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FAN6204AMX Datasheet(PDF) 10 Page - ON Semiconductor |
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FAN6204AMX Datasheet(HTML) 10 Page - ON Semiconductor |
10 / 16 page © 2015 Semiconductor Components Industries, LLC. www.onsemi.com FAN6204A • Rev. 1.1 9 Functional Description V OUT V DET I M V IN/n Primary MOSFET V GS Synchr onous Rectifier MOSFET V CT V LPC t PM.ON I DS I SR /n V IN/n+VOUT t L.DIS t CT.DIS Body diode of SR MOSFET Body diode of SR MOSFET 0.05VOUT 0.83VLPC-HIGH VLPC-HIGH I DS Primary MOSFET IM,av IM,max IM,min V OUT V DET I M V IN/n Primary MOSFET V GS Synchr onous Rectifier MOSFET V CT V LPC t PM.ON I DS I SR /n V IN/n+VOUT t L.DIS t CT.DIS Body diode of SR MOSFET Body diode of SR MOSFET 0.05VOUT 0.83VLPC-HIGH VLPC-HIGH Blanking time (tLPC-EN) IM,max IM,min Figure 18. Typical Waveforms of Linear-Predict Timing Control in CCM and DCM/QR Flyback Linear Predict Timing Control The SR MOSFET turn-off timing is determined by linear-predict timing control and the operation principle is based on the volt-second balance theorem. The volt- second balance theorem states that the inductor average voltage is zero during a switching period in steady state, so the charge voltage and charge time product is equal to the discharge voltage and discharge time product. In flyback converters, the charge voltage on the magnetizing inductor is input voltage (VIN), while the discharge voltage is nVOUT, as the typical waveforms show in Figure 18. The following equation can be drawn: .. IN PM ON OUT L DIS V t n V t (1) where tPM,ON is inductor charge time and tL,DIS is inductor discharge time. FAN6204A uses the LPC and RES pins with two sets of voltage dividers to sense DET voltage (VDET) and output voltage (VOUT), respectively; so VIN/n, tPM.ON, and VOUT can be obtained. As a result, tL,DIS , which is the on-time of SR MOSFET, can be predicted by Equation (1). As shown in Figure 18, the SR MOSFET is turned on when the SR MOSFET body diode starts conducting and DET voltage drops to zero. The SR MOSFET is turned off by linear-predict timing control. Circuit Realization The linear-predict timing-control circuit generates a replica (VCT) of magnetizing current of flyback transformer using internal timing capacitor (CT), as shown in Figure 19. Using the internal capacitor voltage, the inductor discharge time (tL.DIS) can be detected indirectly, as shown in Figure 18. When CT is discharged to zero, the SR controller turns off the SR MOSFET. |
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