How does this circuit work (part of double-conversion UPS)? - Page 1
Forum section: electronics / projects

author: TheMG
author_rank: Frequent Contributor
timestamp: June 03, 2020, 12:10:25 am
content: Please excuse the sloppiness of the schematic diagram, I'm must confess I'm using a pirated copy of DaveCAD and it's a bit buggy.Anyways, in taking a peek under the hood of Eaton/Powerware 9125 UPS, I became curious about how these things work. Almost everything you find online and in technical school about double-conversion (aka "online") UPS, they show a simplified block diagram that basically goes AC in -> rectifier -> inverter -> AC out. In reality, of course it's never that simple. The thing that really got my curiosity about these UPS is the lack of any input rectifier diodes, or at least not in the conventional sense.So I did a little bit of reverse engineering and came up with the diagram attached below. Most of it makes perfect sense to me: Q27 and Q29 in combination with L3 and some filtering capacitors make up the output inverter, and this is driven using a 20kHz PWM drive signal to generate a sinusoidal 60Hz 120VAC waveform at the output. Q1-6,9-15 form a full-bridge chopper at 100kHz through a step-up transformer to generate the +/- 180V DC bus to the inverter when the UPS is running from the 48VDC batteries. To charge the batteries, Q7 and Q11 are switched to use the transformer in reverse, as a step-down, utilizing the body diodes of the MOSFETs Q1-6,9-15 as rectifiers. No problem there.What I can't wrap my head around is how the +/- 180VDC bus is maintained during AC mains operation. The bus voltage remains at 180VDC despite wide fluctuations in AC input voltage, I've tested it on the VARIAC and can get down to 60VAC (half rated voltage!) before the UPS switches to battery operation. Even at 60VAC, 180V is maintained on the DC bus. L2, Q28, and Q26 obviously form the input rectifier circuit. At first, I assumed a synchronous rectifier due to the use of IGBTs, but it's obviously a lot more than that, as it also seems to function as a boost regulator of sorts. Looking at the drive waveform with a scope, these IGBT are also switched at 20kHz PWM, almost seems like the inverter circuit but in reverse. I'm assuming this is also how the input power factor is kept near-unity.The gates of all transistors are driven from a DSP, not from any purpose-designed SMPS drivers or anything like that, so there are no datasheets to get theory hints from, and also the service manuals and schematics for these UPS are unobtainium.Can anyone shed some light on how the highlighted part of the circuit operates as both a rectifier and PFC boost regulator all at the same time with only two transistors?

author: duak
author_rank: Super Contributor
timestamp: June 03, 2020, 01:11:49 am
content: PWM power stages can often transfer power in both directions depending on the voltages and the gate drive to IGBTs. The highlighted circuit is also used in servo drives for motors. During acceleration and running the power is from a DC power source and the flow is to the motor. During deceleration the motor becomes a generator and the power flow is reversed and the energy goes into charging the bulk capacitors. In other words, it could be considered a four quadrant, half-bridge converter with a split power source.Consider the power company to be a motor acting as a generator that we are trying to slow down. By artfully controlling the IGBTs, the power stage acts as a boost converter to charge the bulk capacitors by modulating the current through the inductor. The output side is a similar circuit that takes the Raw DC and develops an AC sinusoidal output.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 03, 2020, 01:54:45 am
content: http://techno-fandom.org/~hobbit/cars/boost-hack/That said, the arrangement of the transistors/diodes on the high voltage side of the transformer does look a bit strange in that it is unipolar when the low voltage side is bipolar.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 03, 2020, 02:03:46 am
content: It's a full wave voltage doubler, except with the inductor and switching, it's able to do so in a continuously variable manner.The waveforms will actually look very similar on the two sides, for obvious(?) reasons. One side will be slightly off the other in duty cycle, so as to draw rather than deliver current, and to account for losses.Tim

author: Phoenix
author_rank: Frequent Contributor
timestamp: June 03, 2020, 11:36:51 am
content: Quote from: NiHaoMike on June 03, 2020, 01:54:45 amOne of the transformer hv windings might be reversed from how it's drawn. Ie they both start on the outside and finish at the center tap. I think that would give the bipolar Flux like a pull-pull.As for the AC side active rectifier. The key advantage of that doubler topology is to carry the neutral connection through to the output - which is very important for a UPS.

author: TheMG
author_rank: Frequent Contributor
timestamp: June 04, 2020, 12:28:43 am
content: I think I see it now... if you replace one of the IGBTs with a plain old diode, it's basically a textbook boost converter. So by using two IGBTs you basically end up with a boost converter that can function on both the positive and negative half-cycles of the AC waveform.Quote from: Phoenix on June 03, 2020, 11:36:51 amThat might very well be the case. For obvious reasons I did not disassemble the transformer to find out.One thing I did note is that the 2000VA and 1500VA models use the transformer in a bi-directional fashion like this, whereas in the 700VA and 1000VA versions, there is a dedicated SMPS circuit for charging the batteries and the main transformer is inactive during mains operation. Perhaps they found it was more efficient that way and the smaller capacity units don't need as high a battery charge power.Either way... lot's of clever circuit design in these things, but also a lot of software magic going on inside that DSP.