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FAN6300A Datasheet(PDF) 11 Page - Fairchild Semiconductor |
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FAN6300A Datasheet(HTML) 11 Page - Fairchild Semiconductor |
11 / 15 page © 2009 Fairchild Semiconductor Corporation www.fairchildsemi.com FAN6300A / FAN6300H • Rev. 1.0.1 11 Operation Description The FAN6300A/H PWM controller integrates designs to enhance the performance of flyback converters. An internal valley voltage detector ensures power system operates at Quasi-Resonant (QR) operation across a wide range of line voltage. The following descriptions highlight some of the features of the FAN6300A/H. Startup Current For startup, the HV pin is connected to the line input or bulk capacitor through an external diode and resistor, RHV, which are recommended as 1N4007 and 100kΩ. Typical startup current drawn from the HV pin is 1.2mA and it charges the hold-up capacitor through the diode and resistor. When the VDD voltage level reaches VDD-ON, the startup current switches off. At this moment, the VDD capacitor only supplies the FAN6300A/H to maintain VDD until the auxiliary winding of the main transformer provides the operating current. Valley Detection The DET pin is connected to an auxiliary winding of the transformer via resistors of the divider to generate a valley signal once the secondary-side switching current discharges to zero. It detects the valley voltage of the switching waveform to achieve the valley voltage switching. This ensures QR operation, minimizes switching losses, and reduces EMI. Figure 17 shows divider resistors RDET and RA. RDET is recommended as 150k Ω to 220kΩ to achieve valley voltage switching. When VAUX (in Figure 17) is negative, the DET pin voltage is clamped to 0.3V. Figure 17. Valley Detect Section The internal timer (minimum tOFF time) prevents gate retriggering within 8µs (3µs for H version) after the gate signal going-low transition. The minimum tOFF limit prevents system frequency being too high. Figure 18 shows a typical drain voltage waveform with first valley switching. Figure 18. First Valley Switching Green-Mode Operation The proprietary green-mode function provides off-time modulation to linearly decrease the switching frequency under light-load conditions. VFB, which is derived from the voltage feedback loop, is taken as the reference. In Figure 19, once VFB is lower than VN, tOFF-MIN increases linearly with lower VFB. The valley voltage detection signal does not start until tOFF-MIN finishes. Therefore, the valley detect circuit is activated until tOFF-MIN finishes, which decreases the switching frequency and provides extended valley voltage switching. However, in very light load condition, it might fail to detect the valley voltage after the tOFF-MIN expires. Under this condition, an internal tTIME-OUT signal initiates a new cycle start after a 9μs delay (with 5µs delay for H version). Figure 20 and Figure 21 show the two different conditions. tOFF -M IN VFB 1.2 V 2.1 V 2.1 m s 38/13 μ s 8/3μ s Figure 19. VFB vs. tOFF-MIN Curve Figure 20. QR Operation in Extended Valley Voltage Detection Mode Figure 21. Internal tTIME-OUT Initiates New Cycle After Failure to Detect Valley Voltage (with 5µs Delay for FAN6300H version) |
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