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FAN6204MY Scheda tecnica(PDF) 10 Page - Fairchild Semiconductor |
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FAN6204MY Scheda tecnica(HTML) 10 Page - Fairchild Semiconductor |
10 / 14 page © 2010 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN6204 • Rev. 1.0.0 10 Figure 19. Simplified Linear-Predict Block The voltage-second balance equation for the primary- side inductance of the flyback converter is given in Equation 1. Inductor current discharge time is given as: . . IN PM ON LDIS OUT Vt t nV ⋅ = ⋅ (2) The voltage scale-down ratio between RES and LPC is defined as K below: () () 43 4 21 2 / / R RR K R RR + = + (3) During tPM.ON, the charge current of CT is iCHR-iDICHR, while during tL.DIS, the discharge current is iDICHR. As a result, the current-second balance equation for internal timing capacitor (CT) can be derived from: .. 5 (( ) ) IN OUT OUT PM ON OUT CT DIS V VV t V t Kn ⋅+ − ⋅ = ⋅ (4) Therefore, the discharge time of CT is given as: . . 5 (( ) ) IN OUT OUT PM ON CT DIS OUT V VV t Kn t V ⋅+ − ⋅ = (5) When the voltage scale-down ratio between RES and LPC (K) is five (5), the discharge time of CT (tCT.DIS) is the same as inductor current discharge time (tL.DIS). However, considering the tolerance of voltage divider resistors and internal circuit, the scale-down ratio (K) should be larger than five (5) to guarantee that tCT.DIS is shorter than tL.DIS. It is typical to set K around 5~5.5. Referring to Figure 18; when LPC voltage is higher than VLPC-EN over a blanking time (tLPC-EN) and lower than VLPC-TH-HIGH (0.05VOUT), then SR MOSFET can be triggered. Therefore, VLPC-EN must be lager than VLPC-TH- HIGH or the SR MOSFET cannot be turned on. When designing the voltage divider of LPC, R1 and R2 should be considered as: . 2 12 0.83 ( ) 0.05 0.3 IN MIN OUT OUT V R VV RR n ⋅⋅ + > + + (6) On the other hand, the linear operation range of LPC and RES (1~4V) should be considered as: . 2 12 () 4 IN MAX OUT V R V RR n ⋅+ < + (7) 4 4 3 4 < ⋅ + OUT V R R R (8) CCM Operation The typical waveforms of CCM operation in steady state are shown as Figure 18. When the primary-side MOSFET is turned on, the energy is stored in Lm. During the on-time of the primary-side MOSFET (tPM.ON), the magnetizing current (IM) increases linearly from IM,min to IM,max. Meanwhile, internal timing capacitor (CT) is charged by current source (iCHR-iDICHR) proportional to VIN, so VCT also increases linearly. When the primary-side MOSFET is turned off, the energy stored in Lm is released to the output. During the inductor discharge time (tL.DIS), the magnetizing current (IM) decreases linearly from IM,max to IM,min. At the same time, the internal timing capacitor (CT) is discharged by current source (iDISCHR) proportional to VOUT, so VCT also decreases linearly. To guarantee the proper operation of SR, it is important to turn off SR MOSFET just before SR current reaches IM,min so that the body diode of SR MOSFET is naturally turned off. DCM / QR Operation In DCM / QR operation, when primary-side MOSFET is turned off, the energy stored in Lm is fully released to the output at the turn-off timing of primary-side MOSFET. Therefore, the DET voltage continues resonating until the primary-side MOSFET is turned on, as depicted in Figure 18. While DET voltage is resonating, DET voltage and LPC voltage drop to zero by resonance, which can trigger the turn-on of the SR MOSFET. To prevent fault triggering of the SR MOSFET in DCM operation, blanking time is introduced to LPC voltage. The SR MOSFET is not turned on even when LPC voltage drops below 0.05VOUT unless LPC voltage stays above 0.83VLPC-HIGH longer than the blanking time (tLPC-EN). The turn-on timing of the SR MOFET is inhibited by gate inhibit time (tINHIBIT), once the SR MOSFET turns off, to prevent fault triggering. Green-Mode Operation To minimize the power consumption at light-load condition, the SR circuit is disabled when the load decreases. As illustrated in Figure 20, the discharge times of inductor and internal timing capacitor decrease as load decreases. If the discharge time of the internal timing capacitor is shorter than tGREEN-ON (around 4.8µs) for more than three cycles, the SR circuit enters Green Mode. Once FAN6204 enters Green Mode, the SR MOSFET stops switching and the major internal block is shut down to further reduce operating current of the SR controller. In Green Mode, the operating current reduces to 800µA. This allows power supplies to meet the most stringent power conservation requirements. When the discharge time of the internal capacitor is longer than tGREEN-OFF (around 5.35µs) for more than seven cycles, the SR circuit is enabled and resumes the normal operation, as shown in Figure 21. |
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