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EFD-25 Datasheet(PDF) 2 Page - Fairchild Semiconductor |
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EFD-25 Datasheet(HTML) 2 Page - Fairchild Semiconductor |
2 / 11 page AN-9737 APPLICATION NOTE © 2011 Fairchild Semiconductor Corporation www.fairchildsemi.com Rev. 1.0.0 • 4/13/11 2 Figure 2 shows typical waveforms of the simplified circuit of a flyback converter with CRM. When the MOSFET (Q1) turns on, the primary current in primary side linearly increases and is clamped at a certain internal level because the FL6961 doesn’t have cycle-by-cycle current limit like a conventional current mode control IC (such as FAN7527B). Its peak level is determined by the primary magnetizing inductance value and the fixed on-time. Instead of the cycle- by-cycle primary current limit, the FL6961 has an over- current protection (OCP) function. If the current sensing signal is larger than internal detection level, the FL6961 doesn’t get output signal for operating the MOSFET (Q1). Figure 2. Key Waveforms of Flyback Converter on CRM The FL6961 has a constant on-time across the whole range. The input average current always follows the peak input current, as shown in the equation: ON PK MOSFET AVG t I I 2 1 ) ( = (1) This is also proportional to the instantaneous input voltage. This means the input current shape is always the same as the input voltage shape. The reverse diode voltage is linearly increased and is equal to: P S IN O DIODE PK N N V V V + = ) ( (2) During the MOSFET off-time, which is also the diode on- time; the input current instantly drops to zero, the diode in the secondary side conducts, and the diode current linearly decreases. The peak current of the secondary side is the same as the multiplication of the primary peak current and turns ratio between the primary side (NP) and secondary side (NS) and naturally decreases to zero. The average current of the secondary side is: off PK S P DIODE AVG t I N N I 2 1 ) ( = (3) Since the diode forward-voltage drop decreases as current decreases, the output voltage reflects the primary winding and adds additional voltage due to overshoot made by resonance between the leakage inductance on primary-side winding and parasitic capacitance on the MOSFET (Q1). As a result, a superimposed voltage occurs on the MOSFET during off-time as: OS R IN off MOSFET V V V V + + = ) ( (4) where VR is the reflected voltage and VOS is the voltage overshoot term. The reflected voltage, VR, is affected by the turns ratio between the primary and secondary side of the transformer and the output voltage, calculated as: O S P R V N N V = (5) Figure 3 shows the ideal waveforms of the primary-side current at MOSFET (Q1) and the secondary-side current at the diode. The input peak and average current on the primary side follows input voltage instantaneously. Normally, secondary-side current on the diode is larger than the primary side because of the turns ratio. Figure 3. Ideal Waveforms time time time IPK ( MOSFET ) IDS (MOSFET Drain-to-Source Current)) ID (Diode Current) VDS (MOSFET Voltage) tON tOFF tS IAVG (MOSFET ) IPK ( DIODE ) IAVG (DIODE ) VIN VR VOS |
Similar Part No. - EFD-25 |
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Similar Description - EFD-25 |
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