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SG6203DZ Datasheet(PDF) 8 Page - Fairchild Semiconductor |
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SG6203DZ Datasheet(HTML) 8 Page - Fairchild Semiconductor |
8 / 13 page Product Specification Synchronous Rectifier Controller for Flyback Converter SG6203 © System General Corp. - 8 - www.sg.com.tw • www.fairchildsemi.com Version 1.0.1 (IAO33.0010.B1) September 24, 2007 OPERATION DESCRIPTION Synchronization The DET pin provides on/off information of the primary switch. As shown in the following reference circuits, one diode connected between the DET pin and the secondary winding of the transformer is used to get this information. Low DET voltage level indicates the secondary rectifier is conducted; therefore, the synchronous MOSFET should be turned on. The voltage on DET pin stays high when the primary switch is conducting and the synchronous MOSFET should be turned off. This innovative feature requires no auxiliary winding and the circuit complexity is greatly reduced. Anticipation and Phase Lock Loop A resistor from the RT pin to ground is used to determine the internal current source reference. When the primary switch is turned on with DET high, an internal one-shot timer outputs high for a duration proportional to the RT resistor. The duration of this one-shot signal can be expressed as: () ) µs ( 24 K R 15 T T RT Ω × = (1) Primary Switch PWM One-Shot Signal Output T The on-time of the synchronous MOSFET is obtained by subtracting the on-time of the primary switch from the one-shot signal duration. If the resistor is too small, the synchronous MOSFET may be turned off when the secondary rectifier is still conducting, which decreases system efficiency. If the resistor is too large, the output is shut off immediately once DET gets high. In SG6203, an internal phase locked loop modulates the duration of the internal one-shot signal to maintain a suitable dead time between the primary switch and the synchronous MOSFET. The duration of the one-shot signal can be extended 80% compared with the original value decided by RT resistor. Current Sense In discontinuous conduction mode, the secondary rectifier turns off before the next switching cycle of the primary switch. Once the secondary rectifier is off, the synchronous MOSFET must be turned off to prevent the reverse energy flow. Therefore, a zero current crossing detector is needed when the converter is operated under DCM. The SG6203 provides two configurations to achieve this: output capacitor ESR method (reference Circuit-A) and current-sensing resistor method (reference Circuit-B). When the secondary rectifier is turned off, the voltage on IN+ is lower than that of IN-. Once this happens, the driving signal for the synchronous MOSFET is turned off. Under-Voltage Lockout (UVLO) The SG6203 has an internal UVLO circuit with hysteresis. The IC is turned on if VCC is higher than 6V. Once turned on, SG6203 is turned off if VCC is lower than 5.1V. When the flyback output voltage is higher than 18V, the VZ pin can be used to provide a stable 18V to VCC pin. |
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