High voltage output SMPS control problems? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: May 04, 2022, 02:17:44 pm
content: We anticipate control problems due to the high voltage output of our 180Vout (390Vin) , 2kW LLC converter. ...And just wondered whether the attached method of "making a high voltage output look like a low voltage output" has any mileage?...It uses a divider and emitter follower to make the 180Vdc "look like" 30Vdc.Is this of any use?(LTspice sim, LTspice .sym and jpeg schem attached should you wish)

author: uer166
author_rank: Super Contributor
timestamp: May 04, 2022, 07:32:42 pm
content: Quote from: Faringdon on May 04, 2022, 02:17:44 pmWhy do you anticipate control problems exactly? Are you sure this is not a solution in search of a problem? That's an awful lot of parts to fix something that may be perceived only.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: May 04, 2022, 08:22:19 pm
content: Quote from: Faringdon on May 04, 2022, 02:17:44 pmLook in chip manufacturers application notes.Andy

author: Faringdon
author_rank: Super Contributor
timestamp: May 04, 2022, 10:26:13 pm
content: QuoteAs you know, error amplifiers etc are down at ~5vdc.....but 180vdc otuput is well above that...so it needs to be divided down before the controller can operate on it...and thats where the problem starts in the control loop.The controller "sees" the output through the divider...and since the divider decimates the heck out of it when vout is high..the controller then struggles a little.I used to work in a HV SMPS place many years ago, and i remeber them saying they were only going to achieve loop bandwidths in the Hz range for some of their HV supplies.Also, i think the added capacitor (as attached) improves things.....it needs the cap, otherwise its just another way of dividing down............at first glance, the one with the cap looks great in terms of transient response etc...and lack of overshoot on startup.

author: uer166
author_rank: Super Contributor
timestamp: May 05, 2022, 03:56:21 am
content: Quote from: Faringdon on May 04, 2022, 10:26:13 pmHave you actually done the math or are you talking out of your ass? Example design: 100pF loading at the FB pin/opto/whatever node, with a 200k total divider resistance. Let's say you make it 200:1 divider, so Rl~=1kOhm, and Rh is 200kOhm.1kOhm with 100pF has a roll-off of1.6MHz. Are you somehow trying to close the loop at above 500Khz on the voltage output? Adjust values as needed, but unless you know that, for whatever reason, the roll-off of a regular old resistor divider is less than your converter BW, why throw all this complexity at this non-existent problem?

author: uer166
author_rank: Super Contributor
timestamp: May 05, 2022, 04:12:59 am
content: Quote from: Faringdon on May 04, 2022, 10:26:13 pmThat sure is a thing for PFC and any boost converter due to a high step-up ratio. PFC can famously have super low loop BW. It has also absolutely nothing to do with the output voltage divider, and instead is about the RHPZ,which does not exist in your design.

author: Faringdon
author_rank: Super Contributor
timestamp: May 05, 2022, 09:10:15 am
content: 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)Quote...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.

author: uer166
author_rank: Super Contributor
timestamp: May 06, 2022, 09:21:51 pm
content: Quote from: Faringdon on May 05, 2022, 09:10:15 amSorry to say but I think you need to revisit basic control loop theory.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: May 07, 2022, 12:53:09 am
content: Hi,There is zero benefit from adding the BJT stage to the divider as illustrated by this model:Both of the dividers have the same division ratio of 72:1 or -37.14dB.At 180V output you will dissipate 2.7W in R1 and R2.The phase advance network can be added and again a passive network can be designed to match the circuit with the BJT.Jay_Diddy_B

author: Faringdon
author_rank: Super Contributor
timestamp: May 08, 2022, 12:24:32 pm
content: Thanks, it looks like the only reason for this "emitter follower with cap" is to save us money in tweaking the upper divider resistor cap compensation components.