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how to design very small 6V power supply @100mA

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A transformer provides isolation for safety.
A switching power supply can operate at a high frequency so the transformer's size can be very small.

I bought some 5V/2A switching power supplies as surplus (2 years old, name brand) for a very low price. They are very small and are a very low weight.
Chargers for cell phones are small switching power supplies.

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I bought my surplus 5V/2A tiny name-brand switching power supplies at a very good price. Their label says that the voltage is adjustable over a wide range but they are sealed so I will cut one open to try it.

I don't know where to buy a high frequency core for the transformer, then it would be a nuisance to wind many turns of wire on the core. Of course, first I would need to refresh my long ago study of transformer theory. I have never made a transformer for a switching power supply.

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if u r going to open up a mobile phone charger i suggest u use it for the power supply u need. most cell phone chargers can provide u with a voltage of 6-7V @500mA, which will be quite enough for ur demands. u could use normal rectifier diodes to drop the excess voltage by connecting them in series (u'll get a drop of 0.6v from each diode) since mobile phone chargers are quite cheap and are optimised for thir ussage, i suggest u use them, it more easy than building ur own. i have an ac-dc adaptor of a chinese make, it has a voltage o/p of 1.5,3,4.5,6,7.5,9 & 12V @1200mA. it uses different feed back resistors to set the voltage. ;)

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The transformer design isn't that bad... 25 turns primary and 5 turns secondary.

For a SMPS, I think you could get away with a Feroxcube ER14.5 core with 3F3 material (inductance factor of 1400nH +/-25%) running at 400KHz on a single ended forward converter. It would have an ~ duty cycle of 46% (so reset wouldn't be a problem). Also the flux swing is limited to 2000 gauss (3F3 saturates around 3500 gauss @ 100C).

Oh... this is all base on a 110VAC half wave rectified RMS voltage of~78V feeding the DC-DC.

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Here's probably more than you want to know about it... it shows a split primary winding of 13-13. This is because the program can't do half turns.

Design Name: Designer:
Date/Time:    06/30/06 09:45:45 Project:  Untitled

Core Family:  ER          Ferrite Core Weight (Grams): 1.800
Geometry: ER14.5/3/7 Total Gap (in): 311.7u
Material Name: 3F3/200K400K_100C Spacer Thickness (in): 155.8u
Manufacturer: PHILIPS        Window Fill (%): 62.67

Eff. Core Area (m^2): 17.60u Min. Core Area (m^2): 17.60u
Winding Length (in): 0.1024 Winding Height (in): 0.1398
Avail. Window (m^2): 9.230u Area Product (m^4): 162.4p
Min. Core Gap (in): 103.5u Volume (in^3): 20.38m
Inside Diameter (m): 4.700m Surface Area (m^2): 580.0u
Mean Length Turn (in): 1.020 Winding Shape: Round
Max. Permeability: 2.400k Max. B, linear u (Gauss): 2.400k
Sat. Flux Density (Gauss): 3.100k Res. Flux Density (Gauss): 1.100k
Mean Mag. Path Len. (m): 19.00m

Flux Swing Type: half wave Input Waveform: pulse
Duty Ratio (Pct.): 0.5000% Current Rise/Fall (Pct.): 0%
Output Power (Watts): 0.6885 Magnetizing Ind. (Henry): 237.2u
Pk. Flux Density (Gauss): 2.509k Core Loss (Watts): 0.2346
AC Flux Density (Gauss): 977.9 Copper Loss (Watts): 259.6u
Ambient Temp. (deg C): 55.00 Core AwAc (m^4): 162.4p
Temp. Rise (deg C): 28.75 Frequency (Hertz): 400.0k
Volts/Turn: 2.754

Trise 28.75 Winding fill % 62.67
Tlevel 0 Total Weight (Pounds): 4.670m
Copper Loss 259.6u Core Loss 0.2346
Bac(max) 100.0k Bac 977.9
Output Power 0.6885 Calculator 4.000
Gap 791.7u Lmag 237.2u
Max Strands 2.000 Min. Turns 1.000
drive winding 1.000 Core weight 1.800
Efficiency 65.89 Round Coef. 0.9900
Winding pitch 1.000 Primary Turns 14.90
Idens(max) 100.0k Bp(max) 100.0k
Vdrop2 4.725m Ploss 47.25u
Vterminal 35.80 Jn 20.13
CuWt 0.2066

Winding Number: 1 2 3
Primary or Secondary: pri sec pri
Volts Specified (Volts): 35.80 12.43 35.80
Volts Average (Volts): 35.80 13.77 35.80
AC Current (Amps):                 10.00m 50.00m 10.00m
DC Current (Amps):                 0 0 0
AC Resistance (Ohms): 0.2925 73.25m 0.4725
DC Resistance (Ohms): 0.1060 62.59m 0.1492
Power Loss, Copper (Watts): 29.25u 183.1u 47.25u
Current Density (Amp/in^2): 129.9 828.0 129.9
Wire Type: HF HF HF
Wire Size (AWG): 39 40 39
Wire Height (in): 4.331m 3.898m 4.331m
Wire Width (in): 4.331m 3.898m 4.331m
Wire Strands: 8 8 8
Turns:               13 5 13
Number of Layers: 6.500 2.500 6.500
Turns per Layer: 2.000 2.000 2.000
Start ID:                 1 3 1000
Finish ID:                 1000 4 2
Pitch:               1 1 1
Layer Insulation (in): 0 0 0
Wrapper Insulation (in): 3.000m 3.000m 3.000m
End Margins (in):               0 0 0
Leakage Ind. Next (Henry): 1.296u 223.9n 0
Leakage(L->Sector) (Henry): 0 0 0
Leakage(Sector<-L) (Henry): 0 0 0
Winding Capacitance (Farad): 5.628p 9.147p 11.08p
Capacitance to Next (Farad): 9.078p 9.979p 0
IR Drop 2.925m 3.663m 4.725m
Copper Loss 29.25u 183.1u 47.25u
Loaded Voltage 35.80 13.76 35.80
Current Density2 20.13 128.3 20.13
Winding Weight 98.78m 36.43m 0.1428

Max Window Fill (%): 100.0 Max Temp. Rise (deg C): 50.00
Max Pk. Flux Dens. (Gauss): 100.0k Max AC Flux Dens. (Gauss): 100.0k
Max Cur. Dens.(Amp/in^2): 645.2k Waveform: square
Auto Margin for Lead Exit: no Pitch (dia/turn): 1
K Conduction: 3.990 K Convection: 710.0
K Insulation: 2.000m K Dielectric: 3.000
Thermal Model Level: 0 Rac Method B&L

Spice Model Name: not saved
Spice Symbol Name: not saved

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