Thanks, but they definetely werent referring to the Boost PFC...they were referring to the SMPS that supplied the HV equipment....Also, they didnt use Boost converters for it...they used LLC's...they said they had to, because in the HV transformers, coupling was poor, so they made use of the leakage inductance.
The guy that said this is one of the foremost HV SMPS engineers in UK......
........i believe you have gathered the info below, but ill just refresh it if its ok......the whole idea of this (as in top post schem) is to make the 180V output "Look like" a 30V output to the controller. Hence the divider to divide the 180V down to 30V.....then this is fed to the emitter follower with Q2.....yes you just look at emitter of Q2 being 0.7V below its base voltage all the time.
The idea i believe is a fantastic way of regulating high voltage SMPS.
I couldnt just divide down the 180v and then feed that to the "controller's divider"...as it would have been too high impedance....so i put the Q2 there....so the "30v divider" sees the output through the emitter of the Q2, which looks like a low impedance........because when you "look" into the emitter of a common collector BJT (Q2), you are "seeing" into a low impedance.....Anyway, what i actually wanted to do, was to transport any ripple/fluctuation voltage on the 180v rail directly down to the "30v rail"...without it being divided down....so that it really does look like a 30v rail.........i wanted to kind of copy/paste the ripple of the 180v rail to the "30v rail"....so i put also the capacitor across the base resistor of Q2....so that at high frequency, i am literally shorting the 180v ripple to the "30v rail" (or trying to)..and the controller will then be fooled into believeing that it really is regulating 30v.
The intial results look very promising......when we try to regulate high voltage...we have to divide it down heavily...and this messes things up for the controller...because it then has to have gain added to overcome the high attenuation of the divider, and thats when problems start...because the gain will act on many things.......not just the thing we want it to....this is why HV SMPS have more limited options for feedback loop regulation.....eg start up vout overshoot is much harder to solve with HV outputs...not only that.....but when you dont do it like the above...then your compensation capacitors across the upper divider resistor have to be 250V rated caps...which is a pain when you need to tweak them........why not just take it all down to 30V, and then work on it there with your standard 50v ceramic compensation caps.?......i honestly believe the above is the untold secret of high voltage SMPS feedback regulation....the only problem might be sourcing high enough voltage NPN's for Q2.......though a Darlingtom would work there.
It is absolutely doubtless that HV SMPS's present more difficulty for SMPS feedback loop regulation.......i used to sit near one of the best HV SMPS designers in UK....he was a top guy at the Marconi place...i overheard him and the softy talking about how they would regulate their new HV SMPS (>13000v output).....they were discussing that they would only be able to manage a feedback loop bandwidth in the "Hz range". (they werent talking about the PFC bit)
Let's say you make it 200:1 divider, so Rl~=1kOhm, and Rh is 200kOhm.
1kOhm with 100pF has a roll-off of 1.6MHz
...As you know, the fact you have a 200:1 divider in the overall loop, means you then have to bring in extra gain to take account of that...but bringing gain into a feedback loop is not simple to do...that extra gain will mess things up in other ways .....making the feedback loop options more limited.
As you know, when you have a 200:1 divider, compensating it is not as simple as just putting in an amplifier, with sufficient gain to take account of the 200:1 attenuation........its just not that simple.........for that degree of attenuation, finding how to put the gain back into the loop is very complex, and always restrictive in terms of possible outcomes....this is why HV SMPS are so non-conducive to good feedback loop compensation.