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FAN103MY Datenblatt(PDF) 10 Page - Fairchild Semiconductor |
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FAN103MY Datenblatt(HTML) 10 Page - Fairchild Semiconductor |
10 / 16 page © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN103 • Rev. 1.0.1 10 Functional Description 0 shows the basic circuit diagram of a primary-side regulated flyback converter with typical waveforms shown in 0. Generally, discontinuous conduction mode (DCM) operation is preferred for primary-side regulation since it allows better output regulation. The operation principles of DCM flyback converter are as follows: During the MOSFET on time (tON), input voltage (VDL) is applied across the primary-side inductor (Lm). Then, MOSFET current (Ids) increases linearly from zero to the peak value (Ipk). During this time, the energy is drawn from the input and stored in the inductor. When the MOSFET is turned off, the energy stored in the inductor forces the rectifier diode (D) to be turned on. While the diode is conducting, the output voltage (Vo), together with diode forward voltage drop (VF), are applied across the secondary-side inductor (Lm ×Ns2/ Np2) and the diode current (ID) decreases linearly from the peak value (Ipk ×Np/Ns) to zero. At the end of inductor current discharge time (tDIS), all the energy stored in the inductor has been delivered to the output. When the diode current reaches zero, the transformer auxiliary winding voltage (Vw) begins to oscillate by the resonance between the primary-side inductor (Lm) and the effective capacitor loaded across MOSFET. During the inductor current discharge time, the sum of output voltage and diode forward-voltage drop is reflected to the auxiliary winding side as (Vo+VF) × Na/Ns. Since the diode forward-voltage drop decreases as current decreases, the auxiliary winding voltage reflects the output voltage best at the end of diode conduction time, where the diode current diminishes to zero. Thus, by sampling the winding voltage at the end of the diode conduction time, the output voltage information can be obtained. The internal error amplifier for output voltage regulation (EA_V) compares the sampled voltage with internal precise reference to generate error voltage (VCOMV), which determines the duty cycle of the MOSFET in CV mode. Meanwhile, the output current can be estimated using the peak drain current and inductor current discharge time since output current is same as average of the diode current in steady state. The output current estimator picks up the peak value of the drain current with a peak detection circuit and calculates the output current using the inductor discharge time (tDIS) and switching period (ts). These output information is compared with internal precise reference to generate error voltage (VCOMI), which determines the duty cycle of the MOSFET in CC mode. Among the two error voltages, VCOMV and VCOMI, the small one determines the duty cycle. Therefore, during constant voltage regulation mode, VCOMV determines the duty cycle while VCOMI is saturated to high. During constant current regulation mode, VCOMI determines the duty cycle while VCOMV is saturated to high. + V DL - L m + V O - N p:Ns I ds I D D Primary-Side Regulation Controller + V w - VDD Gate Vs CS +V F - N A L O A D I o Io Estimator Vo Estimator t DIS Detector PWM Control R CS VAC Ref Ref EA_V EA_I V COMV V COMI R S1 R S2 Figure 21. Simplified PSR Flyback Converter Circuit P pk S N I N ⋅ pk I . Davg o I I = A F S N V N ⋅ A O S N V N ⋅ Figure 22. Key Waveforms of DCM Flyback Converter |
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