LT3757 as a 2KV voltage flyback controller

Question

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!

W.G · Accepted Answer

App note 39 is a good read: 
 https://www.analog.com/media/en/technical-documentation/application-notes/an39f.pdf 
 You might want to try increasing the values of the RC that are connected to the SENSE pin.)

W.G · Answer

In the simulation, if the transformer is not ideal, e.g. with some leakage inductance, the turn-on current spikes would probably affect the operation as well. Personally, instead of the voltage multiplier, I'd go with the stacked windings. Another good read: 
 https://www.analog.com/media/en/technical-documentation/tech-articles/1000-V-Output-No-Opto-Isolated-Flyback-Converter.pdf

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LT3757 as a 2KV voltage flyback controller