Pure sin Vs Modified Sin  - Page 1
Forum section: electronics / projects

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: November 01, 2014, 02:27:46 pm
content: Hi everyone,So as i posted few days ago, am trying to build a 200W UPS i saw a lot of design , i did some test with some simple circuitry, still a basic question remain unanswered ... Can anyone explain to me what are the true advantage of a pure sin wave vs a modified sin wave, does any type of load totally definitely need a pure wave ( The main load in the system am working on it , is a water pomp , a 100W motor ), Thanks

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 01, 2014, 03:37:41 pm
content: As a general rule, motors run just fine on modified sine, but they run noisier. Going to pure since can increase the motor efficiency, but as the typical efficiency of a pure sine inverter is a little lower than modified sine (of the same build quality), the overall efficiency might not increase by much or even decrease. The Prius motors are actually driven with square waves at highway speeds for that reason.

author: Lightages
author_rank: Unknown
timestamp: November 01, 2014, 03:50:27 pm
content: Motors will also run hotter and depending on their build quality they will have lower power output and fail sooner. A modified sine wave, more correctly called a modified square wave, will produce RF noise too.

author: awallin
author_rank: Frequent Contributor
timestamp: November 01, 2014, 03:53:19 pm
content: some laptop/mobile-phone chargers simply refuse to work with square-wave AC. Can anyone explain that?(my guess, which might be completely wrong, is that is messes with PFC in switched-mode PSUs)

author: Marco
author_rank: Super Contributor
timestamp: November 01, 2014, 04:11:31 pm
content: If you use a micro to generate the drive signals then the difference between a modified sine wave and a PWM sine wave (which can be easily filtered to a more or less true sine wave, if you use a transformer it will probably do it for free) is just a software problem.

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: November 01, 2014, 04:45:05 pm
content: So it's not particularly more efficient, but for overall performance , pure sin is better.Quote from: awallin on November 01, 2014, 03:53:19 pmi think you right.Quote from: Marco on November 01, 2014, 04:11:31 pmThat's what i did, i got some decent result , i did just put random components (h. bridge, 230/12 transformer) and wrote a pure sin code with an STM. i managed to power up a small phone charger and a labtop charger (no load on them)

author: oilburner
author_rank: Contributor
timestamp: November 01, 2014, 05:34:21 pm
content: cool! Do you have the schematic and ARM code?g

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 01, 2014, 05:35:07 pm
content: You'll just have to try modified sine and see if the vibration is acceptable. Also try different PWM frequencies for pure sine. 5kHz is a good value if the resulting noise is acceptable.Then there are intermediate steps between classic modified sine and Delta Sigma pure sine, such as multi stepped modified sine (probably not a good choice at low power levels), RL-PDM (Rebecca Liu Pulse Density Modulation), and classic PWM pure sine. RL-PDM can be implemented in a cheap 8 bit microcontroller, but it's basically impossible to change the modulation factor in real time as it relies on precomputed tables. Delta Sigma gives the best output for a given switching frequency (it's actually the same technique used in modern audio amplifiers) but most microcontrollers don't have the hardware to implement it.

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: November 01, 2014, 05:57:10 pm
content: Quote from: NiHaoMike on November 01, 2014, 05:35:07 pmman you talking chinese to me now, i just didn't understand anythingthe signal and the circuit was more for "fun" so it's not really the testing circuit, just that i understand how inverter work since it's the first time i actually have to build one and test the code, pwm signal etc ... and to help me make a decision about the topology that i will put in the final design ( DC/DC boost -> inverter or inverter -> 50Hz transformer )The PWM frequency am using in that crappy circuit is 100KHz , Duty cycle updated every 100µS.I really didn't make any effort to calculate any proper filtering and am useless in theoretic "how it supposed to work" i just knew the general concept make a circuit and burn itso i was pretty much satisfied with those "testing the concept" result, however i didn't quite understand some result ,for example i expected that if i give a 12VDC to the H-bridge then to the 230/12 transformer i will get something around 230VAC output , not even close i just got 110VAC with 100% Duty Cycle at 90° sin wave angle.

author: jahonen
author_rank: Super Contributor
timestamp: November 01, 2014, 06:25:13 pm
content: I have always wondered why modified sine is even called modified sine, it is so much modified that modified square would be more appropriate termRegards,Janne

author: Seekonk
author_rank: Super Contributor
timestamp: November 01, 2014, 06:45:43 pm
content: There is a third option....DC. I have a 12V camp and for a while I was buying 2000/4000W broken inverters to run the fridge etc. These were bundled in sales with smaller inverters. Usual problem was blown FET in H bridge. The smaller ones I just pulled out the H bridge FETs and ran pure 140V DC out to the outlet. Chargers, electronic CFL lamps, CD/DVD players etc work just fine on DC. Anything with a switching power supply. A little more efficient and less noise.Most older inverters use a TL494 switcher to generate on time duty cycle chip to drive the H bridge. One inverter I have operates only the fridge. I've thought about reducing on time or lower voltage after the motor starts. NASA had a patent on this in the 80"s (think it was marketed as The Green Machine) that controlled phase angle and reduced power to match the load.I suggest using a RC network on long lines.

author: Marco
author_rank: Super Contributor
timestamp: November 01, 2014, 06:54:31 pm
content: Quote from: NiHaoMike on November 01, 2014, 05:35:07 pmDelta Sigma with the right parameters will be repetitive just like PWM, so you can still just create a table.

author: Marco
author_rank: Super Contributor
timestamp: November 01, 2014, 06:57:57 pm
content: Quote from: hamdi.tn on November 01, 2014, 04:45:05 pmHey if you have that working, just scale it up and use a full bridge (don't drive the big >30A MOSFETs directly from the micro though, just get a dedicated full bridge driver).Even with a full bridge, 12V DC will only give you 12/sqrt(2) ~= 8.5V RMS ... so take that into account with choosing the transformer.

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: November 01, 2014, 10:19:37 pm
content: QuoteYap SM72295 is the one i selected.12V give 8.5 RMS how i missed thatthanks

author: Marco
author_rank: Super Contributor
timestamp: November 01, 2014, 10:44:04 pm
content: Careful, no shoot through protection on that one.

author: Phoenix
author_rank: Frequent Contributor
timestamp: November 01, 2014, 10:58:27 pm
content: Is that small grey capacitor on the LV side of the transformer? You shouldn't have a capacitor straight on the HBridge PWM output or else you will get very large high frequency current spikes at each switching edge.

author: Kohanbash
author_rank: Regular Contributor
timestamp: November 02, 2014, 12:27:25 am
content: I have seen several times computer systems that would randomly freeze or restart when using a modified-sin inverter and then those problems went away with a pure-sin inverter. I have also seen monitors displays do weird things when connected to modified-sin inverters.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 02, 2014, 02:00:36 am
content: Quote from: Marco on November 01, 2014, 06:54:31 pmDelta Sigma precomputed into a fixed table is more or less RL-PDM. The main disadvantage is that it's basically impossible (or at least very difficult) to change the modulation index in real time. What can be done to use RL-PDM in a VFD is to vary the supply voltage while adjusting the microcontroller clock speed to maintain the target V/Hz ratio. True Delta Sigma is hard to do without hardware that few microcontrollers have (though I presume it's possible with careful coding) but is relatively easy to do in a small FPGA or maybe even a CPLD.A trick to boost efficiency is to use V/Hz scaling. In other words, turn the design into a full fledged VFD.To fix the EMI and standing wave issues with modified sine, add some series inductance (passive PFC chokes work nicely) at the inverter. Pure sine inverters generally also have output inductors, but those can be much smaller thanks to noise shaping largely canceling out harmonics at low frequencies.

author: Marco
author_rank: Super Contributor
timestamp: November 02, 2014, 02:26:23 am
content: Quote from: NiHaoMike on November 02, 2014, 02:00:36 amI don't see why you would want to do that. You might want to change the amplitude of the sine ... but I don't see why you couldn't just keep the pulse centers static and vary the width for that (starting at the start of the next sine of course). Or rather decrease the width, since the table would probably be designed taking into account minimum off time ... so best make the table for maximum necessary amplitude and only decrease the pulse width.PS. with a 50 Hz transformer I don't think he has to worry too much about filtering ... not like you can easily push high frequencies through those. Working directly at mains voltages lets you use smaller magnetics of course, but I doubt he cares much.

author: johansen
author_rank: Super Contributor
timestamp: November 02, 2014, 03:52:07 am
content: The problem with the modified sine wave inverters is they have no mechanism to return reflected power back to the battery.. they literally eat it up in a 5-50 watt resistor.if you want to make the reflected power return to the load you have to have have a switch short out the transformer when both switches are both off. this would require a few extra dollars....--I have even thought about selling a product to do exactly that, but on the ac side of the transformer, basically two igbt's in series to make an ac bidirectional switch, with two sensors wired up to the primary of the transformer.. when the switches turn off, the output igbt turns on. this basically duplicates the effect of driving the transformer from an H bridge instead of a push pull center tapped transformer.. which is about the stupidest way to do it. --deadtime isn't an issue because the leakage inductance of the transformer can absorb the 1us of overlap.--in fact the fets are often turned one from 4.7Kohm resistors.. so you've got a long time available to turn off the igbtseven pure sine wave inverters are built to a price point, they usually have high no load currents due to the fact they are too cheap to install an external HF inductor in series with the primary or secondary of the transformer and the transformer's idle current looks like this:http://johansense.com/bulk/sinewaveinvertermodwoinductorcurrent17mohmshunt.JPGand thishttp://johansense.com/bulk/sinewaveinvertermodwoinductor17mohm.JPGadding an inductor reduces the current to this:http://johansense.com/bulk/sinewaveinvertermodwiinductor17mohm.JPG--yes the double crossing because the transformer is saturated, even at half load which looks like this:http://johansense.com/bulk/sinewaveinvertermodwiinductor17mohm750wattload.JPG

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 02, 2014, 04:59:01 pm
content: RL-PDM is quite complex using a series of pulses to approximate a sine wave such that the low harmonics mostly cancel out and leave less for the output filter to do. Only way I know of to change the modulation index in real time is to store several sets of RL-PDM tables and switch between them.Just turn the MOSFETs off and their integrated reverse diodes will do the trick. Shorting the transformer during the dead time is undesirable as you want it to deflux so that differences in on time don't accumulate into saturation. Also, it's rare to find a low frequency transformer in a modern inverter as it's cheaper to do the step up at high frequency.

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: November 02, 2014, 05:18:05 pm
content: Quote from: Marco on November 01, 2014, 10:44:04 pmRight, no shoot through protection, well have to be careful with PWM wave form, and am too confident that STM timer will handle most of the job.Quote from: Phoenix on November 01, 2014, 10:58:27 pmyes it is a capacitor on the LV side, in serie with a ~500R resistor, well my thinking ( and it can be totally wrong) that the transformer will behave like an inductor so switching will put transistor under stress and i thought to add a snubber.Quote from: Marco on November 02, 2014, 02:26:23 amI don'tQuote from: NiHaoMike on November 02, 2014, 04:59:01 pmI saw this in an AN and a tear-down Dave did it for a UPS, using a low frequency transformer allow ( for both design i saw ) to charge the battery without a dedicated charging circuit, so the additional cost for the 50Hz transformer is not money wasting.

author: Marco
author_rank: Super Contributor
timestamp: November 02, 2014, 05:22:22 pm
content: Quote from: NiHaoMike on November 02, 2014, 04:59:01 pmFor optimal results, but lets say you don't mind being a little suboptimal and just make a table for first order delta sigma with minimum off time constraint ... then you could just decrease the pulse width right?

author: victor
author_rank: Regular Contributor
timestamp: November 02, 2014, 10:45:48 pm
content: Why they don't build inverters like class D amplifiers with a triangle wave to drive the pwm and LC to smooth it to a real sine wave?

author: johansen
author_rank: Super Contributor
timestamp: November 02, 2014, 11:02:55 pm
content: Quote from: NiHaoMike on November 02, 2014, 04:59:01 pmdoesn't work for a push-pull circuit, which is how the majority of 60hz iron transformer based msw inverters operate.dc-dc converter based H bridge output is a different matter.Quote from: victor on November 02, 2014, 10:45:48 pmthey do, they are called pure sine wave inverters. --except they are often too lazy to stick the L in there and just use the leakage inductance of the 60hz transformer.. which is what i complained about in a prior post as well as a thread over at dutchforce and fieldlines.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 03, 2014, 12:02:15 am
content: Quote from: victor on November 02, 2014, 10:45:48 pmDelta Sigma is superior to traditional PWM when it comes to output quality at a given switching frequency.Quote from: johansen on November 02, 2014, 11:02:55 pmThe majority of the inductive kickback gets clamped by the other side and the little remainder (due to leakage inductance) is taken care of by the snubber. In such a design, one trick I like to use is to use a fan as the load in the (diode RC clamp) snubber so the energy dissipated is used to cool the circuit instead of being wasted. If isolation is not needed and the output is DC, it's even possible to use the primary side of the push pull as a forward converter that doubles the input voltage, with the secondary stage connected in series.

author: TarHeelTom
author_rank: Newbie
timestamp: November 03, 2014, 05:03:20 am
content: Quote from: Seekonk on November 01, 2014, 06:45:43 pmBuilt a camper in a regular sized passenger van (but without most of the seats) back before everyone and their brother were producing these. Ran across a small refridgerator which ran on either 12 volt DC or 120 AC. Had a battery charger under it and two extra 12 volts batteries which hooked to the alternator anytime the engine was running. Was young and stupid and hooked the two batteries in parallel. Next time it'll be two huge 6 volt batteries in series.The battery charger failed one day and took out the transistors in the fridge.In troubleshooting it, we discovered that the fridge ran on 40 volts AC. There was a transformer which stepped down the 120 volts to 40 volts, or stepped up the inverted 12 volts to 40 volts.Drove it with the fridge cold for about 6 or 7 years, then sold it.Tom

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: November 06, 2014, 11:43:55 am
content: Some people here suggest to use a High voltage DC in case all equipment is powered by an SMPS, am not sure but since this boost is for domestic use , is there any kind of regulation and special protection for equipment with high voltage DC output ?

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 06, 2014, 02:28:12 pm
content: Some new data centers are using HVDC power distribution. Main problem with HVDC is arcing. A good compromise between AC and DC is a trapezoid wave, which solves the arcing issue, avoids the EMI issues with modified sine, yet is about as efficient as a modified sine inverter.

author: Jeroen3
author_rank: Super Contributor
timestamp: November 06, 2014, 03:31:16 pm
content: Newer smps supplies tend to only draw mains current on the top and bottom of the sine.Those mains LED lights (with an ic for driving it) do this the worst way, creating noises on the line most meters cannot work with.I can imagine trouble when you apply square waves, that is actually a high frequency square if poorly filtered.If you run it through a transformer, it becomes much better. But most smps do not use an 50/60 Hz transformer anymore.Nice images:http://www.vfds.com/blog/what-is-a-vfd

author: nitro2k01
author_rank: Frequent Contributor
timestamp: November 06, 2014, 03:44:00 pm
content: Quote from: NiHaoMike on November 06, 2014, 02:28:12 pmWhat does HVDC mean in this context? The common 48 VDC, or something higher?

author: DanielS
author_rank: Frequent Contributor
timestamp: November 06, 2014, 05:03:55 pm
content: Quote from: Jeroen3 on November 06, 2014, 03:31:16 pmNewer? What you are describing here is what a bridge that dumps energy directly in the bulk decoupling cap would do and that topology has been around for 50+ years.Regulations in many countries are starting to require power factor correction. For PC power supplies, 80+ certification requires a power factor of 0.9 or better, which is only achievable with active PFC. What APFC does is boost the line voltage so the current waveform follows the voltage waveform, which means current is being drawn through most of the cycle instead of only peaks ("top and bottom") where line voltage is higher than the bulk capacitor voltage.

author: NiHaoMike
author_rank: Super Contributor
timestamp: November 06, 2014, 05:39:02 pm
content: Quote from: nitro2k01 on November 06, 2014, 03:44:00 pmAbout 380V DC.

author: David Hess
author_rank: Super Contributor
timestamp: November 06, 2014, 10:47:47 pm
content: Quote from: jahonen on November 01, 2014, 06:25:13 pmI assume because calling it modified sine indicates that the RMS and peak values are the same as a true sine wave source which is the whole reason to use a modified sine anyway.Transformers and motors need to be derated and will run hotter on a modified sine source. Passive lamp ballasts may fail completely.

author: Jeroen3
author_rank: Super Contributor
timestamp: November 07, 2014, 06:29:11 am
content: Maybe you can add "stepped approximation sine" to the title, since the ups I recently bought uses that.Although it's probably just a filtered modified sine.

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: November 07, 2014, 02:36:20 pm
content: well i finally chosen to use boost due to an update that even motor are powered through an SMPS and not directly from the main so no need for a true sine and the additional cost of a 50Hz heavy transformer , and am selecting a stupid push pull transformer , and now am incapable of thinking , i calculated that i need a 36 turn ratio transformer , so if i have 2 turn on both Low voltage output i should have around 72 turn on the high voltage one ? am totally incapable of finding a transformer with that turn ratio in any catalogue of any manufacture " i know", and what the hell this notation " turn ratio 35:2:2:3 " meani guess it's a turn number so the Nps turn ratio is in fact 17 or am totally wrong . SOS

author: DanielS
author_rank: Frequent Contributor
timestamp: November 07, 2014, 04:16:08 pm
content: Quote from: David Hess on November 06, 2014, 10:47:47 pmThere are several variants of "modified sine" output stages out there. The main reason for it that I remember hearing was to reduce the amount of harmonic content and give diode bridges in SMPS time to recover. You can match the RMS value with a square inverter by simply making its output ±120V for 120VRMS.I have an APC BX1000 and its output stage uses an H-bridge with a chunky output inductor (around 50 turns of #14 wire on a 4-5cm long ferrite or powdered iron bar) combined with an additional switched inductor to snub transitions. I do not have an oscilloscope at home (yet) to take a look at what the waveforms are like but the inductors should soften the waveforms by a fair amount when under load.

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: November 07, 2014, 09:23:13 pm
content: So guys any help with the transformer