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Registered Member #230
Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284
Flyback topology is not a good way to go driving a CW due to the leakage inductance spikes which get reflected back through the transformer to the primary which invarialby destroy the switching device. This is why professional high voltage supplies are half or full bridge topology with extensive snubbing to mitigate the leakage inductance of the transformer. Flyback for high voltage supplies is limited to around 30 watts maximum( this is about what the old CRT supplies were rated at) past this point mitigating techniques start costing alot more than moving to half or full bridge
Registered Member #61406
Joined: Thu Jan 05 2017, 11:31PM
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Posts: 268
I don't know how to work it out on paper, but you could use the flyback with just a capative load, and keep adding caps in series to the voltage across the caps doesn't drop much, if the power source is 100 watts with a 10kv output, I would make the capatance reactance for the large enough at minimum to keep the current below 10mA,100kohm
Registered Member #62265
Joined: Sat Apr 28 2018, 11:26AM
Location:
Posts: 24
Plasma wrote ...
I don't know how to work it out on paper, but you could use the flyback with just a capative load, and keep adding caps in series to the voltage across the caps doesn't drop much, if the power source is 100 watts with a 10kv output, I would make the capatance reactance for the large enough at minimum to keep the current below 10mA,100kohm
Thanks Plasma, I understand now that I have to consider capacitive reactance and its effect in loading the driver.
Registered Member #62265
Joined: Sat Apr 28 2018, 11:26AM
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Posts: 24
johnf wrote ...
Flyback topology is not a good way to go driving a CW due to the leakage inductance spikes which get reflected back through the transformer to the primary which invarialby destroy the switching device. This is why professional high voltage supplies are half or full bridge topology with extensive snubbing to mitigate the leakage inductance of the transformer. Flyback for high voltage supplies is limited to around 30 watts maximum( this is about what the old CRT supplies were rated at) past this point mitigating techniques start costing alot more than moving to half or full bridge
John, this is really useful. I have a couple of boards that employ half-bridge topologies using either MOSFETs or BJTs (I haven't looked up the device numbers yet), but being the more expensive option, I have been reluctant to bring them into use prior to understanding what's happening with the cheapo drivers. So if I'm following you, a half-bridge driver feeding a transformer should cope with reflected transients and I should go ahead and employ the more expensive drivers. I'm still unsure as to how the half-bridge topology prevents HV spikes from destroying the transistors so I'll hold off until I get confirmation.
Registered Member #230
Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284
Fly backs work by transfering power when the primary switches off the collapsing field in the core transfers the energy to the secondary which is unlike transformer action leakage inductance or coupling factor appears as an opposing potential so the secondary voltage has to rise to overcome it. ransformers are nearly ideal at 1:1 ratios and hold this to 1:5 or 5:1 but as the ratio climbs so does the leakage inductance and hence the spikes (which are very fast causing dV/dt problems with snubbing and rectifiers et al). Half and full bridge transfer energy as per the ideal transformer ie changing flux in the core effects all windings on that core at the same time --not after switch off as in the flyback (or freewheeling circuit). You can measure your leakage inductance easily you measure your primary inductance twice one time with secondary open circuit and the second time with the secondary short circuit. 100-(Short circuit /opencircuit) gives transformer coupling factor. good coupling factor is 98% +. The missing 2 percent ends up as high voltage spikes and eventually heat.
Probably the best info for multipliers is here They used to have an excel spreadsheet you could download to help with design could not see it when I got this link for you I have measured some 500:1 flyback attempts from others and the coupling factor measured 64% so in a 100 watt convertor this would mean that the snubber diodes resistors and capacitors would dissipate 26watts continuously with huge peak currents and voltages ---exit silicon stage left enter stage right three legged device emitting smoke and returning to beach sand. Using hjalf or full bridge the leakage inductance fights turn off so you turn off early to make sure that there is no current flow when the other side of bridge starts conducting this for voltage fed voltage mode half or full bridge. Current fed current mode half or full bridge you do exactly the opposite you keep the first half of the bridge on until the second half is also into conduction. Current fed current mode in my opinion is the best but it requires a fairly expensive inductor in the current feed circuit but it saves heaps as snubbing is nearly not needed and dv/dt stress almost eliminated
Registered Member #62265
Joined: Sat Apr 28 2018, 11:26AM
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Posts: 24
@johnf: Thanks for a very informative post, and an excellent link which I am looking forward to reading thoroughly.
I'm assuming that the description you give for bridge action is this so-called "zero voltage switching" that my two bridged boards boast. I did try to investigate this ZVS thing but it was all very vague and seemed to be saying nothing more than that it improves the efficiency of the driver.
So in order to get a half-bridge driver and flyback transformer to power a Marx generator directly, I'm guessing I could just use a single 16kV diode (or two in series if needs be), or four diodes for full-wave rectification.
Registered Member #2463
Joined: Wed Nov 11 2009, 03:49AM
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Posts: 1546
You could wind a two turn coil through the window of the flyback, and connect this secondary to a resistor. Try various values that will damp the flyback, 820 ohms ?
Registered Member #62265
Joined: Sat Apr 28 2018, 11:26AM
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Posts: 24
radiotech wrote ...
You could wind a two turn coil through the window of the flyback, and connect this secondary to a resistor. Try various values that will damp the flyback, 820 ohms ?
I've been using simple ferrite core transformers (shown in the photos attached to an earlier post in this thread), but as it happens I've just stripped the LOPT out of a very old monitor I had knocking around, so I'm sure I'll be pursuing this line of activity in due course. Thanks for the suggestion.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
im not sure if internal diodes or caps potted within the "cylinder" type flybacks are playing a factor here. not sure which type of flyback your using.
Registered Member #62265
Joined: Sat Apr 28 2018, 11:26AM
Location:
Posts: 24
Patrick wrote ...
im not sure if internal diodes or caps potted within the "cylinder" type flybacks are playing a factor here. not sure which type of flyback your using.
Thanks Patrick. I've been using simple ferrite core transformers so far, but have stripped a flyback out of an old computer monitor and will be using that at some point in the future.
Edit: Actually you've awakened me to the fact that the output from the LOPT is DC, so I don't need a separate rectification stage and can feed it straight in to the Marx Generator. I'm looking forward to playing with this now.
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Voltage Multipliers keep destroying drivers
