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FAN100 Fiches technique(PDF) 11 Page - Fairchild Semiconductor |
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FAN100 Fiches technique(HTML) 11 Page - Fairchild Semiconductor |
11 / 21 page AN-6067 APPLICATION NOTE © 2008 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.1 • 1/26/10 11 voltage reaches the turn-off threshold voltage of the PSR controller. Therefore, if n a was calculated, the Vo,“B” can be obtained as: ," " 6.75 - fa f a OB a VV n V n +⋅ ⎛⎞ = ⎜⎟ ⎝⎠ (12) where: Vfa is forward-voltage of rectifier diode of auxiliary winding. Vf is forward-voltage of output diode. 6.75V is typically the turn-off threshold voltage of the PSR controller. The maximum duty ratio can be calculated by using a point “B” output condition: ," " .max," " .min," " ," " () () po B f on B in B p o B f nV V d Vn V V ⋅+ = +⋅ + (13) The transformer inductance (L p) is designed specifically for DCM operation and a CC tolerance of +/-10% should be considered. The transformer inductance can be obtained as: 22 ," " .min," " max," " ," " 2 Bin B B p oB o s Vd L VI f η ⋅⋅ = ⋅⋅ ⋅ (14) where: η,”B” is the estimated system efficiency of point “B.” If no values are available, use 0.45~0.5 as an initial value. fs is the PWM frequency. After the primary inductance is calculated, the maximum duty ratio of point “A” can be expressed as: ," " .max," " 2 ," " . min," " 2 OA O P on A Ain A s VI L d VT η ⋅⋅ ⋅ = ⋅⋅ (15) where T s is the switching period. The primary peak inductor current (IPK) of point “A” at full load and low line input voltage condition is: .min," " ," " .max," " in A pkA on A S p V id T L =⋅ ⋅ (16) Determine Primary Inductance Turn Number Based on Faraday’s law and the peak inductor current, the minimum turns for the primary inductance is calculated as: ," " 6 max 10 ppk A pri e Li N BA ⋅ = ⋅ ⋅ (17) where: Bmax is the saturation magnetic flux density, Ae is the effective area of the core-section. The number of turns for the secondary winding is defined as: sec pri p N N n = (18) Once the secondary winding has been calculated, the number of turns for the auxiliary winding is defined as: sec aux a Nn N = ⋅ (19) Determine the Divider Resistor (R1) and Current- Sense Resistor (RS) Once the output voltage VO and auxiliary winding have been defined, the feedback signal divider resistor, R1, can be calculated as: () 12 1 a Of ref n RR VV V ⎡ ⎤ = ⋅⋅ + − ⎢ ⎥ ⎢ ⎥ ⎣ ⎦ (20) where V ref=2.5V, R2 is typically set to 15~20KΩ. As discussed in the Constant Current Output Regulation section, the region of constant current output operation can be adjusted by the current-sense resistor. After the turn ratio (n p) has been determined, the relationship between the output current IO and current sense resistor R s is expressed as: 0.111875 p S O n R I ⋅ = (21) As Figure 27 shows, a design spreadsheet can be used to calculate the transformer design and select the power system components for a first prototype. A 5V/1A design example is shown in Figure 27. |
Numéro de pièce similaire - FAN100 |
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Description similaire - FAN100 |
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