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FAN103 데이터시트(PDF) 2 Page - Fairchild Semiconductor |
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FAN103 데이터시트(HTML) 2 Page - Fairchild Semiconductor |
2 / 16 page AN-8033 © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.1 • 5/6/10 2 2. Operation Principle of Primary- Side Regulation Figure 2 shows the simplified circuit diagram of a primary- side regulated flyback converter and its typical waveforms are shown in Figure 3. Generally, discontinuous conduction mode (DCM) operation is preferred for primary-side regulation since it allows better output regulation. The key of primary-side regulation is how to obtain output voltage and current information without directly sensing them. Once these values are obtained, the control can be accomplished by the conventional feedback compensation method. 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. During the diode conduction time (tD), the output voltage (Vo), together with diode forward-voltage drop (VF), are applied across the secondary-side inductor (L m×Ns 2/ N p 2) and the diode current (ID) decreases linearly from the peak value (I pk× Np/Ns) to zero. At the end of tD, 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 MOSFET output capacitor. During the diode conduction time, the sum of output voltage and diode forward-voltage drop is reflected to the auxiliary winding side as (Vo+VF)× N a/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. 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 an error voltage (VCOMV), which determines the duty cycle of the MOSFET, as shown in Figure 2. Meanwhile, the output current can be estimated through calculation. Assuming that output current is same as the average of the diode current in steady state, the output current can be estimated as: S D S P PK O t 2 t N N I I • = (1) 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 diode conduction time (tD) 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, as shown in the block diagram of Figure 2. Among the two error voltages, VCOMV and VCOMI, the smaller one actually 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. Figure 2. Primary-Side Regulated Flyback Converter p pk s N I N ⋅ pk I . D avg o I I = a F s N V N ⋅ a O s N V N ⋅ Figure 3. Key Waveforms of Primary-Side Regulated Flyback Converter |
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