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Moteur de recherche de fiches techniques de composants électroniques |
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Moteur de recherche de fiches techniques de composants électroniques |
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AN4105 Fiches technique(PDF) 14 Page - Fairchild Semiconductor |
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AN4105 Fiches technique(HTML) 14 Page - Fairchild Semiconductor |
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14 / 22 page  AN4105 APPLICATION NOTE 14 ©2002 Fairchild Semiconductor Corporation 4.2 Discontinuous Conduction Mode (DCM) The appearance of an interval in which the inductor current becomes zero during a switching cycle marks flyback converter operation as discontinuous conduction mode (DCM). As shown in Figure 17, the voltage waveform applied to the inductor, VLm, becomes more complex in DCM. Hence, to avoid difficulties in computation TOFF is not used. Instead, three input and output relationships of a converter are derived by using TOFF * , the time when the output rectifier diode is actually conducting. Figure 17. Flyback converter operating waveforms in discontinuous current mode (DCM). The boundary condition between DCM and CCM is: The following input output relationship in DCM is derived by using the fact that the colored areas A and B of VLm in Figure 17 must always be equal because, in steady state, the average inductor (or transformer) voltage is always zero. . Deriving the above equation again, using Io and the fact that the input and output powers are equal, Vo is obtained as: The following equation represents the input power: where fsw is the switching frequency. 4.3 Flyback Converter Design 4.3.1 Turns Ratio Considerations The turns ratio of an SMPS’s flyback converter transformer is an important variable. It affects the voltage and current levels associated with the primary side switching device and the secondary side rectifier, as well as the number of turns on the transformer and the current through it. A frequently discussed design concept suggests operating at maximum duty ratio when the input voltage is a minimum. For simplified calculations, here it is assumed that operating conditions change as listed immediately below. - Vac input: 85 ~ 265Vac - Vdc (rectified voltage): 100 ~ 400Vdc - Output voltage: 50Vdc - Inductor current: Continuous conduction mode (CCM) operation assumed. The input power taken by the dc source is the product of the dc voltage and average input current. Using a wide duty cycle to deliver equal average current reduces efficiency. A narrow duty cycle increases the effective current on the primary side, increasing the operating temperature of the primary winding and the MOSFET. Also, it is best to decide on a turns ratio, n, based on the device used. If the voltage on the primary side MOSFET is relatively low (e.g., 600V), make n small; if it is on the high side (e.g., 800V), make n large. As the value of n increases, the primary side switching device current and the secondary side rectifier diode voltage decreases Hence, with high output voltage and multiple secondary side outputs, it is advantageous to increase n. t0 t1 t2 t3 VGS VDS ILm ID 0 0 0 0 ON OFF VLm IO TON TOFF TOFF * Vi+nVO Vi Vi nV0 A B Vi+nVO 0 Slpoe=Vi/Lm Slope=nVO/Lm VO+Vi/n VO 0 VD IDS 0 IC 0 IO Slope I i I O + V i 2L m ----------T ON = V O V i -------- T ON T* OFF ------------------- D* 1D* – ---------------- == V i T ON nV O T* OFF = V O V iTON () 2 2 I O n -----L m TON T OFF + () V i + ----------------------------------------------------------------- = P IN 1 2 ---L mILm,(peak) 2 fsw = |
Numéro de pièce similaire - AN4105 |
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Description similaire - AN4105 |
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