Negative-side synchronous rectifier chipU7710 shows its talents

With the improvement of environmental protection awareness and the active promotion of government regulations in many countries, electronic products The specification requirements for power supplies are becoming more and more stringent. With the continuous improvement of the power density of the power supply, the requirements for the secondary rectification of the power supply are getting higher and higher. The rectifier device has developed from the initial Schottky tube rectifier to the synchronous rectification switch Tubes replace diodes to reduce power consumption. Today's recommended is a negative-side synchronous rectification chip U7710 that can improve power supply efficiency!

The negative-side synchronous rectification chip U7710 is a high-performance synchronous rectification switch used to replace the Flyback secondary Schottky diode, with built-in ultra-low on-resistance powerMOSFET to improve system efficiency. U7710 supports "common ground" synchronous rectification configuration, and also supports system discontinuous operation mode (DCM) and quasi-resonant operation mode (QR). U7710 uses output to directly supply VDDpower u>, no auxiliary winding, no VDDcapacitor, reducing system cost. Built-in smart turn-on detection logic to prevent misleading synchronous rectification The 5V output directly supplies power to VDD, no auxiliary winding is needed, the VDD capacitor is omitted, and the system cost is reduced.

After the system is turned on, the negative terminal synchronous rectification chip U7710 internal high voltage< u>LDO draws current from the Drain pin to supply power to the VDD capacitor. When the VDD voltage is lower than the under-voltage protection threshold (3.2V typical value), the chip enters the sleep mode, and the internal synchronization The rectifier MOSFET enters the off state, and the secondary winding current passes through the body diode of the internal synchronous rectification MOSFET to realize continuous flow. When the VDD voltage is higher than the VDD turn-on voltage (3.4V typical value), the chip starts to work. The internal synchronous rectification MOSFET of the chip only It can only be turned on during the freewheeling period of the secondary side.

In the turn-on stage, when the freewheeling stage of the secondary side of the transformer starts, the co-current rectification MOSFET is in the off state, and the secondary side current passes through the body diode of the MOSFET to realize freewheeling. A negative Vds voltage (<-500mV) is formed at both ends. The negative Vds voltage is much smaller than the negative terminal synchronous rectification chip U7710 internal MOSFET turn-on detection threshold (typical value -200mV), so after the turn-on delay (typical value 20nS) The internal MOSFET is turned on.

During the turn-off phase, during the turn-on period of the synchronous rectification MOSFET, the negative-side synchronous rectification chip U7710 samples the voltage across the drain-source of the MOSFET (Vds). When the Vds voltage is higher than the MOSFET turn-off threshold (Typical value -6mV), the internal MOSFET will be turned off after the turn-off delay (Td_off, about 20ns).

At the moment when the internal synchronous rectification MOSFET is turned on, the negative terminal synchronous rectification chip U7710 drain-source ( There will be voltage spikes between Drain-Source). In order to prevent such voltage spikes from interfering with the normal operation of the system and causing chip malfunctions, the chip is integrated with a leading edge blanking circuit (LEB). During LEB time (about 1.12us), turn off The comparator is masked and cannot turn off the internal synchronous rectification MOSFET until the blanking time elapses.