DCAC inverter - Page 1
Forum section: electronics / projects

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: June 20, 2015, 12:53:10 pm
content: Hi everyone,So i been doing an UPS project, i received my board few days ago, so i had a bit of time of play with it and check some stuff, for the most part it work's fine ( good for me) but i have a strange output waveform and am not sure what causing it ( with no load pic , and with SMPS 12W load ).Schematic attached. Please advice

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: June 20, 2015, 12:53:57 pm
content: More pic...

author: Seekonk
author_rank: Super Contributor
timestamp: June 20, 2015, 07:42:25 pm
content: What part seems strange? Looks normal for a MSW converter. Those are just various termination effects.

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: June 20, 2015, 07:59:05 pm
content: Thank you ... finaly someoneit could be me expecting more than reality and since it's basicly my first power thing project i don't really know what my scope is showing me ... but i was expecting a wave form identical for all load and no load, those termination is the strange thing , the command wave is for a modified square wave, my expectation that i shouldn't have any of those termination.With no load i think some capacitive thing is holding the voltage up ! could it be my scope probe !!! it desapeare when i hookup a resistor on the output.But those spike on the 12W load is what i don't understand.Am hoping those picture speak to someone more they do to me. cause am not sure of what am seeing.

author: lewis
author_rank: Frequent Contributor
timestamp: June 20, 2015, 08:23:03 pm
content: The spiky waveform (SMPS 12W load) looks to be caused by ringing in the SMPS's input filter inductor. With a quasi-sine wave you're essentially turning the SMPS on and off very quickly with a fast risetime, and the input filter will ring and generate transient voltages. Try it with SMPSes of different brands and the result will probably be different. If you have an output filter on your UPS not shown on the schematic, this may be the cause.The open-circuit waveform is caused by some capacitance somewhere, either in the probe you're using to measure this waveform, or in some output filter. A 100K resistor across the output should clean it up. The AC output is essentially floating an open circuit when all the MOSFETs are off.

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: June 20, 2015, 08:45:39 pm
content: There is no output filter at all , will try different loads and post the resultif it's the case of input filter ringing .. can you please suggest a way to eliminate this.

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: June 23, 2015, 02:31:27 pm
content: Hi againso i been doing some experiment with the thing and it let the magic smoke out several timeBut one particular experience is what blow out most of the DC AC partfirst i was generating just a 50Hz modified square wave to the driver , but i saw in an ST application note that there DC AC converter use a MSW too but the ON time of the MOS was actually a 16KHz wave once filtered form the 50Hz output.First am not sure why they need switching the ON time of the 50Hz signal !!i tried it out and my mos dead and drivers too.the board was clearly using much more power and the load connected to it ( an old 300W ATX ) refuse to startup ..what i did wrong here

author: Zero999
author_rank: Super Contributor
timestamp: June 23, 2015, 05:01:09 pm
content: Without seeing the application note you're talking about, it's difficult to know.It sounds like it was talking about using PWM and a filter to make a pure sine wave. There are plenty of ways to mess this up.

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: June 23, 2015, 05:18:38 pm
content: AN2794 ,1 kW dual stage DC-AC, Nop i don't think they generate a pure sin it's a MSW

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 30, 2015, 10:08:28 pm
content: Quote from: hamdi.tn on June 23, 2015, 05:18:38 pmIncorrect - that app note describes a pure sine wave inverter. The switches in the H-bridge are switched at 16kHz with the duty cycle following a 50Hz sine wave pattern. The high switching frequency vs. modulation frequency (ie - 16kHz to 50Hz) means you can use a simple/cheap LC filter to integrate the 16kHz PWM'ed square wave output into a decent quality 50Hz sine wave (both the current and voltage waveforms, courtesy of the L and C components, respectively).The scope shots you posted suggest there is insufficient gate drive for the upper switches in the H-bridge. When using a charge-pump type gate driver IC (like the L6386), this either means the bootstrap capacitor is too small or else the IC can't deliver the necessary peak current for the switch. While this is nominally a good application for a charge-pump type gate driver, it has been my experience that these ICs are notoriously unreliable in practice; that said, I normally design power conversion hardware in the 100kw-2MW power range, so that experience doesn't 100% apply at the 1kW or less level.

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: July 01, 2015, 02:12:49 am
content: Quote from: MagicSmoker on June 30, 2015, 10:08:28 pmHi, thanks for those infoI had my doubt about bootstrap cap , but I checked the value , and it seems ok to me.i check tomorrow if the current is enough to charge up the gate.however what worry me about those spiky wave , could it be unintentional switching on of high side mosfet ?

author: Yansi
author_rank: Super Contributor
timestamp: July 01, 2015, 10:08:01 am
content: Any images of the pcb/layout? 1kW inverter is not a toy for a beginner. So I hope the board respect the power electronic rules and ratings.

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: July 01, 2015, 10:30:42 am
content: The application note is 1Kw .. my ups is just 200W

author: Yansi
author_rank: Super Contributor
timestamp: July 01, 2015, 12:52:50 pm
content: OK. But still, can we see the pcb please? It'll help us diagnose possible problems.

author: lewis
author_rank: Frequent Contributor
timestamp: July 01, 2015, 08:01:35 pm
content: Quote from: MagicSmoker on June 30, 2015, 10:08:28 pmWow! That's pretty cool, what sort of stuff?

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: July 01, 2015, 09:05:51 pm
content: Quote from: Yansi on July 01, 2015, 12:52:50 pmThis is the drivers and mos part of itQuote from: lewis on July 01, 2015, 08:01:35 pmyap 100KW - 2MW is pretty impressivei guess it can be used from electric car , electric forklift , or huge UPS for data center.

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 01, 2015, 09:53:42 pm
content: Quote from: hamdi.tn on July 01, 2015, 02:12:49 amOkay, I actually looked at your schematic and there are a few problems with it but the value of the bootstrap capacitor isn't one of them. Although it is generally a good idea to make turn-off happen faster than turn-on, I would simplify the circuit for now and just use a single 39R resistor in series with each gate (in other words, get rid of D15, D16, D18, D19, R47, R51, R58 & R60, and change the value of R45, R48, R55 & R59 to 39R). The trick of using a reverse-biased Schottky to speed up turn-off is a good one, generally, but the L6388 is only capable of sinking 650mA which means the parallel combination of the turn-on and turn-off resistances should be no lower than ~23R. To stay entirely within the specs of the L6388 but double turn-off speed you could make both turn-on and turn-off resistors 47R. In any event, you absolutely don't want/need R48 & R59.Also note that the IRF840 datasheet states switching times with a series gate resistance of 9.1R (this value is usually the minimum, btw); driving the gate with 100R will slow down the switching by approximately 10x. That may be okay in your application (switching frequency is less than a few 10's of kHz), but you need to ensure there is sufficient deadtime between switches in each bridge leg to prevent cross-conduction and, regardless, larger values of series gate resistance increase susceptibility to turn on of the supposed-to-be-off switch from Miller current transients.Quote from: hamdi.tn on July 01, 2015, 02:12:49 amIt could be Miller-induced turn-on of the off switch, yes, or it could be from simple cross-conduction (ie- the next leg turns on before the previous leg has finished turning off), or from excessive inductance and/or insufficient decoupling capacitance in the DC link.To check for cross conduction you simply measure the DC link current into the H-bridge with no load on its output; it should be 0A, or no more than a few 10's of mA at this power level. To see if Miller current is a problem you need to watch the gate waveform of both switches in each side of the H-bridge (ie - the upper and lower of one side). Finally, check for DC link collapse by watching the DC link with AC coupling on your scope. You might want to use a second channel to watch the gate of a low side switch and use that waveform for triggering.

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 01, 2015, 10:00:59 pm
content: Quote from: lewis on July 01, 2015, 08:01:35 pmThese days, switcher locomotive drive systems. GE and Siemens have all but exited the switcher market because the volume is too low, but for my company it's like Christmas come early.And yeah, it is pretty cool... Except when something blows up, because when that happens at this power level you usually end up on anti-anxiety meds afterwards.

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: July 01, 2015, 11:24:23 pm
content: Quote from: MagicSmoker on July 01, 2015, 09:53:42 pmthanks a lot for all of thisi will check all that once i get replacement components i post what i get

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: November 20, 2015, 09:31:25 am
content: Hi, again, long time since i started this topic and i finally find a bit if time to do an other test run.so i did what magicsmoker suggested and modified the schematics, and changed boot cap value cause it wasn't enough, it didn't change a single thing.i explainwith no load the output keep on falling slowly until the low side MOS short it to ground.i observed gate waves on scope, am totally sure that the gate can see the 100Khz switching . but the H-bridge doesn't ,i tried different frequency, different wave form ( true sin, modified square ) i only get what am posting on the first post, a square wave if no load is connected.and by the end of the day, the driver is dead once again with no clear reason.---i return to a simpler circuit by using an L268 H-bridge, driven by same 100Khz waves i can totally see the switching on the H-bridge, and after adding an LC filter a quite nice sin wave is out ... but i don't understand why it simply collapse when i connect a load, even if the load is not that demanding (observed with inductive loads) .i finally killed it too ,when connecting a 24VAC contractoram getting my ass kicked on this project and am not sure what am doing right or wrong, what piss me off the most is that the full circuit is taken of an application note, i used a similar component and not getting any results.help please

author: mich41
author_rank: Regular Contributor
timestamp: November 20, 2015, 10:07:59 pm
content: The 84W trace looks nasty. After low-side turn-off, there is a fast rise to roughly 0.6*VCC, then brief fall to 0.4, back to 0.6 and gradual fall to 0.5, until the high side turns on.This dip to 0.4*VCC reallyreallylooks as if the low side switch was turning back on for a moment. If you verified that Miller effect doesn't distort gate waveforms (I hope you probed them at the FET, not at the driver IC...), maybe the FETs turn on due to excessive Vds rise rate disturbing their parasitic BJTs? See "dv/dt capability"herefor details. The rise rate will naturally be fast if load current is high and/or the load is inductive.You may need to drive the FETs slightly softer.

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: November 21, 2015, 09:23:30 am
content: am sorry but am not quite following what you wrote, please explain

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: November 21, 2015, 11:52:17 am
content: To explain a bit more what am going throughI soldered a chip on a prototype board as shown on sch.png, just a half bridge with low voltages ,the goal is just to test the chipnow what am getting is what i suppose to be a correct wave form on the gates both high and low side ( it's a 100Khz pulse blocs alternate at 50Hz) but the out pin is just not returning to 0V until the low side switch on.

author: Phoenix
author_rank: Frequent Contributor
timestamp: November 21, 2015, 12:02:17 pm
content: Quote from: hamdi.tn on November 21, 2015, 11:52:17 amIt looks like you don't have a load connected.

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: November 21, 2015, 12:09:12 pm
content: well i don't , but that doesn't explain why i should measure any voltage if the high side mosfet is off

author: MagicSmoker
author_rank: Super Contributor
timestamp: November 21, 2015, 01:04:45 pm
content: Here are the reasons that one or more bridge MOSFETs can fail (in no particular order):1. Excessively long switching times - in this voltage/switching frequency range the voltage across each MOSFET should take 50-100ns to transition from off to on and vice versa. Note that this should be measured while under a meaningful load (at least 10%, ideally >50%). Also note that too fast a transition time causes lots of EMI, ringing/voltage overshoots and exacerbates problems with body diode conduction (see below).2. Cross conduction - check DC current into bridge with no load (including shunt [e.g. - capacitive] filter components); should be close to 0mA. Adjust deadtime between bridge legs until it just gets to 0mA (you don't want too much deadtime, either).3. Body diode conduction - the intrinsic inverse parallel diode in all MOSFETs (even ones that claim to be fast) is pathetically slow compared to a purpose-built fast diode. With a purely resistive load, ideal (near-zero) deadtime and cross-conduction, the body diodes will never conduct in a full bridge. But with a reactive load (more specifically, leading power factor [ie - capacitive], which can come from the output filter in a pure sine wave inverter!) and/or excessive dead time (typically >200ns) the body diodes will start conducting current, and they then take a long time to turn off (reverse recovery), and the entire time the body diode tries to turn off it presents a direct short to the MOSFET in the other leg which has just turned on.4. Excessively narrow pulse width - this problem tends to occur at very low load in a bridge using regular PWM (as opposed to phase shift modulation). How narrow is too narrow depends on a lot of other variables, but limiting the minimum pulse width to 5% of the switching period is a good start (and yet another case of too little load being a problem).5. Overvoltage/ringing during turn off - caused by undamped stray inductance in between switches in a bridge leg or between the DC link capacitor and the bridge. These inductances form resonant tanks with the sum of the effective output capacitance of the switches in a bridge leg. This, then, is primarily a physical layout problem. Even with good layout, though, RC dampers across each switch and/or the transformer primary are frequently needed (note that quasi-resonant transition and phase-shifted bridges put these strays to good use, and the latter topology is also very good at dealing with reactive loads).6. Finally, the obvious stuff like insufficient gate drive voltage, excessive drain current, too low a voltage rating relative to supply, etc.

author: mich41
author_rank: Regular Contributor
timestamp: November 21, 2015, 02:46:50 pm
content: Quote from: hamdi.tn on November 21, 2015, 12:09:12 pmYou charged AC output connector up to VCC and now where is the charge supposed to go if all mosfets are off and there's no load? I mean, itwillleak (through mosfets, through the scope, through air, pcb and whatnot) butslowly.You may want to read some introduction to the fundamentals somewhere... Also, connect some resistors, inductors and capacitors to a low voltage source (5V or whatever), observe the waveforms when they are switched on and off and make sure you understand why the waveforms look the way they do.

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: November 21, 2015, 10:08:59 pm
content: Quote from: mich41 on November 21, 2015, 02:46:50 pmno i can't agree about that, cause i tested other H-bridge, like with the L298 am getting a clean 30VAC sadely the L298 is not capable of high voltage as high as 350VDC, even directly with a micro-controller output with no bridge driver at all (VCC=1.5V, so MOS can be trigered by 3.3V uC output) i still can see on the bridge or the half bridge, an identical wave form to the driving wave, so it's not that.the AC output must float so no voltage at all, it's like a scope being connected to nothing, i don't see why it has to be slowly decaying voltage. it's exactly this capacitive phenomena that i can't explain, certain thing if i can't see the HF signal on the bridge output on the scope neither will the LC filter.unless someone can give me an explanation to that, i can't see why this should happen.I will not be able to do any tests until next month but am thinking to test an other high side - low side driver and in a second time build my own high side driver with some transistors, and see what i get.Quote from: MagicSmokerThanks for that, i will focus on point 3 and 5.

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: March 20, 2016, 12:34:24 pm
content: hi everyone,since i annoyed everyone here with this ,here where i'am this project.Finally some decent looking wave form, with a clear distortion in it, not the most smooth sin wave i ever seenHere it's powering a contactor coil and two SMPS both powering different circuit.I added some feedback circuitry to it, so it keep the voltage around 230VAC.so far so good, thanks everyone for your help