Stability problems with LTC3757 used as boost converter

Hello!

I want to use the LTC3757 as booster (with a strong NMOST and diode). It's working and it's stable. But when I turn on the load the regulation will get unstable, because:

  • The output caps are 5uF ceramic.
  • Then: power ferrit bead.
  • 470uF electrolytic capacitor.
  • Then: power ferrit bead.
  • An H-Bridge, stabilized by 5uF ceramic.

And now the problem: The half bridge outputs into a tank circuit with a resonance frequency of 150 kHz. And we get a DC load of approx. 0.5A and an AC load of  2A. So there is some energy fed bock into the capacitors. And in this case the regulation of the LTC3757 loses the stable point.

yellow: output ac current, 2A/div
blue: output half bridge
magenta: switch voltage (unstable duty cycle)

What can I do? Making the control slower by putting some nF to the compensation network pin. Filtering the feedback voltage divider (I already use 10nF added to the feedback network to get a corner frequency of approx 3kHz)?

Best regards and thanks for the help!