Readers are advised to proceed with the construction of the presented circuits only after understanding the concepts from the core. Not adhering to this can lead to failures and frustrations.

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Tuesday, May 1, 2012

How to Design a H-Bridge Circuit for Modified Sine Wave Inverters

In electronics H-bridge circuit refers to a configuration consisting of four individual switching devices like transistors or mosfets, such that these can be switched through external discrete signals from respective stages of the involved control circuit.

The above configuration is bridged or rigged in a form which resembles the letter "H" and hence the name H-bridge.
 The above special formation has a specific reason behind it.

Looking at the circuit diagram we see that the two arms of the bridge consists devices which are complementary to each other.

While switching, the complementary pairs from each of the arms switch together, meaning the NPN device from the left arm and the PNP device from the right arm of the bridge conduct together in response to an external applied signal.
Similarly the other complementary follow the switching pattern and the two pairs conduct in tandem at a given frequency.
The arrangement generates a push pull effect in the connected transformer winding of the inverter which in turn generates a required one full cycle of AC at the output of the transformer.

The H-bridge configuration has another great advantage with respect to the making of modified sine wave inverters.

The special arrangement of the devices make it possible for driving the devices through two individual signal sources, one which decides the switching rate of the output while the other decides the RMS value of the output from the transformer.

The given circuit idea shows how a H-bridge circuit may be designed for a particular modified sine wave inverter application.
 The two gates carry the 50 Hz signal from the source to T1 and T3 such that they conduct alternately.

Transistors T2 and T4 ae fed with the PWM pulses which switch ON together, however when T1 is conducting, T2 should remain shut OFF and similarly  when T3 conducts T4 should remain OFF, the two diodes from the respective NAND gates takes care of the issue and allow only the relevant transistors to carry out the PWM switching.

Thus the NPN transistors are responsible for producing the regular 50 or 60 Hz pulses while the PNP transistors are responsible for breaking the square waves as per the PWM sections.

The resultant pulses generate the exact intended, optimized modified sine wave waveform at the output of the transformer.


  1. Hi, congratulations on this configuration you can give the list of materials and see it says 12 + and 12 - this configuration is symmetrical with power? 12 + 12 - and gnd?
    thank you very much.


    1. Hi,

      Here I have just tried to explain the concept, I'll be publishing one complete example circuit based on the above concept very soon, +12 and -12 refers to the battery poles, so there's no GND involved here.


    2. Thanks for congratulating me, appreciate that.

  2. Hi Swagatam

    Thanks for all your generous efforts. I am waiting for an example circuit utilizing above. I am also waiting for load independent 220v support in your modified square wave inverters otherwise they are perfect to utilize and experimentation.

    Thanks again for each and every circuit diagram and explanation.



    1. Hi Sam,

      Great to see you back after some time! Yes I'm trying hard to design the circuits that you have suggested, once I make them I won't waste a second, and post them immediately for you....

      Thanks very much for keeping me motivated.

      Best Regards.

  3. Hello again Mr Swagatam, I have to tell you that this is a nice article. I have previously explored h-bridge inverter designs but I left them because they mostly produce square waves and am glad you have invented a way to to produce modified sine waves using this configuration. a couple of questions though,

    1. At what frequency will the pwm be generated and what is the relationship between supply frequency and pwm frequency?
    I have q great deal of interest in designs implemented using ic 555 because it is easily and readily available here in Nigeria so am very much interested in this design. I went hunting for 4049 which was used in one of your inverter designs but I couldn't get it. couldnt find 4017 decade counter either.

    1. Thank You Kopje,

      The supply frequency is the one which is fixed as per the country specs, either 50 or 60Hz, PWM frequency may vary as per our selection.

      The PWM frequency should be generally 100 times higher than the supply frequency, however there's no standard rule because it's after all adjustable as per the needs.


  4. Hello Mr Swagatam, thank you very much for your response. a few more quick questions.
    1. Won't it be necessary to add a low pass filter after the h-bridge to filter out the pwm frequency? Because from my understanding, you will have two frequencies (50hz + pwm frequency) at the primary of the transformer. Or is it unnecessary to add a low pass filter?
    2. Will it be possible to post an image of what the output waveform will be like?

    And thank you very much for your time and effort. You have been of immense help. Cheers.

    1. Hello Kopje,

      A low pass filter is not required here, because the question of filtering PWM does not arise, rather we want the PWM to remain in the waveform.

      I'll will try to update the waveform image soon.


  5. Okay sir....thank you very much. I have decided to implement this design for a 5KV inverter using parallel mosfets to increase output power. I will update u the result of m work when am done. Thank you very much sir. Also waiting for the waveform image.

    Much regards sir.

  6. Sir, I want to design a simplest FM transmitter circuit.. Will u help me out..

    1. Ho Pradeep,

      You can try the circuit shown in this link, the second diagram:


  7. Mr Swagatam, good day to you sir. I am wondering if there is a way I can send you the schematic of my pwm inverter design which is based on your design.

    1. Hi Kopje,

      Good day!
      send it to

  8. Dear Sir Swagatam,

    thank you for your very nice circuit, and for sharing your knowledge with us, it's really great!

    I read in one of your comments that you were going to publish a complete version of this circuit with +/-12 V as supply. Did you already publish this? Where can I find it?

    I have to control the temperature of a little box using AC current, and I think that this solution can be attractive for me. How can I control the amplitude of the output sinusoidal wave though?

    Thank you again for your kindness.


    1. Dear Fabrizio,

      Thank you very much for appreciating mu work!

      If you could explain your need in details then probably I can come up with a much more suitable idea than the above, because I think there are much easier options than the above with similar implementations.


    2. Hi Mr Swagatum
      I've been reading a lot of your posts on inverter circuits which control the centre tapped transformers (12v-0-12v to 220v) and came accross the H-bridge inverter circuit diagram for modified sine wave,which controls the normal 12v-0v to 220v transformer.I would like to know if you've published the complete example circuit based on the below concept

      I would like to build a 500w inverter with this concept,as the centre tapped transformers are very expensive and I have easy access to the normal transformers

    3. Hi Robin,
      The above concept is based only on my assumptions and has never been tested practically, therefore I am sorry I cannot provide you with the example.

      By the way you may refer to this circuit also, which is much simpler and looks more feasible.


    4. Hi Swagatam
      I'm still going to persevere with my original goal of including PWM in a H-bridge,the above circuit was maybe to complex for me to build at first,but now that we have built a simple H-bridge using P and N channel mosfet's and built a 400W mosfet PWM inverter (also been able to vary the output voltage as required)maybe now we can attempt the above circuit?
      The final circuit should have a good quality modified sine wave(maybe to run soffisticated electronic equipment) and have automatic voltage regulation for varying loads.
      If I can show my appreciation for what you have done for me in any way,let me know
      Regards Robin

    5. Hi Robin,

      Your relentless hard work and valuable inputs are all commendable.

      I appreciate every bit that you have provided to this blog and the interested readers, and we all thank you for that.

      By the way if you would like to contribute your own written articles for this blog, you work will be highly appreciated.

      Best Regards.

    6. Hi Swagatam
      With your guidance and expertise I've been able to build the above concept and it works.
      If you can foresee any problems please comment and fine tune it,it is by no means perfect.
      I will send you a circuit diagram to,there is a 1k resistor missing just before the base of BC547.
      You can publish it if you want.
      Cheers Robin

    7. Hi Robin, thanks a lot!

      The above circuit was posted a long time ago and since then until now my knowledge level has increased considerably, so surely the above circuit has room for improvements. I'll check your circuit and study it.

      By the did you see your waveform images, posted here:


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