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FAN6208 Datasheet(PDF) 9 Page - Fairchild Semiconductor |
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FAN6208 Datasheet(HTML) 9 Page - Fairchild Semiconductor |
9 / 18 page © 2010 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN6208 • Rev. 1.0.1 9 Figure 10.SR Gate Timing Diagram Figure 11.DETL Debounce (Blanking) Time Gate-Shrink Functions In normal operation, the turn-off instant is determined by subtracting a dead time (tDEAD) from the measured SR conduction duration of the previous switching cycle, as shown in Figure 10. This allows proper driving timing for SR MOSFETS when the converter is in steady state and the switching frequency does not change much. However, this control method may cause shoot-through of SR MOSFETs when the switching frequency increases fast and switching transition of the primary- side MOSFETs takes place before the turn-off command of SR is given. To prevent the shoot-through, FAN6208 has gate-shrink functions. Gate shrink occurs under three conditions: (a) When insufficient dead time is detected in the previous switching cycle. When the DETL goes HIGH within 125ns of detection window after SR gate is turned off, the SR gate drive signal in the next switching cycle is reduced by tSHRINK-DT (about 1.25µs) to increase the dead time, as shown in Figure 12. Figure 12.Gate Shrink by Minimum Dead Time Detection Window (b) When the feedback information changes fast. FAN6208 monitors the current through the opto- coupler diode by measuring the voltage across the resistor in series with the opto-coupler diode, as depicted in Figure 13. If the feedback current through the opto-coupler diode increases by more than 20% of the feedback current of the previous switching cycle, the SR gate signal is shrunk by tSHRINK-FD (about 1.4µs) for tD-SHRINK-FD (about 90µs), as shown in Figure 14. Figure 13.Typical Application Circuit |
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