5.9 Synchronous Rectifiers

Test conditions:

V_IN = 800V, V_OUT = 15V, I_OUT = 180A, Po = 2700W

Channel details:

Channel 1: SR gate-source voltage

Channel 2: Transformer primary current

Channel 3: Transformer primary voltage

Channel 4 : SR drain-source voltage

SR Switching ON

Commutation of PSFB-CDR from phase 1 to 4 shows that SR1 goes from not conducting and turned OFF to fully ON and conducting in phase 4. In phase 1, SR1 Vds is equal to VIN/N (Turns ratio) of the transformer. In phase 2, the voltage applied to transformer primary reduces from V_IN and swings to 0V. This results in drain-source voltage of SR1 to also reduce to 0V. At this point, SR1 is switched ON and turns on with full ZVS.

SR Switching OFF

Analyse SR2 as it goes from ON and fully conducting to switched OFF. SR2 is switched OFF from phase 3 to 4. In phase 3, SR2 is conducting approximately output current. SR2 is switched OFF right before phase 4 at which point SR2 MOSFET channel stops conducting and its body diode hits reverse recovery from the increased Vds and becomes reverse biased. Both SRs turn OFF with ZVS but still have body diode conduction losses due to dead time and reverse recovery losses.