Help Needed Finding Full Bridge DC/DC Converter Output Impedance

#1

Hi All,

I’m trying to determine the output impedance of a Full Bridge Converter using the procedure outlined in this video. But, the results don’t really make sense to me, and I have a few residual questions.

  1. The video above mentions that this method of testing output impedance will work for most converter topologies. This implies that there are some topologies where this won’t work. How do I know which topologies this method won’t work for?

  2. Should I be testing this with a load-resistance attached? If so, what value should I pick? I want my converter to work with a decent range of load sizes.

  3. Does anyone have any intuition on what size injection currents ought to be? I know the video mentions using 5-10% of load current. Why this value? Also, how should I calculate this if I want a decent range of load currents?

  4. I would hope that the output impedance would follow a relatively smooth curve. The results don’t quite look like that. Any thoughts on what’s going on here? I’m injecting about 10% of maximum output current.

Any insight is greatly appreciated!

(I tried to upload a picture with my post, but the forum does not want me to do that. I will try to post my circuit as a reply.)

#2
  1. The video above mentions that this method of testing output impedance will work for most converter topologies. This implies that there are some topologies where this won’t work. How do I know which topologies this method won’t work for?

If it has a DC steady state it will work. Something like a PFC might be tricky.

  1. Should I be testing this with a load-resistance attached? If so, what value should I pick? I want my converter to work with a decent range of load sizes.

Yes with load impedance you will need to run with different loads, this curve will change with different loading.

  1. Does anyone have any intuition on what size injection currents ought to be? I know the video mentions using 5-10% of load current. Why this value? Also, how should I calculate this if I want a decent range of load currents?

it needs to be small signal. so we need to inject a perturbation that we can measure but won’t force the converter to do something like switch into DCM or some other boundary condition

  1. I would hope that the output impedance would follow a relatively smooth curve. The results don’t quite look like that. Any thoughts on what’s going on here? I’m injecting about 10% of maximum output current.

You’ll need to play with the settings. there is another video that gives tips on how to trouble shoot the sweeps.

Please post your schematic.

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