Transformer Operating Frequency = 25.6Khz T=39uSec
Sine output is constructed from 256 varying width pulses per half cycle of output,
ie 50hz* 512 = 25.6khz pulse rate. To prevent flux walking problems in the push pull
transformer circuit each side is driven alternatly with equal width pulses. The half sine
outputs are multiplexed back as a full sine wave and then filtered.
Because the modulated pulses don't reach 100% maximum width, allowance for this
means the primary voltage is reduced to about 0.9 of maximum possible. The high
frequency greatly reduces the core size and winding quantity of the heavy gauge
primary needed to carry the current.
Sec power = 1500watts Pri Power = 1500/98% = 1530 watts
Sec volts = 240 v rms Pri volts = 48*0.9 = 43.2
Sec current = 6.25 amps rms Pri current = 1530/43.2 = 35 amps
Txfr Area Product = Core area.Winding area (cm^4)
= {Power/(K *delt B(Tesla) * freq(Hz)}^4/3 (cm^4) K=0.015
F=25600
= {1530/(0.015 * 0.7 * 25600} ^ 1.333 B=2*.35
= {1530/268.8} ^ 1.33
= 10.1 (cm^4)
Epcos E55 core csa = 3.57 sq cm.for 1.7 cm * 2.1 cm or csa 0.000357(m^2)
E55 Winding csa = 1.9 sq cm Product = 7cm^4
Turns/volt = 1
__________________________________ mks units
Root (Pi* B max * core csa * freq) core csa = sq metres,
B = webers/sq metre(Teslers)
F = Freq (Hz) K = 1 core stack
= 1
____________________________
root (pi*0.35*.000357*25600)
= 0.31 TPV
Pri Turns = 48v*.315 = 15.12 (15) turns per side, Bifilar wound.
Sec Turns ratio Vo/Vi = 240*1.414(Peak)+ diode drops=1v)/43.2 * 15.12 = 119 turns.
Allowing for txfr efficiency, say 95% brings the turns to 119/.95 = 125 turns per side.
Winding Wire Gauges:
Let copper losses = 10 watts primary + 10 watts secondary = I^2 R then
Res Secondary = 10/6.25*6.25 = 0.25 ohms maximum.
Power is divided by centre tapped windings which allows 0.5 ohm/winding
Sec mean diameter = 30mm. Metres/ohm = pi*30*126turns/0.5= 23 mets/ohm
Sec wire diameter = 0.71mm or 22 swg.
Res Primary = 10/22*22 = 0.02 ohms
Res Primary = 10/35*35 = 0.008 ohms
Power is divided by centre tapped windings which allows 0.016 ohm/winding
Pri Mean dia = 27mm. Mets/ohm = pi*27*15 turns/.016 = 79 mets/ohm
Pri wire diameter = 1.2mm = 18 swg (16 better)
Switching pulse widths (F=25600hz):
Maximum = 39.06 usec * 90% = 35.15usec
Minimum = 35.15 usec * 1/128 = 0.274 usec = 274ns (128 pulses for 90 degrees
sine wave)
Switching Transistors:
Primary side must be able to stand off 2* 48 plus a healthy margin.
FQP32N20C, 200v, 30Amp cont, tr & tf ~20 nsecs
Switching losses = 3 watts. Channel on losses = (38 mv* 8 amps)say 600 at 20 = 12watts.
Total 15 watts each driver.
Secondary side must be able to stand off 640 v plus margin.
Fairchild FQ8N100C will block 1000v and pass 8A cont 30A pulsed, Rds = 1.4 ohm at 4A
Rectifiers: MUR8100E are 1kv working
Transformer Core:
Ferrite core set at Farnell Epcos E55 B66335GX127 @ £3.06 for half
core *2. Former = £2.21.
Transformer wire (18swg 1.2mm at Maplins = £6.9)