LT3757 as a 2KV voltage flyback controller

Hi!

  I'm in the process of testing a 2KV power supply using an LT3757 device.  It has the basic sample high voltage circuit shown in data sheet (see below):

  except with a voltage multiplier diode configuration after the transformer:

  In the lab, with an open load at the output, load net Out3 measures 1.1KV instead of 2KV as predicted by simulation.

Feedback (FBX) pin reads .9V all the time, and never gets to 1.6V as expected.  Is there something I'm missing for proper biasing the LT3757 part?

Again, simulation runs fine with open load conditions.

Thanks!

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