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LT3757A 5V to 170V Flyback Design Inquiries

Category: Hardware
Product Number: LT3757A

Hi everyone,

I'm designing a flyback converter using the LT3757a to boost a 5V input to 170V:

With a 40mA active current load on the output, I'm getting a pretty clean output at 170V:

However, I'm seeing several issues on the other stages:

  • Big current spikes on my input source:

  • These input current spikes correspond to these huge current spikes and voltage oscillations on the primary coil side:

The input caps don't do anything after startup in terms of smoothing out the current spikes. I'm pretty sure this is a modelling issue where I haven't taken some parasitics into account, but I'm not sure how to fix this.

I specified a coupling coefficient of 1 for the inductors, so there shouldn't be any leakage inductance causing the oscillations. 

If I could get some help with the design to either fix these issues or verify that this behavior is expected, I would appreciate it!

EDIT: so I found that all of these issues improved after increasing the inductance values in my coupled inductor. By changing them to 10uH/1000uH, the oscillations went away and the current spikes decreased to about 5A. Increasing the inductance values further reduced the current spikes on the input to about 3.5A, and further increased in inductance saw very little change.

I assume this indicates that my inductors were saturated, but I don't understand how the oscillations/current spikes relate to this.


Added further data
[edited by: soohan at 8:48 PM (GMT -4) on 17 Mar 2023]

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