Opto-isolated SMPS with high voltage output needs special feedback circuit.

[Faringdon]

Hi,
When we do SMPS with high voltage output, like 180Vdc, then its more difficult to avoid ending up with a very unfortunately low  feedback loop bandwidth….
Therefore, for eg an opto-isolated SMPS as attached, its very important to do the feedback loop like in  Circuit “B” of the attached. Circuit “A” has two paths from Vout to the error amplifier, and these paths 'fight' against each other……and in solving this you end up with an unfortunately lower feedback loop bandwidth.

(LTspice simulation and jpeg schem as attached, shoudl you wish)

So do you agree, Circuit “B” is better? 

(Also attached is the LTspice Sym and Sub files for the TL431...should you wish for  them)

[T3sl4co1l]

1. Just for efficiency's sake, using a secondary side aux winding or tap isn't a bad idea.  Then you can draw some ~mA without big resistors or dropper zeners.

2. In what way is the bandwidth reduced?  The supply dropper shown provides a feed-forward path.  It's a hybrid between an all-zener divider (no error amp / ref) and a conventional error amp approach.  As such, adjust the resistor values to get the right amount of pole-zero cancellation or whatever.

Of course in the #1 case, you wouldn't have this effect.

3. Can use a lower current ref like TLV431 (or the higher voltage relatives) to save current in general, and to make back the low bandwidth of the phototransistor, it can be cascoded.  Some controllers do this out of the box, others can be wired (here, add a resistor from COMP to FB and tie opto from VREF to the summing node FB), still others you might need the added hardware (an explicit transistor?).  Photodiode types can also be used (or photodiode-strapped phototransistor, when using 6-pin types), needing significant current gain but better performance can be had with an appropriate amp than the phototransistor alone.

Also, note that EMI can be much worse with high voltage outputs, as the edges from both windings fight each other and common mode can get pretty nasty.  Transmission line transformers help.

Tim

[Faringdon]

2. In what way is the bandwidth reduced?  The supply dropper shown provides a feed-forward path.  It's a hybrid between an all-zener divider (no error amp / ref) and a conventional error amp approach.  As such, adjust the resistor values to get the right amount of pole-zero cancellation or whatever

Thanks.....
Page 45 of the below explains the inferiority of the "A" approach...
https://www.onsemi.com/pub/Collateral/TND381-D.PDF

...But the inferiority of the "A" approach to feedback  compensation of SMPS is further detailed in Christoph Basso's book. The relevant pages are as copied here, apologies for the bad camera.
Its clearly stated about the inferiority of the "A" method. Its made clear that you cannot adjust component  values to make "A" better than "B" (or even as good as "B"). "B" can always be made better than "A"....and with HV SMPS, you need all the help that can be gotten with the feedback loop....to try and stop it being otherwise so lethargic.

If you read the page 279.....Basso refers to the "A" approach as a "tragedy"....and goes on to say "B" method is better.

As you know, the "A" approach is only ever used because of cost , and cost alone......because it doesnt need the extra bias supply its  cheaper.

The feedback becomes , as you know, more critical with high voltage output SMPS's....and we need all the help  we can get...so the "B" way seems the best for HV output?

Page 279 of Basso's book "Designing control loops for linear and switching power supplies" discusses the limitations that "A" places on feedback loop compensation......as you know, for HV output SMPS, you need all the  feedback loop "goodys" that you can get....so the  "A" approach has to be rejected in favour of "B"?