Monday, November 5, 2012

Research on transformerless AC-AC (sine wave) conversion- Part 1: AC-AC Buck


In Bangladesh, mains AC line voltage may vary quite a lot, sometimes down to about 180V (or maybe lower!) and sometimes up above 220V. Thus, using a voltage regulator/stabilizer is necessary so that the output voltage is fixed, if not exactly at 220V, somewhere around that. 210-225V is an acceptable range. The common method of AC-AC voltage regulation/stabilization is to use relays to switch the AC input voltage into different tappings of a transformer, so that the voltage is stepped up or down by an amount, as required, depending on the input voltage.

There are 2 things in this circuit that could be improved:
1) Output voltage variation - Output voltage may sometimes vary by 10V or maybe even up to 20V. While, in most cases, it may be acceptable, a tighter regulation would certainly be better.
2) Cost - This type of stabilizer becomes quite expensive due to the large 50Hz transformer required. Also labor costs are high due to winding transformer with multiple tappings.

So, for the last six months, I have been researching on AC-AC "transformerless" converters, attempting to convert the line voltage to provide an output voltage either higher or lower than the input voltage, as required. If input voltage is lower than required (the reference voltage is approximately 220VAC), then the voltage is stepped up. If input voltage is higher than required (the reference voltage is approximately 220VAC), the voltage is stepped down. However, I've been trying to do this without the use of any 50Hz transformer, which is quite bulky, large and expensive.

My first idea was to first convert the line voltage to DC, step it up and then use SPWM (sinusoidal pulse width modulation) on a MOSFET with feedback to provide a sine wave output with the required voltage (approximately 220VAC).

However, I thought, there must be a simpler way. There must be a way to control the line AC without having to convert to DC. There must be a way to achieve regulation without the use of 5 MOSFETs.

Then, I thought of the buck-boost converter. If it could be done on DC, it can also be done on AC.

But that was the problem. How would it be done on AC? I would need a high voltage high frequency device - a high frequency high voltage switch that I can control with a microcontroller. In the DC-DC buck-boost converters, MOSFETs and diodes are used with DC. But, they can not be used in the same way with AC since direction of current is alternating and I can not directly control the flow of AC with a single MOSFET.

So, I decided to break it up. Instead of attempting to construct the DC-DC buck-boost converter at once, I thought of first constructing the buck converter to test it out to see if my idea of direct line voltage control was correct.

I analyzed the DC-DC buck converter so that I could make a similar circuit for AC: an AC-AC buck converter.

Here is the DC-DC buck converter (the switch can be a BJT, MOSFET or IGBT):

Here, you can see the current flow when the switch is on:

Here, you can see the current flow when the switch is off:


If you are not clear about how buck converters work, you should read up on them. Many tutorials can be found online and there are many books on SMPS available. These are 3 of the books I learnt from (and they're great books):
Power Supply Cookbook - Marty Brown
Switching Power Supplies Demystified - Raymond Mack Jr
Switching Power Supply Design - Abraham Pressman

I started thinking about porting the same DC-DC buck concept for AC-AC buck converter. The switch could no longer be a single BJT, MOSFET or IGBT as they are unidirectional switches (can control flow of current in only one direction) and so can not be used for AC. The diode also needed to be replaced with a switch that would be turned on when the main switch was off, and vice versa. Why? Because in the DC-DC buck converter, direction of flow of current when the switch is off will be the same at all times. However, in an AC-AC buck converter, direction of flow of current will alternate every half cycle. So, one half cycle, the diode would be forward biased, but the next half cycle, it would be reverse biased.

So, here is the AC-AC buck converter circuit:

Notice how the diode is now replaced with a switch - SW2. Also, notice how no electrolytic capacitor is used at the output. The reason is obvious - the output is AC, so we can not use a capacitor that has polarity.

If you have understood how a buck converter works, the next 4 diagrams showing the flow of current should be self-explanatory in demonstrating an AC-AC buck converter.

Current flow when SW1 is on in the positive half cycle:

Current flow when SW1 is off in the positive half cycle:

Current flow when SW1 is on in the negative half cycle:

Current flow when SW1 is off in the negative half cycle:

That seemed reasonable - it should work, I thought. But, what would I use for SW1 and SW2. I can't use triacs since they're too slow and there's no way they can be used with frequencies of tens of kiloHertz. Plus, they would just latch on when fired. So, I had to somehow use MOSFET(s) or IGBT(s). But how?

Well, with some clever circuitry, it's possible to control AC loads with MOSFETs/IGBTs. And I showed how here: http://tahmidmc.blogspot.com/2012/11/controlling-ac-load-with-mosfet.html

For convenience, I'll summarize it here.



Perhaps you don't see how it works now. But consider the two diagrams below, which show the flow of current during the two AC half cycles. I'm sure you'll get it better then.



As you can see, due to the bridge rectifier, the MOSFET always "sees" a DC voltage as the drain is always positive with respect to the source. Thus, with this combination of the bridge rectifier and MOSFET, by controlling a DC switch - the MOSFET, you can control the AC current flow.

So, I could replace SW1 with a bridge rectifier and MOSFET, and SW2 with another bridge rectifier and MOSFET.

Initially when testing, I hadn't included SW2 and found that it caused the inductor to saturate. After a few seconds, volt-time averaging wasn't happening. This is because the inductor's flux is not "reset" and it just stores the energy which is not dissipated when SW1 is off.

Then, I included SW2 and everything worked well. There was no more problem. You must remember that since frequency of operation is 20kHz, a normal bridge rectifier can not be used. For the bridge rectifier, ultrafast diodes must be used.

Now comes the part about control - how to control the voltage.

In the AC-AC buck circuit, SW2 acts like the diode in a DC-DC buck converter. So, our buck conversion and chopping is done by SW1.

In a DC-DC buck converter, the output voltage is given by:

Vout = Vin * DC [where DC = Duty Cycle]

So, if duty cycle is 50% or 0.5, the output voltage is half the input voltage.
Is that also correct for AC-AC buck converter? It should be, since it uses the same principle. And, yes, it is.

Here's the principle of operation:
A microcontroller (I used PIC16F684) detects zero-crossing. Upon zero-crossing, PWM is started. The selected frequency is 20kHz. It's the maximum frequency I can get with a PIC16F684. Let's take an example with 50% duty cycle. So time period is 50us. Every cycle (20kHz, not the 50Hz one), SW1 is kept on for 50% of the time. So, for 25us, SW1 is kept on. Then, SW1 is turned off. A small deadtime is provided and SW2 is kept on for the remainder of the cycle to act as the catch diode. At the end of the cycle another dead time is provided before SW1 is turned on. The deadtime (or deadband delay) is provided so that SW1 and SW2 aren't on at the same time.

So, what happens is, the 50Hz AC wave is broken down into 400 parts and pulse width modulation is carried out over each part. Each part represents a time of 50us. So, when half of each of the 400 parts is kept on and the other half is kept off, due to the LC filter which will do the volt-time average, you will get a sine wave output - the same waveshape as that of the input - but with half the amplitude. So, the input AC voltage has become halved in amplitude, without a distortion of the waveshape. The duty cycle can be varied to obtain other voltages.

These 2 diagrams illustrate the above example:


Signal from microcontroller (in red)


Chopped Input Voltage (in red)
Filtered Output Voltage (in blue)


These 3 diagrams should clear your doubts:



In the AC-AC buck converter, feedback is implemented. The microcontroller senses the output voltage and increses or decreases the duty cycle to adjust the output voltage as required, to deal with line/load variations.

Here is the prototype at work (generating an output voltage of 115VAC from 230VAC input):



The output waveform without proper filtering:

The output waveform with proper filtering (the "jerkiness" of the waveform is due to the slow response of the camera sensor):

So, I've shown and explained AC-AC buck conversion. In the next part (part 2), I will show and explain the boost part. With the buck and boost parts combined, a transformless AC-AC converter can be created, which when controlled precisely by the microcontoller, can scale the line voltage up or down without having to regenerate the sine wave, thus outputting a wave that has a different amplitude but same waveform as that of the input wave.

My research on this topic is still going on and once I am done, I will post part 2 of my research (boost conversion) on transformerless AC-AC (sine wave) conversion.

40 comments:

  1. Brilliant post! I've heard about this concept but it's hard to find much about it. I hope to experiment with this idea at some point. Thank you for the excellent details.

    - John

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  2. Thanks John. I'm glad I could be of help.

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  3. Awesome Tahmid, good effort, when part 2 will come ? God pless you.

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  4. nice thamid and good explanation , How much load will take this circuit , i mean is there any Equation for that?

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  5. http://www.edaboard.com/thread278006.html#post1191671

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  6. The maximum load it can take depends on the diodes and the MOSFET itself. The larger they can handle, the larger the overall combination can handle.

    However, for very high currents and loads, you may find it suitable to use 2 MOSFETs instead of 1 MOSFET and 4 diodes, due to losses in the diodes.

    Regards,
    Tahmid.

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  7. Hi thamid . can u make it for buck- Boost both config?

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  8. Hi,

    The original idea was to make buck-bost configuration. I intended to first test them separately and then combine them. However, as I have been extremely busy, I have not managed to complete and test the boost section.

    Regards,
    Tahmid.

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  9. Hi Tahmid,
    Nice concept, but I doubt whether it would work with inductive loads. During the dead time, there is no way to release the stored energy.

    Regards,
    Prasanth.

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  10. The energy is released through SW2. The deadtime is kept minimal. Its effectivenes could be experimented and if unnecessary, it could be removed all together.

    Regards,
    Tahmid.

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    Replies
    1. very nice - could you post full schematic showing both SW1 and SW2 with bridge?

      J

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  11. Hi Tahmid,
    Removing the deadtime is not a practical solution as there is always a risk of shootthrough condition and trying to stop the current flowing through an inductor will generate very high voltages, whatever short duration the off time be. In dc converters, the freewheeling diode comes for help in such situation.

    Regards,
    Prasanth.
    prasanthkumar77@gmail.com

    ReplyDelete
  12. There will always be a "dead time" due to the nature of the PWM generator (usually microcontroller for such purposes). However, it is to be determined if this small dead time is sufficient or not.

    SW2 does act like the free wheeling diode.

    Long time testing of the circuit with inductive load will reveal more details and I will post the results once I've done it.

    Regards,
    Tahmid.

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  13. A cool 'Out of the BOX' thinking.

    Hats off! Tahmid.

    Thanks a lot. This open a wide range of power saving possibilities.

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    Replies
    1. Thanks! I hope you put this to use.

      Regards,
      Tahmid.

      Delete
  14. Tahmid, possible to use this circuit for AC fan control?
    If yes, could you pls post a schematic?

    Thanks.

    ReplyDelete
    Replies
    1. Further work is required for work with inductive load. I'll post more on this later.

      Regards,
      Tahmid.

      Delete
  15. Hi Tahmid,
    This is a nice effort, you are doing a great job.Could you be kind enough to give us detail schematics?.
    I have written to you about this project to your email for assistance, i will be glad to see your reply.
    Regards,
    Tayo.

    ReplyDelete
    Replies
    1. I still haven't tested this extensively. Further testing is required to know about the configuration's limtiations and effectiveness.

      Regards,
      Tahmid.

      Delete
  16. Hi Tahmid, great project, please keep it up. There is a German Prof. who invented the HERIC concept, tranformerless high efficiency ... he achieved 99% and patented his work. A company called Sunways are selling these HERIC GTIs. Amazing stuff. I think his name was Heribert Schmidt. Rgds, Selim

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  17. Hi Tahmid,
    out of the box thinking by using dc-dc converter for transformer less ac conversion. well my concern is that this works well with single phase how to modify the same circuit for 3-phase/(multi-phase)ac source?

    Regards,
    Shruthi

    ReplyDelete
    Replies
    1. I still need to test more extensively to understand its limitations and effectiveness. After that, I'll think about 3-phase / multi-phase.

      Regards,
      Tahmid.

      Delete
  18. what mosfet/diodes did you use? and can you show me how to connect mosfets and diodes for high loads/currents?

    Very Nice and helpful blog btw

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    Replies
    1. For initial testing, I used MUR460 diodes and IRF840 MOSFETs.

      Regards,
      Tahmid.

      Delete
    2. And when are you surprising all of us with boost converter too, because most of the time our voltages are lower then required...

      Delete
  19. and one more thing, what inductor and capaxitor you used? what should I use if I want to make 1 kw 220 to 110v buck converter

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  20. Hi, good job!
    How did you do the gate driver for mosfets?

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  21. Hi Tahmid,
    I think you can use bi-directional switch instead of this. Just use 2 Mosfets only, and directly series connect to the load.
    Mosfet must have internal drain source diode (freewheel). Here the sample http://www.powerfactor.us/bridgeless-pfc.html

    Regards,

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  22. This comment has been removed by the author.

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  23. the line load before rectifier gives unequal rectification of the two half cycles

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  24. The isolation transformers help in the smooth operation of the electronic devices and leads to proper distribution of the power in the electronic devices. Thus they are installed in business units on a large scale.

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  25. Hello Tahmid,
    Excellet post,just amazing..i am really going to use this for my inverter project, can you please provide with the drive circuit for mosfet for pic16f684?

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  26. Hi Tahmid,
    Is it possible get constant DC output i mean only positive half cycle at the output if input AC is Varying from 0 to 120VAC.

    Regards,
    Divya

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  27. This comment has been removed by the author.

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  28. I just found your post today. This is just brilliant! Did you make progress on this idea (more phases) ? Did you get measurements to check overall efficiency of this circuit ? How about thermal losses ?

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  29. Hello Tahmid,
    This is wonderful project,in fist time i see it, i 'll try to make same project, but when i just turn on ac source, igbt and fuse are immediately dead. It 's important for me so i 'm very glad if you can give me some advise ? I look forward to hearing from you soon.

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    ReplyDelete
  31. Hello Tahmid any further update on this project?
    Can you please share complete circuit for this part-1 for me to experiment further.

    ReplyDelete