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AN107
Designing a 2 Transistor Forward Converter
Using a CMI E 2000q Core

By
Colonel Wm. T. McLyman

To view the Wire Table click here (this will open a new browser window).

Figure 1. Two transistor forward converter.

The two transistor forward converter is shown in Figure 1. This type of converter topology is used for powers under 200W. The dynamic BH loops for the single-ended, forward converter and the push-pull converter are shown in Figure 2.

Two Transistor Forward Converter Transformer Design Specification

  1. Input dc voltage max ................................................................................
  2. Input dc voltage nom ................................................................................
  3. Input dc voltage min .................................................................................
  4. Output voltage .........................................................................................
  5. Output current .........................................................................................
  6. Frequency ...............................................................................................
  7. Converter efficiency .................................................................................
  8. Maximum duty ratio .................................................................................
  9. Regulation ...............................................................................................
  10. Operating flux density ..............................................................................
  11. Diode voltage drop .................................................................................
  12. Window utilization ...................................................................................

Note: Bac = DB/2

Vmax = 32 volts
Vnom = 28 volts
Vmin = 24 volts
Vo = 5 volts
Io = 10 amps
f = 100 kHz
µ= 98 %
Dmax = 0.5
a = 1.0 %
DBm = 0.1 tesla
Vd = 1.0 volt
Ku = 0.4

 

Figure 2. The dynamic BH loop comparison between a single-ended, forward converter and a push-pull converter.

Figure 3. Typical single-ended forward, converter waveforms.

The waveforms shown in Figure 3, are typical waveforms of the single-ended forward converter. The collector current Ic is shown in Figure 3-A, and the magnetizing, Im, is shown in Figure 3-B. The inductor L1 current, IL, made up from the rectifier CR3, and the commutating rectifier, CR4, are shown in Figure 3-C. The collector voltage, Vc is shown in figure 3-D.

Select a wire so that the relationship between the ac resistance and the dc resistance is 1:

The skin depth in centimeters is:

Then, the wire diameter is:

Then, the bare wire area Aw is:

From the Wire Table, number 26 has a bare wire area of 0.001280 centimeters. This will be the minimum wire size used in this design. If the design requires more wire area to meet the specification, then, the design will use a multifilar of #26. Listed Below are #27 and #28, just in case #26 requires too much rounding off.

Wire AWG
Bare Area
Area Ins.
Bare/Ins.
µW/cm
#26 0.001280 0.001603 0.798 1345
#27 0.001021 0.001313 0.778 1687
#28 0.0008046 0.0010515 0.765 2142

 

Step No. 1 Calculate the total period, T.

Step No. 2 Calculate the maximum transistor on time, ton.

Step No. 3 Calculate the secondary output power, Po.

Step No. 4 Calculate the total input power, Pin.

Step No. 5 Calculate the electrical conditions, Ke.

Step No. 6 Calculate the core geometry, Kg .

Step No. 7 Select from the data sheet a E 2000q core comparable in core geometry, Kg.

Core number ........................................................................................
Manufacturer .......................................................................................
Magnetic material ................................................................................
Magnetic path length ............................................................................
Core weight .........................................................................................
Copper weight .....................................................................................
Mean length turn ..................................................................................
Iron area .............................................................................................
Window area ......................................................................................
Area product ......................................................................................
Core geometry ...................................................................................
Surface area .......................................................................................
TEA0113Q
CoreMaster International
E 2000q
MPL = 6.44 cm
Wtfe = 18.0 grams
Wtcu = 22.3 grams
MLT = 4.1 cm
Ac = 0.36 cm2
Wa = 1.539 cm2
Ap = 0.554 cm4
Kg = 0.0196 cm5
At = 38.5 cm2

Step No. 8 Calculate the low line input current, Iin.

Step No. 9 Calculate the primary rms current, Iprms.

Step No. 10 Calculate the number of primary turns, Np.

Step No. 11 Calculate the current density J using a window utilization Ku = 0.40.

Step No. 12 Calculate the required primary bare wire area, Awp.

Step No. 13 Calculate the required number of strands NSp.

Step No. 14 Calculate the primary new µW per centimeter from the number 26 AWG.

Step No. 15 Calculate the primary winding resistance, Rp.

Step No. 16 Calculate the primary copper loss, Pp.

Step No. 17 Calculate the transformer secondary voltage, Vs.

Step No. 18 Calculate the number of secondary turns, Ns.

Step No. 19 Calculate the secondary rms current, Is.

Step No. 20 Calculate the secondary wire area, Aws.

Step No. 21 Calculate the number of secondary strands, NSs.

Step No. 22 Calculate the secondary new µW per centimeter from the number 26 AWG.

Step No. 23 Calculate the winding resistance, Rs.

Step No. 24 Calculate the secondary copper loss, Ps.

Step No. 25 Calculate the total copper loss, Pcu.

Step No. 26 Calculate the regulation, a for this design.

Step No. 27 Calculate the window utilization, Ku.

Step No. 28 Calculate the milliwatts per gram, mW/g.

Step No. 29 Calculate the core loss, Pfe.

Step No. 30 Calculate the total loss, core Pfe and copper Pcu, in watts P.

Step No. 31 Calculate the watt density,y.

Step No. 32 Calculate the temperature rise in degrees C.

Step No. 33 Calculate the transformer efficiency, h.

 

BIBLIOGRAPHY

Colonel William T. McLyman, Transformer and Inductor Design Handbook, Second Edition, Marcel Dekker Inc., New York, 1988.

Colonel William T. McLyman, Magnetic Core Selection for Transformers and Inductors, Second Edition, Marcel Dekker Inc., 1997.

Colonel William T. McLyman, Designing Magnetic Components for High Frequency, dc-dc Converters, Kg Magnetics, Inc., 1993.

For information regarding the above Books and Companion Software for Windows 95', 98' and NT, contact:

Kg Magnetics, Inc.
38 West Sierra Madre Blvd, Suite J
Sierra Madre, Ca. 91024
Phone: (626) 836-7233, FAX: (626) 836-7263
Web Page: www.kgmagnetics.com
Email: sheassoc@pacbell.net

 

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