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Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
he wants to ballast a HV 40khz transformer? yet people are telling him to use inductive ballast recalculated for 40khz, what happens with high inductance and a non-sinusoid/squarish HV wave? is there a potential cemf coming back to kill his device?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Nope, inductive ballasting works just fine with high frequency from an inverter. CFL low energy lamps drive their tubes at about 30kHz with a little half bridge, and a ferrite-cored ballast inductor the size of your pinky tip replaces the big iron lump found in an old-fashioned fluorescent light fixture.
All practical transformers have leakage inductance, which looks like an inductor in series with the primary. High voltage transformers have more of it: the spaces required for insulation are also spaces for leakage flux to sneak through. When you're designing a HV, HF transformer, the leakage inductance can provide more ballast than you want.
Registered Member #2405
Joined: Fri Oct 02 2009, 12:59AM
Location: Wisconsin
Posts: 140
Patrick wrote ...
he wants to ballast a HV 40khz transformer? yet people are telling him to use inductive ballast recalculated for 40khz, what happens with high inductance and a non-sinusoid/squarish HV wave? is there a potential cemf coming back to kill his device?
There is always a potential for back EMF in any inductor. Thats why MOV's are used as well TVS and any other protection you can find. All I am doing is throwing ideas out there without experimenting or testing it myself all I can do is suggest things to experiment with. Dont know exactly what level he is even trying to limit too. Could use Diodes for protection too. Just throwing out ideas.
Registered Member #2405
Joined: Fri Oct 02 2009, 12:59AM
Location: Wisconsin
Posts: 140
Dr. Brownout wrote ...
Patrick wrote ...
he wants to ballast a HV 40khz transformer? yet people are telling him to use inductive ballast recalculated for 40khz, what happens with high inductance and a non-sinusoid/squarish HV wave? is there a potential cemf coming back to kill his device?
There is always a potential for back EMF in any inductor. Thats why MOV's are used as well TVS and any other protection you can find. All I am doing is throwing ideas out there without experimenting or testing it myself all I can do is suggest things to experiment with. Dont know exactly what level he is even trying to limit too. Could use Diodes for protection too. Just throwing out ideas.
I though he was talking about protection around the transformer windings, the same reason freewheel diodes are used thats why I mentioned it the way I did. But the the ballast should pose no threat.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
In a half bridge or full bridge, diodes are connected across the transistors to return inductive energy to the DC bus. The diodes come free (though usually not too good) in MOSFETs. Most IGBTs have a "co-packaged" diode added.
The only time back EMF is a worry, is when using a single switch, as in the classic self-destructing flyback driver.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
so for his frequency (40khz), he can reduce the amount of inductance form the painfully low 60Hz freq, presumably he only needs to pass small current, as in steves CFL inverter comment. But should he use powdered metal, or ferrite for this application... or does it even matter?
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