought a failed diode wouldn't reduce the voltage that much?).

I then replaced this NEXT failed (

) Eldor with a Phillips AT2076, and surprise surprise, after about 10 minutes of operation on 36VDC, it again experienced a similar failure, except with an light audible snap and the voltage output dropped significantly, meaning another failure.

First I just assumed the ZVS driver was too powerful at 36V, given flyback transformers operate well out of their range in a ZVS, however I also wondered whether grounding the flyback made failure more likely. I thought that grounding the flyback causes a greater internal voltage stress as opposed to leaving the both HV outputs floating. The evidence I have that suggests otherwise is the first above mentioned flyback was also an Eldor brand flyback (although maybe higher voltage rated than the second), which endured a much longer lifetime on the ZVS in an earthed setup, probably 1-2 hours total operation.

I'm interested to hear other people's experience on this and perhaps receive a nice technical explanation?
I am currently in the 2nd year of electrical engineering, but we won't be studying a course in high voltage and grounding till next year at least.

Erlend^SE

Sun Sept 03 2017, 09:47PM

Registered Member #1565 Joined: Wed Jun 25 2008, 09:08PM
Location: Norway
Posts: 159

Well, how far can it start a spark?
The transformers are generally rated 30 kV+

ZVS is known for lots of current (way above rating), so you can actually cook the transformer.
If you want the voltage, a flyback driver would work better.

Patrick

Fri Sept 15 2017, 02:40AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Kiwihvguy wrote ...

I am currently in the 2nd year of electrical engineering, but we won't be studying a course in high voltage and grounding till next year at least.

Your asking some complicated questions since i can see your exact setup. Generally ive always had the best luck with driving a flyback as it was intended by the manufacturer.

That being said, Im looking into custom planar ferrite transformers for the benefits of all of us here on the forum. The internals of the flybacks are always a problem for us as they have magic components and values inside.

You can look up my work here on the forum somewhere.

Patrick

Fri Sept 15 2017, 02:41AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Patrick wrote ...

Kiwihvguy wrote ...

I am currently in the 2nd year of electrical engineering, but we won't be studying a course in high voltage and grounding till next year at least.

Your asking some complicated questions since i cant see your exact setup. Generally ive always had the best luck with driving a flyback as it was intended by the manufacturer.

That being said, Im looking into custom planar ferrite transformers for the benefit of all of us here on the forum. The internals of the flybacks are always a problem for us as they have magic components and parasitic values inside.

You can look up my work here on the forum somewhere.

radiotech

Mon Sept 25 2017, 02:52PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

An answer from the past, when a flyback transformer was used to provide current for circuits
in a TV set:

A flyback transformer has a duty cycle rating. TV technicians might replace a burned out one,
only to find the new one lasted a few weeks. Mad customer! And apart from the CRT, the
flyback transformer was a very costly item.

I have a very costly analyzer to test TV scan circuits. Made useless the moment flat screen
TV sets appeared.

For NTSC (American) sets this duty cycle was around 30 uSec. That is the current from the driver was on
for that time, for each scan line.

To protect the flyback transformer from burning out, find out how they were
used in a TV set, or monitor. Lots of books out there.

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