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  1. #1
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    Default Sine wave power inverter

    Hi PICers,

    I want to design a 24V DC to 50Hz/240AC power inverter. My choice of PIC is the 18F452 (HWPWM). But I am not sure whether the ADC conversion time would be short enough for dynamic correction.

    Any thoughts

    Regards

    Sougata
    The Andig Technologies

  2. #2
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    Default

    Sougata,

    I don't think a PIC is the right choice.

    I would use one of the SMPS-Controllers that are designed for this this kind of application.
    regards

    Ralph

    _______________________________________________
    There are only 10 types of people:
    Those who understand binary, and those who don't ...
    _______________________________________________



  3. #3
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    Unhappy Why not?

    Dear Ralph,

    Thanks for the reply. I don't know which SMPS controller your are talking about. But if you are talking about PWM controllers like SG3524/25 or even TC140 from the microchip family then I am afraid the only possibility of generating a sine wave is a resonant tank at the secondary side. I am talking about weighted sine through PWM lookup table. The Hardware PWM coupled with high speed would be choice. The trend is using DSP (from TI or AD) which have onboard peripherals specially useful for this type of applications. There are a family of PICs which has EPWM (enhanced) circuitry onboard but recent ones are not supported by pbp and I am not so conversant with either asm or C. In fact microchip had an appnote using the 17C familiy for an online UPS. You can go through AC induction motor appnotes to understand what I mean

    Regards

  4. #4
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    Default

    I made once a ac generator which was capable for automatic syncronising with an external signal for 45 till 65 Hz (zero crossing) and it had an output from 0 to 230V sinewave (input 0-10V). I used a pic16F873 on 20Mhz. So it is possible. I cannot give you the source because I made some promissis about the it for not given the design away. I say it is possible, just try !

  5. #5
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    Smile Thanks mat for the boost

    Hi there,

    What I have planned is having a sinelook up table calling periodically through timer overflow interrupt and a closed loop voltage monitoring to adjust the value of the lookup table on the fly to minimize voltage drifts and sinewave distortion. Now the problem is that the PIC takes altleast 21us for the ad conversion then comes the calculation part. Also I am unsure which PWM freq. to use (over audible range) and how to design the parallel mosfet power output stage. For overload protection I am going to use a different comparator and do a pulse by pulse limitimg through high priority interrupts. What is the best way to protect the mosfets. I don't know how to design a RCD snubber but my scope shows ringing and peaks during switch off in a normal square wave design. I beleive they are the reason for MOSFET heat up. I have searched and read quite a lot appnotes from SOA limiting to optimizing gate drive but still experience counts. So if anybody out there please help. By the way mat I don' t want a word from your source can you at least guide on the flow.

    Regards

    Sougata

  6. #6
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    Default

    I will draw a block diagram and I will post it here.

  7. #7
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    Default Thanks again Mat

    Hi mat,

    That would be of great help to all the pic-ers here.

    Regards

    Sougata

  8. #8
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    Default

    Here is the block diagram for the output stage.
    When you use the SG3524 at the inputstage to make the 320V dc out of the 24Volts.
    I will also make a block diagram for the sw but that will take a while.
    Attached Images Attached Images  

  9. #9
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    Default

    It is very hard for me to make a flowchart of the program without telling how the programm works. So here is a small tekst.
    There is a table of 200 numbers what is the pulse width of the pwm.
    Timer 1 is loaded with a 16 bit number eg $FE5F.
    every interrupt the PWM gets int new number out of the table.
    The table number is set to the new number (old number + 1)
    if number => 200 then number = 0..

    I will look if it is a problem to post the table. If not then I will post the data of the table.

  10. #10
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    Default Could you explain a little bit more

    Hi Matt,

    I will work the other way round. Rather I would try to post a flow chart (I am not so good at it) so that you and other members can go through it to detect any possible flaws or betterment suggestions. BTW thanks for the block. I was out-of town hence the delay in reply

    Regards

    Sougata

  11. #11
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    Default

    Normaly I also make the flowdiagram first and then write the program and when it is a simple aplication i don't make a flowdiagram.This program was made a few years ago so I have to look to the comment and in that time I did not make a flowchart.
    Here is the data of the pwm of a sine wave in 200 steps.

    128,132,136,139,143,147,150,154,158,161,165,169,17 2,176,179,182,186,189,192,195,
    199,202,204,207,210,213,215,218,220,223,225,227,22 9,231,233,235,237,238,240,241,
    242,243,244,245,246,247,247,247,248,248,248,248,24 8,247,247,247,246,245,244,243,
    242,241,240,238,237,235,233,231,229,227,225,223,22 0,218,215,213,210,207,204,202,
    199,195,192,189,186,182,179,176,172,169,165,161,15 8,154,150,147,143,139,136,132,
    128,124,120,117,113,109,106,102, 98, 95, 91, 87, 84, 80, 77, 74, 70, 67, 64, 61,
    57, 54, 52, 49, 46, 43, 41, 38, 36, 33, 31, 29, 27, 25, 23, 21, 19, 18, 16, 15,
    14, 13, 12, 11, 10, 9, 9, 9, 8, 8, 8, 8, 8, 9, 9, 9, 10, 11, 12, 13,
    14, 15, 16, 18, 19, 21, 23, 25, 27, 29, 31, 33, 36, 38, 41, 43, 46, 49, 52, 54,
    57, 61, 64, 67, 70, 74, 77, 80, 84, 87, 91, 95, 98,102,106,109,113,117,120,124

    The data is the on period of the pwm.

  12. #12
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    Default

    I disassembled a DOA APC ES500 UPS the other day, and found that it contained a 28bit PIC (can't remember which). There were other ICs between the PIC and the output FETs (SG????'s).

  13. #13
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    Arrow Thanks for the LU table

    Hi everybody,

    Thanks for the table. The schematic block that you have posted indicates that you are using an already boosted battery voltage to drive an IGBT bridge. I am trying to drive it straight into a transformer for boost up. I wonder if this is going to work.

    By the way batee any PIC with an onboard HWPWM and ADC will do. You are damn right about the APC UPS.

    Regards

    Sougata

  14. #14
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    Default

    When I understand it good, you are going to switsch the 24 V over the bridge into an output transformer, and the voltage what is cumming out of the transformer, will pass the low pass filter for making a sinewave.
    I hope you will find a good output transformer for it !!!???

  15. #15
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    Smile I have my own transformer manufacturing unit

    Hi Matt,

    I understand your concern but I am hopeful here since I have got my own transformer manufacturing unit. So from paper to product and fine tuning issues I have got a plus point over here.However I am not sure of the actual losses and bulk of the design for practical use so I might need to design a battery boost circuit.

    Regards

    Sougata

  16. #16
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    Default

    Please let me know if it works and how you did it. And if you have some questions about the hardware, just let me know, maybe I can help.

  17. #17
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    Smile I will send you my final design

    Okay Matt,

    That would be great. I need to design a RCD snubber and I don't know the maths may be you can help. I am concerned about the switch off peaks.
    I would send you the entire schematic as soon as I finish it up. It will take some time but I will keep in touch.
    Regards
    Sougata

  18. #18
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    Default

    As an aside... talking about Transformers... the 50 year Copyright (1953) has expired on the Radiotron Designers Handbook. I use this as my bible for Transformer Design and it's abosolutely brilliant - the principles in design are as valid today as they were back then. You can download it from the internet from various places (including the link below) - but be warned it's about 70Mb total pdf...

    http://elysiansound.com/goo.php

    Most Transformer manufacturers have forgotten most of this stuff (and it reflects in the abysmal products they produce). So, if you're into rolling your own (or commercially designing), then you'll love this, if you're not, then it's a sure cure for insomnia.

  19. #19
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    Default

    Give me please some data about the switching frequency of the system and inductances of the transformer then I will sent you snubber data

  20. #20
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    Smile Melanie that's great

    Hi there,

    Melanie thanks for the link. I live in a place where dial up is the only available internet. But I can manage to download it. Matt I would send you the details as soon as my prototype is ready.

    Thanks guys for keeping this thread active.

    Regards

    Sougata

  21. #21
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    Default

    You're lucky, they've broken it down into chapters so you don't have to be on-line for a week straight, just grab a section at a time each time you go on-line to do something else.

  22. #22
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    Default Is there any better site?

    Im trying to download the CHAPTERS from the site.But it is extremely difficult to open.Going to home page most links are not correct and site is under renewal.Can anybody be please send link to mirror site(s)?
    Thanks in advance.

    Also please let me know if anybody has used transformer after the H-Bridge and whether the operating frquency of transformer supposed to be output sinewave or the switching frequency? In case the fequency is that of output,
    then we loose advantage of switching and having small transformers like in smps.








    Quote Originally Posted by Melanie
    As an aside... talking about Transformers... the 50 year Copyright (1953) has expired on the Radiotron Designers Handbook. I use this as my bible for Transformer Design and it's abosolutely brilliant - the principles in design are as valid today as they were back then. You can download it from the internet from various places (including the link below) - but be warned it's about 70Mb total pdf...

    http://elysiansound.com/goo.php

    Most Transformer manufacturers have forgotten most of this stuff (and it reflects in the abysmal products they produce). So, if you're into rolling your own (or commercially designing), then you'll love this, if you're not, then it's a sure cure for insomnia.

  23. #23
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    Default Another site

    I have found other site which is better than one I was telling about:
    http://www.oldradioz.com/manuals/rdh4/

    After going thru the Table of contents only Chapter 5 & 6 are
    concerned with Transformers. Other are for Valve radios & components.
    It is excellent material for those who may not have seen valves intheir
    lifetime.
    I suppose that there are few more sites which have design guides
    for transformers and inductors using ferrite cores which will be found
    in most recents equipments.

  24. #24
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    Default Sine inverter

    Dear Sougota,
    You are possibly trying to use the Micro (PIC XXXXX) as part of the control loop to regulate the amplitude of your sine wave.
    The alternbative approach is to build the control loop around SG 3524 PWM control IC & use you PIC XXXX micro to supply the set point to SG 3524.
    SG 3524 will provide you with a very fast control action with minimum No. of components.
    The point of using PIC micro in your application is to generate variable amplitude variable frequency sine wave using the PWM command in PIC Basic Pro.
    You can also use a normal iron core transformer to check the results then convert to ferrite the ferrite core will icrease the effiiciency of your design.
    A center tapped transformer is a good choice!!!
    Reagrds.
    Suded.

  25. #25
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    Cool Power Design circuitery simulations

    For Simulation of power circuit and also recognized
    in most universities is the PSPICE software;you
    could find the software and the environmental package
    in the following two sites:

    http://www.ousetech.co.uk/winspice2/index.html
    http://www.5spice.com/

    all you need in power designing in addition to snubbers
    simulations, etc... .

  26. #26
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    Unhappy I didn't get the issue of the SG3524

    Hi Suded,

    If I have understood right then you are suggesting that I build a hf oscillator around the SG3524 then use the PIC18F452 to supply the closed loop control voltage to the SG3524 (the PIC HWPWM acts as a DAC).

    Now the issues are,

    1. I already have two HWPWMs built into the PIC

    2. The SG3524 allows dead-time programming through a simple resistor but while switching phases I again need a few logic chips to get it done.

    3. I have designed the software around interrupts. I am doing an AD-Oversampling which bruce, tim and melanie suggested in the MELAB forum.For a pulse by pulse current limiting (so that the MOSFETs don't blow up) I am using a comparator to sense the current and generating an INT to turn off the MOSFET drives.(Heh! Heh! a digital copy of the SG3524 current limit scheme).
    So why use the SG3524 as the ad-sample and dac conversion will add up to the loop error.

    The only thing that concerns me is the THD introduced while driving inductive loads as my routines are not so fast to correct that in realtime. I tried but the results are erratic (thats because of lack of knowledge from my side). The PIC is running at 40MHz (10MIPS,10MHz@HSPLL). Now I am concentrating on reducing noise on the AD line, using interrupt based AD (PBP tends to waste time while doing AD).

    BTW can anybody shed some light on the time consumed by LCDroutines. Because while processing interrupts with PBP I found glitches while the LCD was being updated.

    Thanks to psdayama and doctor for the links. As far as PSPICE goes I am not sure whether my knowledge is sufficient to handle them (I don't have an engineering background)

    Regards
    Sougata

  27. #27
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    Default

    That's why I used a serperate pic for the display.

  28. #28
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    Talking I also did it earlier

    Hi there,

    Yes matt. I did it earlier with two PIC18F452 communicating by the USART. What I did was adapt an asm INT UART routine to handle the remote display found from the MELABS page.

    Regards

  29. #29
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    Default sinewave inverter

    hi saugata
    i need three phase sine wave generation circuit i have the full knowledge of how to drive msosfet and igbt in three phase configuration.
    pramod


    Quote Originally Posted by sougata
    Hi Suded,

    If I have understood right then you are suggesting that I build a hf oscillator around the SG3524 then use the PIC18F452 to supply the closed loop control voltage to the SG3524 (the PIC HWPWM acts as a DAC).

    Now the issues are,

    1. I already have two HWPWMs built into the PIC

    2. The SG3524 allows dead-time programming through a simple resistor but while switching phases I again need a few logic chips to get it done.

    3. I have designed the software around interrupts. I am doing an AD-Oversampling which bruce, tim and melanie suggested in the MELAB forum.For a pulse by pulse current limiting (so that the MOSFETs don't blow up) I am using a comparator to sense the current and generating an INT to turn off the MOSFET drives.(Heh! Heh! a digital copy of the SG3524 current limit scheme).
    So why use the SG3524 as the ad-sample and dac conversion will add up to the loop error.

    The only thing that concerns me is the THD introduced while driving inductive loads as my routines are not so fast to correct that in realtime. I tried but the results are erratic (thats because of lack of knowledge from my side). The PIC is running at 40MHz (10MIPS,10MHz@HSPLL). Now I am concentrating on reducing noise on the AD line, using interrupt based AD (PBP tends to waste time while doing AD).

    BTW can anybody shed some light on the time consumed by LCDroutines. Because while processing interrupts with PBP I found glitches while the LCD was being updated.

    Thanks to psdayama and doctor for the links. As far as PSPICE goes I am not sure whether my knowledge is sufficient to handle them (I don't have an engineering background)

    Regards
    Sougata

  30. #30
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    Default sinewave inverter

    i went through all options to make a sine wave inverter but it was defficult to make it in basic language.

  31. #31

    Default

    Attached is a sinewave generator from PIC outputs going high 1 at a time. Hope this helps.
    Attached Images Attached Images

  32. #32
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    Smile Go through the M'CHIP Appnotes

    Hi Pramod,

    Microchip has a very comprehensive appnote AN843 that demonstrates how to use a PIC18Fxxx to generate three phase PWM signals. Since the basic 18F series has onto two hardware PWMs inbuilt you have to rely on software PWM for the third. Definitely not through the Pbasic PWM function. As you are aware that designing a sine wave inverter entirely in PIC Basic is difficult (due to limited interrupt capabilities and latency issues) you have to structure your software around assembly. I am attaching a pbp source code alongwith the sinetable and firing concept. It may help. I have heavily commented the code to facilitate understanding.Let me know if it helps you out.I am sorry due professional restriction I cannot give you the entire code. But we can communicate through this forum so that other users of this forum may also benefit.

    Microchip makes some special PICs in the PIC18F2331/4331 series which have an enhanced PWM hardware that supports programmable dead time. It also includes 3 or more PWMs. Coupled to that are ADCs which have a much lower conversion time. This would give you the lowest THD (tighter and faster correction loop) and still have enough room left for some housekeeping such as pulse by pulse overload limiting, overtemperature protection, line monitor, battery monitor, serial com, etc.



    By the way you mentioned that you are well familier with MOSFET drive techniques.Could you enlighten me on that. May be this forum is not the right place (off topic). You can drop me a private message so that I can give you my email id.

    I have desingned the MOSFET driver on a totem-pole circuitry. However I am not satisfied with the slope.I believe a steeper slope would effectively reduce switching losses. I tried MOSFET drivers from Microchip too but they blew up. Possibly due to latch up or improper grounding of the PCB traces. They are costly too here in Kolkata. Photovoltaic drivers with active discharge is not available here in Kolkata. Also I need to understand why my MOSFETs blew up during initial testing phase while the inverter was unloaded.

    regards

    Sougata
    Attached Files Attached Files

  33. #33
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    Default I didn't get the issue of the SG3524

    Hi Saugata,
    I think that for controlling the amplitude a hardware approach is better
    as so many delays and errors can creep in. There is a Buck Boost circuit with
    two mosfets and an inductor and few diodes. The DC input to H bridge can be
    controlled with B/B circuit with reqd. time delays generated by Micro output(which doesn't have to be so fast as changing per cycle of sinewave)
    The shape and frequency then can be generated by normal 2 PWM outputs(Actually one is enough just route the output to another branch with a gate and this also have dead time built in).
    I still wonder why anybody is not responding about my querry of transformers. The output of Inverter is serries of pulses at high frequencies with polarity reversal at 50-60 Hz. So is the transformer used designed for
    50 Hz operation or high frequency like say 20KHz? There is huge difference
    in size of transformers.
    regards,
    purushottam dayama



    Quote Originally Posted by sougata
    Hi Suded,

    If I have understood right then you are suggesting that I build a hf oscillator around the SG3524 then use the PIC18F452 to supply the closed loop control voltage to the SG3524 (the PIC HWPWM acts as a DAC).

    Now the issues are,

    1. I already have two HWPWMs built into the PIC

    2. The SG3524 allows dead-time programming through a simple resistor but while switching phases I again need a few logic chips to get it done.

    3. I have designed the software around interrupts. I am doing an AD-Oversampling which bruce, tim and melanie suggested in the MELAB forum.For a pulse by pulse current limiting (so that the MOSFETs don't blow up) I am using a comparator to sense the current and generating an INT to turn off the MOSFET drives.(Heh! Heh! a digital copy of the SG3524 current limit scheme).
    So why use the SG3524 as the ad-sample and dac conversion will add up to the loop error.

    The only thing that concerns me is the THD introduced while driving inductive loads as my routines are not so fast to correct that in realtime. I tried but the results are erratic (thats because of lack of knowledge from my side). The PIC is running at 40MHz (10MIPS,10MHz@HSPLL). Now I am concentrating on reducing noise on the AD line, using interrupt based AD (PBP tends to waste time while doing AD).

    BTW can anybody shed some light on the time consumed by LCDroutines. Because while processing interrupts with PBP I found glitches while the LCD was being updated.

    Thanks to psdayama and doctor for the links. As far as PSPICE goes I am not sure whether my knowledge is sufficient to handle them (I don't have an engineering background)

    Regards
    Sougata

  34. #34
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    Default Have you seen the code at all

    Hi Dayama,

    You are right that only a single PWM can be used and done exactly the same. It is modulated with a sine table (32 samples). The transformer needed is a 50Hz type. The transformer, output choke and capacitors form a low pass filter (actually the sinewave is integrated) and the output waveform is a sinewave.

    Your approach of using a B/B converter would rather complicate things up and also be costly. That is basically a dual conversion design if I understood right. With a MCU you can tune , upgrade your design without actually increasing the production cost. If you are in India then you may be aware of the latest buzz of "DSP" sinewave inverter. Here the approach is same.However a DSP gives you much better ADs and fast computation with dedicated hardwares (for e.g a 32 x 32 Multiply) so you can keep your sinewave undistorted with inductive or mixed load.

    I could not post the whole code due to professional restriction but here is what it does :

    1. On Interrupt it fetches the sinetable vaue (indexed) for PWM

    2. Offset it with ERROR from previous and current sample (a PID style error loop)

    3. Turn on the MOSFETs with the calculated value.

    What I don't do is dynamically correct a voltage error in the same index pointer. My ADC conversion time does not allow me to do that. To mess up things more I actually take 4 samples and do and average to deal with noise. Then I have a nifty LCD routine, a battery monitor, RS232 interface (to shutdown my computer and inverter when not needed)

    BTW where is Pramod who kicked this thread into existence again.

    Regards

    Sougata

  35. #35
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    Default Have you seen the code at all

    Dear Sougata,
    I haven't seen the entire code at all but just one table of delays for
    sinewave. Also I have faint recollection of the block diagram.
    Truly speaking Im not much PIC er. Im doing everything with ATMEL
    micros. I have found this forum while searching for sinewave inverter.
    I think it doesn't matter which company U use but design is more importatnt.
    So I can understand code very well.
    I also have Transformer Unit but it is for HT Instrumentation type.
    I think that varying amplitude with delays and error adding will work only
    maximum upto 70% of basic amplitude. After that U will have gross THD due
    to most of delays at zero crossing very small.(It appears in class D amplifiers
    of analog kind). I haven't seen whether such approach works with truly
    variable amplitude or from 0 to 120% of the amplitude.
    What I m saying is not double conversion but switchng of Buck Boost
    mosfets with 2 ports from same PIC outputs and CORRECT amplitude there.(See attached bmp file.) In this s1 and s2 are mosfets whose timings is controlled to vary the voltage.
    Im in India but near Mumbai so here not much Inverter brouhaha is there.
    Melanie has agreed that this DSP method is not so good although everybody
    is claiming otherwise.
    I am also looking for design in which there should be no such bulky
    transformer of 50Hz. If I have read right then proper driving of mosfets
    would result in 20-40Khz output which is double modulated. this can use
    high frequency transformer with centre tapped secondary and demodulator cum filter circuit which will give 50Hz output!!
    As presently the transformer is costing about 50% of the total invertor
    cost and one can imagine savings. Also as many circuits are having SMPS type charger for battery the advantage is obivious.(Compare APLAB 30V 1A
    regulated supply with 250W computer smps,)
    Hope to have more discussion on this.
    regards,
    dayama
    Attached Images Attached Images  

  36. #36
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    Default Gotcha!

    Hi Dayama,

    It is possible to reduce the size of the transformer. Looks like a class-d amp solution to me. A simple filter would reconstruct the sinewave and remove the HF component.Did you find anything or anyone having done it on the net. I will also search and probably would like to develop one. But some practical start point is needed.

    Regards

    Sougata

  37. #37
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    Default Class D amplifier

    Here is the link:They make readymade PWM amplifiers but don't ask cost!!!!!
    http://eportal.apexmicrotech.com/mai...m_overview.asp

    I asked their reps in Mumbai and they said each IC costs Rs.25,000/-
    But one can make PWM output like that and then have just High freq. txr or
    direct output with filter.
    regards,
    Dayama





    Quote Originally Posted by sougata
    Hi Dayama,

    It is possible to reduce the size of the transformer. Looks like a class-d amp solution to me. A simple filter would reconstruct the sinewave and remove the HF component.Did you find anything or anyone having done it on the net. I will also search and probably would like to develop one. But some practical start point is needed.

    Regards

    Sougata

  38. #38
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    43

    Default Is this thread dead?

    Hello everybody (Saugata,Mel xxxx)
    I wonder why there is no activity here since last month!
    I have just now made a inverter of Delhi type with sinewave output.
    The transformer for 500VA output is whopping 10kg in weight.
    You all may be discussing sine wave with PIC but practically
    there is no use if such big transformers have to be used and adding
    to cost about 60% of the project.
    I have hit an idea to circumvate this and will discuss if anybody
    is interested.
    HAPPY DIWALI!!
    dayama

  39. #39
    Join Date
    Feb 2005
    Location
    Kolkata-India
    Posts
    562

    Smile Well go on please

    Hi Dayama,

    I actually wrote the code for a customer's specific requirement and know any further development is entirely personal and my current schedule does not permit actually working on it. Currently I have a running code with fair sinewave output. However THD measurements and transient response characteristics were not done. Also to use it like a UPS I need to design a fast transfer circuit. The sugar cube relays are fast enough but fails with increasing and mixed type load. I have incorporated an interrupt based AC Fail detection circuit that checks the following:

    1. Improper Line Frequency
    2. Unhealthy AC-Line Voltage
    3. Sudden brown/black out

    When I compl



    As far as the transformer issue goes I beleive it will take much time and effort which is beyond my scope right now. Also in Kolkata it is very difficult to source good quality ferrite cores.

    As I told you previously we need to have a high frequency carrier and modulate it with a 50Hz sine table. (Which I am doing anyway). Use of the high frequency should theoritically be used to drive a ferrite cored transformer (read low cost light weight) so a low pass filter in the secondary should reconstruct the modulating 50Hz.

    BTW I am attending the Microchip Masters Seminar at Lonavala on 29th nov. are you?

    Thanks for keeping this thread alive and giving me a kick to ponder further.

    Regards

    Sougata

  40. #40
    Join Date
    Jun 2005
    Location
    Mumbai,India
    Posts
    43

    Default Thanks

    Hi Saugata,
    Its OK to be late than never. I think U should stop the work on high frequency
    as there are more complcations. Instead I suggest that whats Ur code for
    current work is alright.
    I am planning to make inverter with Half Bridge
    output which is direct to Load and not thru transformer. Problem is everybody is inverting at low frequency of 50Hz from battery. And use same
    transformer for charging battery when on mains.
    If we make high frequency >40 Khz inverter with
    square wave and output is rectified and made high voltage DC then with dual supplies (+/-) we can use IGBT/MOSFET half bridge to drive sinwave output
    directly without transformer. This will save the output transformer cost and ferrite transformer would cost hardly 100/-. Also for charging battery same type SMPS circuit can be used and its wattage can be max 120W which would cost hardly 200/-.(Old PC SMPS 250W are available for 150/- in market)
    I am still designing the circuit. I will post the block diagram here as soon as
    it is ready.
    When you are working with this type circuit U can have slightly expensive
    circuit which doesn't require a relay -sugar cube or any type as U are just switching DC sources with help of diodes and they are very fast!
    Basic plan is
    MAINS POWER---|----->SMPS WITH CHARGE CONTROL--> BATTERY->-|
    |_____>RECTIFIER --->|<RECTIFIER--<SQR.INVERTOR<|
    |
    50HZ PWM INVERTOR WITH HALF BRIDGE OUTPUT
    |
    LOAD WITH SMALL FILTER

    This ckt will work like true online UPS still expensive just as much as ordinary
    invertors.But U can have Ur DSP alright and also PICs other a/d lines can be used for battery charge management as well!
    For cheaper type the Mains rectifier for dual supply can be ommited and
    instead sugar cube relay used for switching outputs. Half wave +/- rectifier
    requires costly HV elcos.
    For good ferrites ask for dealer of Cosmo Ferrites. U check their list on web page. There is another manufacture Hinodaya.
    I won't be attending M'Chip Masters course as I am just beginer in this field but not to programming. May be I will attend next time when I hope to get some nuances and I am working on Transformer Ratio test set(comparator
    type) and want to use PIC for advanced features.So I think that I will also be
    familiar with PICBASIC then.
    Wish all the best for conference.My place is abt 2hrs from Lonavala
    regards,
    psdayama

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