***ABOUT SMPS*** - Page 1
Forum section: electronics / beginners

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: February 27, 2024, 08:35:06 pm
content: Hello,by time i will add here more questions. I dont have anyone to ask, so big thanks for reply!I dont understand how can one EA sink current at TL494. If you look in functional diagram there are diodes in the way.This is written in datasheet:https://www.ti.com/lit/ds/symlink/tl494.pdf"Both high-gain error amplifiers receive their bias from the VI supply rail. This permits a common-mode inputvoltage range from –0.3 V to 2 V less than VI. Both amplifiers behave characteristically of a single-endedsingle-supply amplifier, in that each output is active high only. This allows each amplifier to pull up independentlyfor a decreasing output pulse-width demand.With both outputs ORed together at the inverting input node of thePWM comparator, the amplifier demanding the minimum pulse out dominates. The amplifier outputs are biasedlow by a current sink to provide maximum pulse width out when both amplifiers are biased off."#1 How can be "ANALOG OR" realized?Ive tried to measure compensator transfer function with added 47R before resistor divider and connected generator across this resistor, then take Vout as "Vin" and voltage at comparator input as "Vout". results on bode plots were unusable, to much distortion. Another problem is that EA does not have negative rail voltage so i cant measure response of EA alone with added DC offset it has no way how to bring this DC offset at negative EA input.#2 Is there way to bode plot compensator transfer function without getting SMPS turned on with load connected?#3 Is true that TL494 use OTAs instead of classic OAs?#4 Is possible to add external EA for PWM comparator within TL494? "Analog or" will be needed i guess =)#5 Does anyone here successfuly finish project with TL494 completely stable at DCM and CCM "buck derivate topologies"#6 Has anyone experience with bode plots on rigols MSO series usage on SMPS?Are results sufficient?I have problem with TL494 compensation at DCM. When i did control to output bodeplot i get about +90 degrees phase shift at low frequencies, type 3 noninverting compensator has -90 degrees at low frequencies. Well i calculated it with classic OA, if tl494 has OTA then my results are wrong becouse my aproach was to calculate voltage at nonivnerting input, then calculate current through resistor connected paralel at inverting input to ground and then multiply it with total impedance (EA output to ground). I will be thinking about forced CCM, if it does not add to much complexivity.#7 Since TL494 does not offer 180 phase shift, i though about SG3524, but i would like to has two EA to adjust voltage and current.So is possible to have external EAs at SG3524?, how can i do it?There will be something like "inverted analog or i guess =)".I am also thinking about average current mode control instead of voltage mode but CMC is more suscetible to noise and its not good at light loads.I am constructing high voltage, high power, high current, high everything... =D SMPS, i want voltage and current adjusting from zero to nominal value.#8 What do you think about CMC, is it preferable for regulated smps?https://www.allaboutcircuits.com/uploads/articles/Keim_10_13_Fig_3.jpgHere is schematic with average current mode control, to add current control i will need add another feedback loop for current and i need to "OR" output from voltage loop EA and current loop EA together at noninverting input of "current error amplifier in the schematic".#9 Am i right?I think its enough for now. Thaaanks

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 27, 2024, 08:38:49 pm
content: Always check the appnoteshttps://www.ti.com/lit/an/slva001e/slva001e.pdfUnusually, they actually go into near-complete detail, giving an at least representative transistor-level schematic for all blocks. A diode-OR is of course meaningless without some pull-down; the diodes are in fact emitter followers, and a 300uA sink provides pulldown (p.9, Fig. 9).Hah, typo, the inverting input shows an upside-down NPN. Clearly they meant a PNP there.Classic voltage-mode op-amp, no OTA here. Perhaps it can be abused as an OTA (e.g. compensation by R+C from COMP to GND), but this will depend on output current limits and likely give a nonlinear response (i.e. requiring low R and large C, just for, at best conditional effect, not general smooth and linear operation).The two internal op-amps are useless, one of them will be I mean, for current mode; you need them cascaded, not paralleled. Simply strap one off as disabled, and add an external op-amp. Usually this will be a TL431 ref, which is better described as an op-amp with open-collector output and conspicuous yet highly stable Vios.You might also want a more specific title for this thread...Tim

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: February 28, 2024, 05:20:13 pm
content: Quote from: T3sl4co1l on February 27, 2024, 08:38:49 pmAre you making reference to this image:https://www.allaboutcircuits.com/uploads/articles/Keim_10_13_Fig_3.jpg?There are OA cascaded. OA with inputs across rsense is just diferential one. Lower OA in the right is EA for voltage loop (lets call it EAV), then EA on the left is for current control (EAC).How this works? So EAV has its own reference on the noninverting input and want to has same amount of voltage on inverting pin, so it will adjust output voltage on noninverting pin of EAC. EAC has its inverting input connected to voltage equal rsense (probably multiply it with some factor, like 10 or so...). Every load has its own current which will flow when its connected on the output of PSU. EAV is trying to find current which will match output voltage and EAC is doing same job. EAC is trying to find voltage on its own output when both inputs match. Output voltages of EA are defined by A*(Vnon_inv-Vinverting) so it starts with max output and decreased it i guess? So it will overshoot current and then settle.But how is current adjusting added? Thanks

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 28, 2024, 08:13:39 pm
content: Yes, this diagram. Match up blocks. You already have part of the "power stage" in the TL494. You have the comparator and a ramp generator. You have a direct connection from a pair of op-amps to the comparator: this is your current error amp. The other one cannot be used (unless maybe as a U/OVLO, or some other process variable, temp limit, etc.) so probably just gets strapped off. Voltage error amp has to be a separate chip, and can be as simple as a BJT if you don't need much for regulation, but preferably is an op-amp proper.TL494 doesn't have a current sense amp so you need to add one externally; the diff amp in the diagram can be a current sense/shunt amplifier, Hall effect sensor, pair of current transformers in the power stage, etc.All the EAC is doing is comparing inductor current to the setpoint, and that setpoint comes from the EAV. Imagine you were controlling, by hand, a current source to regulate output voltage: you'd be adjusting this setpoint manually (pot from VREF to GND, say). But you're looking at VOUT in relation to VSET and adjusting it accordingly, which an EA can do automatically, and much faster than human reaction time.And we can limit the EAV's output range, so it only ever commands a current setting that is within nominal design range.We physically cannot overcurrent the power stagethis way, not under normal operation at least.Put another way: section off the power stage, PWM, EAC and sense amp. Wrap all of that into a block. What does it do? It takes in some power (how much, doesn't matter, other than for a buck, Vin > Vout), sends out a current, and that current is in proportional to the input voltage (current setpoint / EAV output). We have a transconductance amplifier.Which also means we can connectunlimited power stages in parallelif they are of this type: where the inductor current is controlled locally, in loop with its respective EAC. Such a circuit is highly scalable.Finally, for dynamics, notice the gm amp (the whole power stage + EAC block) has a dominant-pole response cutting at a well-defined frequency, given by inverter gain, inductance, and EA compensation. We put this inside the voltage loop, which has a dominant-pole response cutting at a somewhat lower frequency, given by gm amp response, the capacitor, and EA compensation.This pole can be higher than 1/sqrt(LC), thus we have better performance (lower output impedance, and to higher frequencies) than a voltage-mode control which has to have the compensation pole below the LC resonance.Overshoot is not at all assured: the EA can approach steady-state gradually and settle in, or too quickly and overshoot. There is no RHP zero as in a peak current mode control (although I don't actually know the details, and probably analysis is a bit worse in DCM; ACMC somewhat assumes CCM, or more to the point, is easiest to understand in CCM), so a simple 2nd or 3rd order overall response, of whatever desired prototype (i.e. Butterworth, etc.), is possible.PCMC tends to have overshoot, or a compromise in response (slower than might otherwise be expected), due to the RHP zero at Fsw/2, making it a non-minimum-phase system; the control pole is thus forced to be at least a bit lower than this (maybe Fsw/5 or something?). But this is fine, as using electrolytic capacitors for output filtering, they generally need to be more than large enough to meet this, and thus the voltage loop compensation pole is low enough to avoid the zero. A zero can also be added to the compensation (pole-zero or type 2 compensator) to win back further phase margin.Which, speaking of PCMC, almost every diagram you see of UC3842 has an R||C in the feedback network, which is... just bizarre. It's intentionally throwing away DC gain (and thus regulation and PSRR) just to make it a tiny bit easier to compensate (the limited gain at any frequency, means somewhat less chance for oscillation). Maybe not bizarre, being the best word for it, but definitely lazy. Lazy by both author and reader: another one of those oft-repeated errors that's just good enough to pass, and no one stops to question it. Well, I do, and I disagree.(The resolution is trivial: put the R+C in series instead of parallel, and adjust values until compensation is optimal.) (There is a further quirk of this, were the R||C sometimes is shown after an optoisolator, when compensation has already been realized on the secondary side around a TL431 or whatever. Which is just silly; the COMP pin can be driven by opto directly, or the error amp strapped as inverting-gain-of-1 with a pair of resistors, no compensation needed or desired. I think I've seen this a few times in TL494-related schematics too, additional compensation after the error amp proper. Go figure.)Tim

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: March 02, 2024, 10:19:29 pm
content: Thanks for detailed description!I'm a little confused about the behavior of error amplifiers.If you look at the block diagram of the TL494 (https://circuitdigest.com/sites/default/files/inlineimages/u2/TL494-Inverter-IC.png), you can see that when the OR after the PWM comparator is at HIGH logic level, output switching is disabled. If we look at the ACMC diagram againhttps://www.allaboutcircuits.com/uploads/articles/Keim_10_13_Fig_3.jpgwe see that the EAV will try to null the voltage difference at its inputs and thus increase the output voltage according to the equation gain*(NotInv-Inv). NotInv = Vref, Inv will be equal to 0V when the circuit starts. Same thing happened with the EAC. When we get to the PWM comparator, we see that its output is HIGH when the voltage at its non-inverting input is greater than the voltage at its inverting input, i.e. at the input to which the Vramp is connected. Vramp varies from 0V to 3V aprox. And at the non inverting input we have output of EAC which starts at higher voltage then peak voltage of Vramp i guess. I'm struggling a little with when the output of the operational amplifier changes the polarity of the voltage derivative, i.e. if the voltage will rise or fall. That is, when the circuit starts, the maximum voltage at the output of the error amplifiers corresponds to zero pulse width, i.e. the voltage at the output of the source cannot increase and the result of the equation Gain*(NotInv-Inv) of the operational amplifier will still be the same because the voltage at the inverting input will not change. This confuses me a bit. I though EA need some response when it sets its output high, by response i mean something that "tells it" that output voltage of power supply start to change, but in this circuit increasing voltage at EA output makes duty cycle actually smaller and when EA output start at its maximum voltage it cant has response then, so i am confused a bit with -> when EA decide to go down with its own voltage to trigger output response. I hope you can understand what I mean =)Thanks,

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 03, 2024, 01:43:24 am
content: Well if it's polarity you have a problem with, just swap it aroundTL494 is a bit weird in that its PWM variable (V(COMP)) is "reversed", but it can simply be whatever it needs to be, and you swap the op-amp inputs to close the loop with overall negative feedback.Helps to go around the loop, note each unit's sign/polarity, multiply them all together, and if after traversing the complete loop, you get negative, then it's negative feedback, and that's all you need.Tim

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: March 03, 2024, 09:24:27 pm
content: So i can disable op amps in tl494 by setting inverting input at higher voltage level than noninverting input, so letsay Inverting inputs is connected at Vref and noninverting ones are grounded, by doing so i have 5V feedback pin which is equal to 0% duty cycle. Someone mentioned through PM that i can use internal EA in inverting mode which i dont think so. I think there is way to add inverting EA by disabling internal op amps, add BJT NPN to the feedback pin and drive it with external EA configured as inverting amplifier. Am I right? Thinking about response of PSU, since BJT need small current to shut down feedback pin to ground level, so i am curious how EA will handle it =D

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 03, 2024, 09:30:18 pm
content: Just strap off one and use the other, no need to mess around with back-driving the pin.

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: March 03, 2024, 09:40:44 pm
content: Using one internal op amp, and disabling second one for ACMC will be better aproach. I am just trying to understand how it will behave with BJT controling feedback pin =).(trying to convert tl494 for usage with external EAs connected in inverting mode)Edit:During night i have realized that its impossible to introduce negative feedback when ramop voltage is connected to inverting pin. BJT + inverting EA give positive feedback, and with negative, it will not work anymore.