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From this Reference Design I have two questions:

  1. They are doing PSR (Primary Side Regulation) with LM5020. However, according to the datasheet LM5020 is a Current Mode PWM Controller which was not designed for PSR, compared to other controllers like UCC2871x for example, that clearly specify that are optimized for PSR. So, would LM5020 work in PSR at the expense of less accuracy or something?

  2. Is the output Pi filter (C8, L1, C9) needed? Could I remove L1 and leave just a capacitor at the expense of more noise?

  • This circuit is for three phase high voltage. Once fully rectified the peak DC voltage will exceed the HV rating of the UCC2871X. – Robin Iddon Jul 10 '20 at 06:46
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    You see that the LM5020 based circuit had to take significant steps to produce VDD for the switcher from the primary. Once you look past that, there isn't so much difference between the two. Without the pi filter the output will be a mess. All the switching pulses will appear in the output. – Robin Iddon Jul 10 '20 at 07:03
  • @RobinIddon Thanks. Just an additional question. The input varistors have a common point. Where does that common point go? Normally one puts varistors between line to line, or line to neutral. But in this case the varistors go from the line to nowhere. –  Jul 10 '20 at 13:11
  • The varistors will work in pairs to shunt voltage surges on one conductor to the others. I actually don't understand the 3phase + neutral rectifier circuit in the reference, I missed that aspect when I looked at it. I wonder whether there is more info on that at TI somewhere. It's like it's designed for either single L-N or three phase usage. – Robin Iddon Jul 11 '20 at 17:44

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