shon35us said:
I'm still not understanding what you're doing. Them=dual secondarys from each other? Mostly all switchers xfrmers are handwound.
3M makes tape for this purpose in standard widths for the most popular bobbins like ETD. Your magnetics distributor can point you in the right direction. If the tape is not the correct width, your build up gets too high to support the 4mm overlap in a continuous tape wind.
Use 4mm margins from each of the windings to bobbin walls. Approved tape in the correct width will be at least > 2 layers min.
Secondaries lower than SELV do need additional insulation from each other.
Teflon tape or non-metalized mylar (polyester) works. Teflon and Polyester have high breakdown tempertures and also have fairly high breakdown voltages. More expensive films and tapes that can be used are Polyimide and Nomex. These have very high thermal breakdown tempertures, and ideal for transformers operating in severe conditions.
As infinia discussed you have to becareful with uneven overlapping when you need to wind many layers as the overlap will cause the layers to become uneven, which will eventually make it impossible to add layers without large gaps (causing exccessive Inductance leakage aka large voltage spikes,, and lower coupling efficiency).
If your just putting together a simple experimental low voltage prototype, you can also use Kraft Paper or even plain copier. You can increase the dielectric value of the paper by applying a coat of polyurethane to plastize the paper (brushed on). You cut the paper to match the size of spool so that there are no overlapping.
I also recommend applying a coat of electrical varnish (or at least polyurethane or polyurethane glue) to each layer to secure the windings and to reduce audio noise when the transformer is operating. Windings that are not secured will vibrate at the switching frequency and generate noise. This vibration can also rub off the enamel insulation over time. OEM transformers are usually use vacuum impregnated varish to secure the windings. The transformer wounded spool is sumerged in electrical varish in a vacuum chamber, when the air is vacuumed out of the chamber it pulls out the air between the windings. When the air is let back in to the chamber, varnish is pulled into to all of the gaps and holes in the windings. Then the spool is baked to set the varnish.
Polyurethane isn't electrical varnish as it has a much lower breakdown temperature, lower thermal conduction value, and lower diaelectic value. However its better than nothing. Dow Corning manufactures electrical varnish, but its not cheap and its difficult to obtain from distributors in small quantities. It also has a shelf-life of a few years.
As infinia discussed you have to becareful with uneven overlapping when you need to wind many layers as the overlap will cause the layers to become uneven, which will eventually make it impossible to add layers without large gaps (causing exccessive Inductance leakage aka large voltage spikes,, and lower coupling efficiency).
If your just putting together a simple experimental low voltage prototype, you can also use Kraft Paper or even plain copier. You can increase the dielectric value of the paper by applying a coat of polyurethane to plastize the paper (brushed on). You cut the paper to match the size of spool so that there are no overlapping.
I also recommend applying a coat of electrical varnish (or at least polyurethane or polyurethane glue) to each layer to secure the windings and to reduce audio noise when the transformer is operating. Windings that are not secured will vibrate at the switching frequency and generate noise. This vibration can also rub off the enamel insulation over time. OEM transformers are usually use vacuum impregnated varish to secure the windings. The transformer wounded spool is sumerged in electrical varish in a vacuum chamber, when the air is vacuumed out of the chamber it pulls out the air between the windings. When the air is let back in to the chamber, varnish is pulled into to all of the gaps and holes in the windings. Then the spool is baked to set the varnish.
Polyurethane isn't electrical varnish as it has a much lower breakdown temperature, lower thermal conduction value, and lower diaelectic value. However its better than nothing. Dow Corning manufactures electrical varnish, but its not cheap and its difficult to obtain from distributors in small quantities. It also has a shelf-life of a few years.
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