How is this topology/circuit called (solar inverter)? - Page 1
Forum section: electronics / projects

author: lordnoxx
author_rank: Contributor
timestamp: December 03, 2020, 04:11:52 pm
content: Hello everybody,I recently came across a broken solar inverter. So I thought...ok..at least I can reverse engineer the power path and find an expected topology or I can learn something new.And I think the letter is the case here. OK...chances were good to find something new (for me at least) as a first sight on the PCB revealed a center tapped transformer which is fed at the center tap by an series inductor. So...long story short...here is the circuit:So I guess it is some kind of "isolated current fed push-pull converter". But what I don't understand at the moment:1. Why is there a synchronous buck converter at the input?2. As far as I know, current fed topologies do not net much input capacitance. But here we have 6000µF.3. How is MPPT handled with this input circuit? Are the current "snippets" the controller receives via the current transformer enough to get an good idea of the actual solar panel current?4. How the hell can this circuit operate without any bulk capacitance right after the secondary site bridge rectifier?5. Gives C1 some resonating effect and thus influences the whole circuit operation in some non-obvious way?Thanks in advance for your hints, tips, and knowledge sharing.

author: Wolfram
author_rank: Frequent Contributor
timestamp: December 03, 2020, 04:31:08 pm
content: The synchronous buck is likely synthesizing the mains waveform, in the full wave rectified form. The buck inductor also doubles as a DC link choke for the current-fed push pull inverter. The capacitor across the secondary indicates that this stage could be resonant to lower the switching losses. The full bridge on the output unfolds the rectified sine wave into a bipolar one, completing the mains waveform synthesis.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 03, 2020, 05:26:40 pm
content: Well, what size is the transformer?Current-fed isn't common, but can be useful at times. Here, it would seem it saves on output filter capacitance.A current-fed forward converter, like this, is complementary to a conventional current-mode forward converter. The only thing that's different is where the filter choke is placed, primary or secondary. (Obviously, replacing diodes with MOSFETs as needed -- and of course we can make a conventional forward converter with synchronous rectification that supports bidirectional power flow, so that's nothing special at all, indeed such a design includes both cases, depending only on the direction of power flow -- making this merely a semantic distinction!)I might be inclined to make a design like this, with better filtering at the center tap, just to reduce losses in the transformer; maybe they figured those losses just aren't worth saving. Depends. Or even better, I'd make one with a bulk supply (DC link) and inverter output, so the transformer doesn't have to be stupid huge. But maybe that's more complicated, since you need an SMPS controller that's line-facing, not just a single buck doing whatever. So maybe they arrived at a solution like this, more through a combination of simplifications, than of optimizations.Is there no input current shunt, or output shunt or CT? Only the one on the buck?If only the one, yeah, it can still be measured that way -- input current is simply the average of that reading -- but it's not as easy, due to the very peaky waveform. (Note: there should be a diode into that burden resistor.) And, I would argue more importantly: since you're not reading the current during the flyback phase, it's harder to control output current -- which I don't like, I would much prefer an average current mode control for something like this.I kind of get the feeling there may be even more simplifications at work here -- maybe they're even doing a complete no-ass voltage mode control, and just faulting out on peak current flow or something like that.As for MPPT, that can be done in whatever way. Even the leftover mains ripple might be enough to do it -- all you need is some repetitive ripple, and a measurement of the fundamental and 2nd harmonic components in the input voltage and current. Drive fundamental in one (in this case, voltage), read all 2nd harmonic (nulled fundamental) in the other (current), there you go. If fundamental is not nulled, then the phase it's higher on is the direction the setpoint needs to be adjusted towards. And if not, then it's still fine to make small changes (like, dither the output power every second or so) and do a slow control that way. And every so often, stop and explore a wider range to check for global maxima, if you're into that sort of thing.You'd have to watch it in operation to figure out how much functionality it has, I suppose. (Or, you know, look up whatever chips it's using for control, which, if that includes an MCU, you'd have to reverse engineer to see how it works directly, and good luck with that, sorta... I mean, it's plenty possible, just likely not worth the effort.)Tim

author: lordnoxx
author_rank: Contributor
timestamp: December 03, 2020, 05:56:11 pm
content: QuoteInductor L1 and transformer L2-L3-L4 are wound on identical E50 cores. Thus I think the core of the transformer might be a little to small to transfer a 50 Hz half-wave.EDIT: It is a 500W solar inverter. For 230V mains.

author: aiq25
author_rank: Regular Contributor
timestamp: December 03, 2020, 06:55:40 pm
content: Interesting design. Never same this topology before.One comment I wanted to make is, for an inverter project I worked on previously, for automotive applications, we put in an input boost converter to stabilize the voltage going into the power stage. This helped with start/stop events and also it helped to reduce the number of turns required for the transformer. They might be doing something similar? Interesting to see no bulk capacitors on the secondary side, like you mentioned.

author: RoGeorge
author_rank: Super Contributor
timestamp: December 03, 2020, 07:05:06 pm
content: How is it called? Best I can do is this:https://www.ti.com/lit/ml/sluw001g/sluw001g.pdf

author: capt bullshot
author_rank: Super Contributor
timestamp: December 03, 2020, 07:40:49 pm
content: I've seen this before, in a "Gridfit" solar inverter.There's a current regulator that controls M1/M2. M3 and M4 are simply turned on / off alternating with afair half the switching frequency of M1/M2.This way the current through L1 is transformed as AC to the secondary (line) side of the transformer. The diode brigde rectifies this, while the Mosfet bridge steers that current into the power line with the correct polarity. As it is a current, no smoothing capacitor is required here.The setpoint of the current regulator is modulated by a full wave rectified sinusoidal waveform synchronized to the power line. A small uC generates this, and also does the MPPT algorithm. Can't remember whether the uC has more inputs or if it's just the output of the current sense transformer. I remember the whole thing was quite optimized in terms of parts count and signal isolation (only two optocouplers between DC and AC side). One of the optocouplers transferred the analog setpoint signal from line side to the DC input side.The large bulk capacitor at the input is required because the solar panels deliver constant DC current, while the regulator input is said full wave sinusoidal waveform. So one needs some capacitance here to buffer that.

author: Phoenix
author_rank: Frequent Contributor
timestamp: December 04, 2020, 02:10:19 am
content: Quote from: lordnoxx on December 03, 2020, 04:11:52 pmIt looks like C1 filters the high frequency currents from the push-pull so they don't cause the output rectifier diodes to switch at the high frequency, then the diodes to rectify the 100Hz component only.

author: uer166
author_rank: Super Contributor
timestamp: December 04, 2020, 02:50:44 am
content: Quote from: blueskull on December 04, 2020, 02:22:20 amInterested..Any reference designs or papers on this specific topology? (no matter where the inductor is located), or is this something you naturally arrived to.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 04, 2020, 03:07:59 am
content: Like I said -- it's just your basic full-wave forward converter with choke output filter, turned around.The buck into choke is equivalent to a boost on the output, which you wouldn't normally see, because why bother with anything more, you've already used diodes, just adjust the transformer ratio to give an excess voltage range, PWM it down to nominal, and be done. But you could do this just fine, if you wanted to. (Maybe it would be desirable on a wide-range bench supply?)Well, turning it around, we can't use diodes on the primary side, we have to use controlled switches. In particular, we must use something to deliver a controllable current, and a peak or average current mode buck will do alright for that. The secondary side can again be as simple as possible, transforming what used to be a primary side H-bridge into a secondary side FWB (or 1/2B into FWD, or PP into FWCT), no filter choke, straight into caps.Tim

author: David Hess
author_rank: Super Contributor
timestamp: December 04, 2020, 02:17:01 pm
content: I know it as current fed push-pull, and parallel resonate in this case as well? Tektronix evolved this topology in their 22xx and 24xx series oscilloscopes back in the 1970s starting from a voltage fed push-pull. Current fed has the advantage of enforcing balanced flux in the transformer.I suspect current fed typologies are easier to use in grid-tie applications because of their inherent current limiting.