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AN4149 Datasheet(PDF) 7 Page - Fairchild Semiconductor |
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AN4149 Datasheet(HTML) 7 Page - Fairchild Semiconductor |
7 / 19 page APPLICATION NOTE AN4149 7 ©2005 Fairchild Semiconductor Corporation [STEP-9] Determine the wire diameter for each winding based on the RMS current of each output. The RMS current of the n-th secondary winding is obtained as where Dmax and Ids rms are specified in equations (6) and (9), Vo(n) is the output voltage of the n-th output, VF(n) is the diode (DR(n)) forward voltage drop, VRO is specified in STEP-3 and KL(n) is the load occupying factor for n-th output defined in equation (2). The current density is typically 5A/mm2 when the wire is long (>1m). When the wire is short with a small number of turns, a current density of 6-10 A/mm2 is also acceptable. Do not use wire with a diameter larger than 1 mm to avoid severe eddy current losses as well as to make winding easier. For high current output, it is recommended using parallel windings with multiple strands of thinner wire to minimize skin effect. Check if the winding window area of the core, Aw (refer to Figure 6) is enough to accommodate the wires. The required winding window area (Awr) is given by where Ac is the actual conductor area and KF is the fill factor. Typically the fill factor is 0.2~0.25 for single output applications and 0.15~0.2 for multiple output applications. If the required window (Awr) is larger than the actual window area (Aw), go back to the STEP-6 and change the core to a bigger one. Sometimes it is impossible to change the core due to cost or size constraints. In that case, reduce VRO in STEP-3 or increase fs min, which reduces the primary side inductance (Lm) and the minimum number of turns for the primary (Np min) as can be seen in equation (7) and (10). [STEP-10] Choose the proper rectifier diodes in the secondary side based on the voltage and current ratings. The maximum reverse voltage and the rms current of the rectifier diode (DR(n)) of the n-th output are obtained as where KL(n), VDC max, D max and Ids rms are specified in equations (2), (4), (6) and (9), respectively, VRO is specified in STEP-3, Vo(n) is the output voltage of the n-th output and VF(n) is the diode (DR(n)) forward voltage drop. The typical voltage and current margins for the rectifier diode are as follows where VRRM is the maximum reverse voltage and IF is the average forward current of the diode. A quick selection guide for the Fairchild Semiconductor rectifier diodes is given in Table 3. In this table, trr is the maximum reverse recovery time. Table 3. Fairchild Diode Quick Selection Table I n () sec rms I ds rms 1D max – D max ----------------------- V RO K Ln () ⋅ V on () V Fn () + () -------------------------------------- ⋅ =20 () A wr A c KF ⁄ = (21) V Dn () V on () V DC max V on () V Fn () + () ⋅ V RO ---------------------------------------------------------------- + =22 () I Dn () rms I ds rms 1D max – D max ----------------------- V ROKLn () V on () V Fn () + () -------------------------------------- ⋅ =23 () Ultra Fast Recovery Diode Products VRRM IF trr Package EGP10B 100 V 1 A 50 ns DO-41 UF4002 100 V 1 A 50 ns DO-41 EGP20B 100 V 2 A 50 ns DO-15 EGP30B 100 V 3 A 50 ns DO-210AD FES16BT 100 V 16 A 35 ns TO-220AC EGP10C 150 V 1 A 50 ns DO-41 EGP20C 150 V 2 A 50 ns DO-15 EGP30C 150 V 3 A 50 ns DO-210AD FES16CT 150 V 16 A 35 ns TO-220AC EGP10D 200 V 1 A 50 ns DO-41 UF4003 200 V 1 A 50 ns DO-41 EGP20D 200 V 2 A 50 ns DO-15 EGP30D 200 V 3 A 50 ns DO-210AD FES16DT 200 V 16 A 35 ns TO-220AC EGP10F 300 V 1 A 50 ns DO-41 EGP20F 300 V 2 A 50 ns DO-15 EGP30F 300 V 3 A 50 ns DO-210AD EGP10G 400 V 1 A 50 ns DO-41 UF4004 400 V 1 A 50 ns DO-41 EGP20G 400 V 2 A 50 ns DO-15 EGP30G 400 V 3 A 50 ns DO-210AD UF4005 600 V 1 A 75 ns DO-41 EGP10J 600 V 1A 75 ns DO-41 EGP20J 600 V 2 A 75 ns DO-15 EGP30J 600 V 3 A 75 ns DO-210AD UF4006 800 V 1 A 75 ns TO-41 UF4007 1000 V 1 A 75 ns TO-41 V RRM 1.3 V Dn () ⋅ > (24) I F 1.5 I Dn () rms ⋅ > (25) |
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