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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
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.
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.
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 På.
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.
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.
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