CFL Inverter transformer insulation problem

[betkro]

I have an inverter project for emergency lighting with two 110W CFLs. The circuit uses bipolar transistors powered from 12VDC in a self-oscillating push-pull topology. Both lamps must be insulated from each other, which requires to have isolated secondary windings. Lamps are connected through 1nF/1600V polipropylene capacitors. Transformer data: Standard ferrite E-core 30/15/7 (mm). 7+7+2/330+330 turns. Freq. 30KHz. Enameled wire is 180ºC and 2500V class.

The circuit efficiency is reasonable good. However, insulation leakages happen frequently between turns from different layers in the same secondary winding. The winding is made manually and layers are not perfect (turns from different layers may become side by side). Secondary steady state voltage on each 110W CFL lamp is about 800Vpk-pk, and when 32W CFL lamps are used, secondary voltage is about 400V, however insulation problems still happen.

When the two secondary lamp circuits are left independent (isolated from each other) operation is OK. Unfortunately, this in not true for real operation, where any secondary end is connected to any end of the other, thus causing the referred problem.

Any ideas will be welcome.

betkro

 

[sreid]

MWS Wire Industries

http://www.mwswire.com/

produces and stocks magnet wire with up to a four layer insulation build up.

 

[BrianG]

If you have any room on the bobbin, the old fashioned way to prevent this sort of breakdown problem is to separate each copper layer with very thin self-adhesive mylar tape (usually yellow in colour and was made specifically for the purpose). This prevents turns from the different layers ending up next to each other, and hence build up of high potential difference between them.

 

[cbarn24050]

Rearrange your windings, sec1,pri,sec2. This split sandwich arrangement gives you lower leakage eddycurrent losses as well.

 

[OperaHouse]

Any reversing field can cause corona effects in a trapped air space of a winding or capacitor. This causes the air in these spaces to "glow" and eventually deteriorate the insulation. This is the reason a 2,000V DC capacitor can fail when only used on 240VAC. Increasing the insulation thickness will help, but I would look a t methods to reduce the spike level of the inverter or rise time.

 

[sreid]

BrianG's suggestion of using Mylar tape between all winding layers is probably the "Lead Pipe Chinch" fix for the shorting problem. Mylar Tape can be purchased from EIS (Electronic Insulation Specialities).

http://www.eis-inc.com/

 

[BrianG]

sreid: as I'm from the UK I don't understand the phrase "Lead Pipe Chinch". I trust that means it's OK?

 

[sreid]

BrianG,

Yes, "Lead Pipe Chinch" means "Guranteeded to Work." What would be the British Slang equilent?

 

[BrianG]

Hi sreid,

Well, there are lots of phrases in British slang that mean "guaranteed to work" or similar. A couple of the more printable ones are "stone bonker", "dead cert".