T
treez
Guest
Hello
We are using the LT8309 synchronous rectifier for a PFC’d flyback on 80-135VAC input…26vdc output. We are using it like in the attached LTspice schematic (which needs to be run with the attached textfile “bcmpfcfile” in the same folder….the sim file is called “bcmpfc”)
As explained on page 9 of the LT8309 datasheet, there is a fault mode of this synchronous rectifier….this occurs when in DCM as follows…when the secondary coil has discharged, the primary drain voltage can ring down and turn on the primary fet’s intrinsic diode………when the ringing primary current then reverses, the primary fet diode will turn back off again, but due to its reverse recovery it wont turn off straight away, and so some “Phantom” primary current will flow……when it stops flowing, the secondary will then take its turn and flow current…..this can drive the secondary VDS voltage such that the sync rect IC turns on again when it should not….and then the primary fet may turn on at the same time giving unwanted cross-conduction.
The ways given to solve this are either to use a schottky parallel to the primary fet, or to set the turns ratio of the flyback such that the primary drain voltage cannot ring down to ground…neither of these options are open to us because our max primary drain voltage is too great for a schottky…and no matter what we set the turns ratio to…there is always going to be a time when the primary drain voltage can ring down to ground…(because the primary bus voltage falls very low due to PFC operation with little primary bus capacitance).
Do you know any other flyback sync rect IC’s which offer mitigation of this problem?
LT8309 datasheet:
https://www.analog.com/media/en/technical-documentation/data-sheets/8309fa.pdf
We are using the LT8309 synchronous rectifier for a PFC’d flyback on 80-135VAC input…26vdc output. We are using it like in the attached LTspice schematic (which needs to be run with the attached textfile “bcmpfcfile” in the same folder….the sim file is called “bcmpfc”)
As explained on page 9 of the LT8309 datasheet, there is a fault mode of this synchronous rectifier….this occurs when in DCM as follows…when the secondary coil has discharged, the primary drain voltage can ring down and turn on the primary fet’s intrinsic diode………when the ringing primary current then reverses, the primary fet diode will turn back off again, but due to its reverse recovery it wont turn off straight away, and so some “Phantom” primary current will flow……when it stops flowing, the secondary will then take its turn and flow current…..this can drive the secondary VDS voltage such that the sync rect IC turns on again when it should not….and then the primary fet may turn on at the same time giving unwanted cross-conduction.
The ways given to solve this are either to use a schottky parallel to the primary fet, or to set the turns ratio of the flyback such that the primary drain voltage cannot ring down to ground…neither of these options are open to us because our max primary drain voltage is too great for a schottky…and no matter what we set the turns ratio to…there is always going to be a time when the primary drain voltage can ring down to ground…(because the primary bus voltage falls very low due to PFC operation with little primary bus capacitance).
Do you know any other flyback sync rect IC’s which offer mitigation of this problem?
LT8309 datasheet:
https://www.analog.com/media/en/technical-documentation/data-sheets/8309fa.pdf