LTC3769 Synchronous SEPIC configuration

Hi,

I am using LTC3769 controller for my synchronous SEPIC project. I have done full calculation and simulation for this configuration. And it works for simulation. However, when I am doing bench testing, Vds of synchronous MOSFET voltage rise quiet slowly which might cause shoot through problem. I have tried to parallel a Schottky diode with my MOSFET to exclude the reverse recovery effect of the MOSFET body diode. But the problem still remains. The detail schematic and waveform are attached below. 

Overall Pulse CH.1 Syn FET Vg - GND; CH.2 Syn FET Vs - GND; CH.3 Main FET Vds; CH.4 Syn FET Vd - GND

Main FET turn-off

Main FET turn-on

As you can see from last waveform, the main FET seems turning on before syn FET turned off. Large negative voltage overshoot found on Main FET Vds. Do you have any suggestion for this problem?

Best regards,

Mike

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  • Hi Victor,

    Yes, it is a boost controller. But normally a boost controller can be easily configured as a SEPIC converter, what feature makes this one not suitable for syn SEPIC topology?

    Best regards,

    Mike

  • +1
    •  Analog Employees 
    on Jul 16, 2020 7:40 PM 9 months ago in reply to Mike Zeng

    The sync FET driver you are using is not in LTC3769 datasheet, so it's unclear how the "Top Gate Off to Bottom Gate On Switch-On Delay Time" and "Bottom Gate Off to Top Gate On Switch-On Delay Time" will be affected. Besides there's significant ringing on the gate of the sync FET, so actual state of the FET channel is not clear as well. You may take a look at the sync FET driver. We have wide selection of the buck-boost converters that might meet your spec. The DC2234A covers the synchronous SEPIC, if this topology is preferable.  

Reply
  • +1
    •  Analog Employees 
    on Jul 16, 2020 7:40 PM 9 months ago in reply to Mike Zeng

    The sync FET driver you are using is not in LTC3769 datasheet, so it's unclear how the "Top Gate Off to Bottom Gate On Switch-On Delay Time" and "Bottom Gate Off to Top Gate On Switch-On Delay Time" will be affected. Besides there's significant ringing on the gate of the sync FET, so actual state of the FET channel is not clear as well. You may take a look at the sync FET driver. We have wide selection of the buck-boost converters that might meet your spec. The DC2234A covers the synchronous SEPIC, if this topology is preferable.  

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