vorezky04, Give people a few days to read this to check for mistakes.

Guessing N87 material, there is a flux chart here. It is in a format that is hard to read but it looks to me like 1400 Gauss is best place to run this core.

http://www.epcos.com/web/generator/...F/PDF__N87,property=Data__en.pdf;/PDF_N87.pdf
170VDC nominal

70Khz

ETD49 Ae = 2.114 CM squared

1400 Gauss flux (.14T)

Primary turns = (Vpri x 100,000,000) / 4 x frequency x Gauss x Ae

Primary turns = (170VDC X 100,000,000) / (4 x 70,000 hz x 1400 Gauss x 2.11 Ae)

Pri turns = 20.55 turns, make 20 turns.

Secondary you want 2 x 70vdc output. 70vdc x .707 = 49.49vac + diode drop 2v = 51.49, about 52VAC.

Secondary turns = (20 Primary turns x 52VAC secondary) / 150VDC primary low = 6.93 turns. The 6.93 turns is the total turns in secondary, giving you about 10 volts per turn. You need an even number of turns for the center tap winding. So you can use 3 + 3, giving you about +-60VDC, or 4 + 4, giving you about +- 80VDC.

70Khz skin depth = 10.72mils x 2 = 21.445 mils, metric equivalent .5mm Diameter wire = 400CM

400CM / 350CM/A (hot design) = 1.14A per strand.

Want 10 amp output x 1.2 multiplier for diodes = 12 amps minimum.

I am uncertain if you have to add in more current for 70% efficency and power factor.

12amps / 1.14 = 10.52 strands of .5mm wire, make it an even 10.

P = I x E, 12 amps secondary x 52VAC secondary = 624W

624W / 170 primary volts = 3.67A, because of poor power factor and efficency, the primary will see about 30% more current. 3.67A x 1.3 = 4.78A primary current.

Using the .5mm wire at 1.14A per strand. 4.78A primary / 1.14A strand = 4.2 strands use 4 strands.

Primary 20 turns .5mm x 4 strands

Secondary 4 + 4 turns .5mm x 10 strands

Lets see how wire fits on the ETD-49 core.

Width of the winding area is 36.16mm, subtract 1.3mm per side for bobbin = 33.62mm, subtract 4mm per side for VDE voltage specification = 25.62mm for winding area.

Primary 20 turns .5mm x 4 strands = 40mm, to wide for single layer. Use 2 layers 10 turns each on inner and outer layers of transformer.

Secondary 4 + 4 turns .5mm x 10 strands = 40mm, to wide for single layer. Use 2 layers 4 turns each on inner layers of transformer.

We have 5.62mm left over on each layer we can use for winding’s. If we add more strands to fill it up you will have a cooler running transformer.

Primary 20 turns .5mm x 5 strands = 50mm, or 25mm per layer leaving only .62mm.

Secondary 4 + 4 turns .5mm x 12 strands = 48mm or 24mm per layer leaving only 1.62mm.

The new CM/A is.

Pri. 5 strands x 400CM = 2000CM / 4.78A Pri. = 418CM/A

Sec. 12 strands x 400CM = 4,800CM / 12A Sec = 400CM/A

This will be a very nice transformer. 500CM/A is considered a good design point for continuous use. And you are using this for music power so not as many CM (circular mils) are needed. I am very happy the way it turned out.

One last thing I did not have a bobbing to measure for the width i had to guess its dimensions. Check yourself and let us know how much room there is.