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Friday, March 15, 2013

How to Design an Inverter - Basic Circuit Tutorial

The post explains the fundamental tips and theories which may be useful for the newcomers while designing or dealing with basic inverter concepts. Let's learn more.

What's an Inverter

It's a device which converts or inverts a low voltage, high DC potential into a low current high alternating voltage such as from a 12V automotive battery source to 220V AC output.

Basic Principle behind the above Conversion

The basic principle behind converting a low voltage, DC to high voltage AC is to use up the stored high current inside a DC source and step it up to a high voltage. This is basically achieved by using an inductor, which is primarily a transformer having two sets of winding namely primary (input) and secondary (output). The primary winding is meant for receiving the direct high current input while the secondary is for inverting this input into the corresponding high voltage low current alternating output.

By alternating voltage we mean a voltage which switches its polarity from positive to negative and vice versa many times a second depending upon the set frequency at the input of the transformer.

Generally this frequency is a 50Hz or 60 Hz depending upon the particular country's utility specs.

An artificially generated  frequency is used at the above rates for feeding the output stages which may consist of power transistors or mosfets or GBTs integrated with the power transformer.

The power devices respond to the fed pulses and drive the connected transformer winding with the corresponding frequency at the given battery current and voltage.

The above action induces an equivalent high voltage across the transformer secondary winding which ultimately outputs the required 220V or 120V AC.

How the Transformer Converts a given Input

As discussed above, the transformer will have two winding one primary and the other secondary, The winding react in such a way that a current applied at the primary winding electromagnetically induces a proportionately relevant power at the secondary winding. so if the primary is rated at 12V and the secondary at 220V, the 12V input to the primary would induce and generate a 220V across the secondary terminals.

However the input to the primary cannot be a direct current, meaning though the source may be a DC, it must be applied in a pulsed form or intermittently across the primary, or in the form of a frequency at the specified level, we have discussed this in the previous section.

This is required so that the inherent attributes of an inductor can be implemented, according to which an inductor restricts a fluctuating current and tries to balance it by throwing an equivalent current into the system during the absence of the input.

Therefore when the DC is applied, the primary stores this current, and when the DC is disconnected from the winding, allows the winding to kick back the stored current across its terminals. However since the terminals are disconnected, this back emf gets induced into the secondary winding, constituting the required AC across the secondary output terminals.

The above explanation thus shows that a pulser circuit or more simply put, an oscillator circuit becomes imperative while designing an inverter.

How to Design an Oscillator Circuit for an Inverter

An oscillator stage is perhaps the simplest part in an inverter circuit. It's basically an astable multivibrator configuration which can be made through many different ways.

You can use NAND gates, NOR gates, devices with built-in oscillators such as IC 4060, IC LM567 or just utterly a 555 IC. Another option is the use of transistors and capacitors in standard astable mode.

The following images show the different oscillator configurations which can be effectively employed for achieving the basic oscillations for any proposed inverter design.

In the following diagrams we see a few popular oscillator circuit designs, the outputs are square wave which are actually positive pulses, the high square blocks indicate positive potentials, the height of the square blocks indicate the voltage level, which is normally equal to the applied supply voltage to the IC, and the width of the square blocks indicate the time span for which this voltage stays alive.








The Role of an Oscillator in an Inverter Circuit

As discussed in the previous section, an oscillator stage is required for generating basic voltage pulses for feeding the subsequent power stages.

However the pulses from these stages can be too low with their current outputs, and therefore it cannot be fed directly to the transformer or to the power transistors in the output stage.

In order to push the oscillation current to the required levels, an intermediate driver stage is normally employed, which might consist of a couple of high gain medium power transistors or even something more complex.

However today with the advent of sophisticated mosfets, a driver stage may be completely eliminated.

This is because mosfets are voltage dependent devices and does not rely on current magnitudes for operating.

With the presence of a potential above 5V across their gate and source,  most mosfets would saturate and conduct fully across their drain and source, even if the current is as low as 1mA

This makes conditions hugely suitable, and easy for applying them for inverter applications.

We can see that in the above oscillator circuits, the output is a single source, however in all inverter topologies we require an alternately pulsing outputs from two sources. This can be simply achieved by adding an inverter gate stage (for inverting the voltage)  to the existing output from the oscillators, see the figures below.







Understanding Inverter Topologies (How to Configure the Output Stage)

In the above sections we learned about the oscillator stages, and also the fact that the pulsed voltage from the oscillator goes straight to the preceding power output stage.

There are primarily three ways through which an output stage of an inverter may be designed.

By Using a:

Push Pull Stage (with Center Tap Transformer)

Push Pull Half-Bridge Stage

Push Pull Full-Bridge or H-Bridge Stage

The push pull stage using a center tap transformer is the most popular design because it involves simpler implementations and produces guaranteed results. However it requires bulkier transformers and output is lower in efficiency.

In this configuration, basically a center tap transformer is used with its outer taps connected to the hot ends of the output devices (transistors or mosfets) while the center tap either goes to the negative of the battery or to the positive of the battery depending upon the type of devices used (N type or P type).

A half bridge stage does not make use of s center tap transformer.


A half bridge configuration is better than a center tap push pull type of circuit in terms of compactness and efficiency, however it requires large value capacitors for implementing the above functions.



A full bridge or an H-bridge inverter is similar to a half bridge network since it also incorporates an ordinary two tap transformer and does not require a center tap transformer.

The only difference being the elimination of the capacitors and the inclusion of two more power devices.

A full bridge inverter circuit consists of four transistors or mosfets arranged in a configuration resembling the letter "H".

All The four devices may be N channel type or with two N channel and two P channel depending upon the external driver oscillator stage that's being used.

Just like a half bridge, a full bridge also requires separate, isolated alternately oscillating outputs for triggering the devices.

The result is the same, the connected transformer primary is subjected to a reverse forward kind of switching of the battery current through it. This generates the required induced stepped up voltage across the output secondary winding of the transformer. Efficiency is highest with this design.

The following diagram shows a typical H-bridge configuration, the switching are made as under:

A HIGH, D HIGH - forward push
B HIGH, C HIGH - reverse pull
A HIGH, B HIGH - dangerous (prohibited)
C HIGH, D HIGH - dangerous (prohibited)


The above explanation provides the basic information regarding how to design an inverter, and may be incorporated only for designing a ordinary inverter circuits, typically the square wave types.

However there are many further concepts that may be associated with inverter designs like making a sine wave inverter, PWM based inverter, output controlled inverter, these are just additional stages which may be added in the above explained basic designs for implementing the said functions.

We will discuss them some other time or may be through your valuable comments.









50 comments:

  1. hello swagatam,
    Is there any circuit to convert 12 v square wave to 12v sine wave???

    ReplyDelete
    Replies
    1. search for "Bubba Oscillator" it is a perfect sine wave oscillator circuit.

      Delete
  2. Hello Sir Swagatam,

    Please give me a suitable high amps circuit/inverter {12Vdc to 220Vac} that can power up a TV/PC , Fan and a fluorescent light simultaneously bcoz power interruption occurs most of the time here in our place.

    My power source is a 12V car Battery.

    Thank you Sir.

    ReplyDelete
    Replies
    1. There are plenty of inverter circuits in this blog, please type "inverter circuit" in the search box, you will be able to find many suitable circuits....

      Delete
    2. with due to respect SIR,i am faysal. Really your work is good. Many many thank you for your work.Sir, i need a circuit on automatic water tap with very familiar materials so that i can make it easily.if have enough time on your hand and it would be possible for you,,,,please to make me a design on automatic water tap.
      Thank you very much,SIR.

      Delete
    3. Thanks Faisal,

      The sensor section can be easily designed, but the mechanical side won't be my area of expertise so will be difficult.

      Delete
  3. Hi Mr.Majumdar,
    What must be the input for A,B,C,D in above inverter diagram.
    (is it output from 555 oscillator)

    ReplyDelete
    Replies
    1. from any of the oscillators above having two outputs...

      Delete
  4. Hi which mosfet pairs must be given the same input from oscillator

    ReplyDelete
    Replies
    1. for the H-bridge circuit?? the info is given just above the diagram.

      Delete
  5. Hi Sir!you said A half bridge configuration is betterthan a center tap push pull type of circuit in terms of compactness andefficiency, however it requires large value capacitors for implementing. Now What Are The values of C1 and C2 in the half bridge network diagram. are they suppose to be large electrolitic capacitors?

    ReplyDelete
    Replies
    1. Hi Hassan,

      C1 or C2 can be around 1000uF/50V, they should be non polar type.....you may put two 2200uF/25V caps in series for getting the above value....connect the like poles together for making the required non polar capacitors.

      Delete
  6. A and B are given same pulse
    C and D are given same pulse.is this right

    ReplyDelete
    Replies
    1. A and D are given with the same pulse

      B and C with the other alternate pulse

      Delete
  7. Thank you sir! can i use outputs of ic cd4047 pin 10(Q)for A and D,
    pin 11(Q') for B and C

    ReplyDelete
    Replies
    1. yes it may be tried, but be sure to connect a fuse with the battery positive, in case something goes wrong it would protect the mosfets.

      Delete
  8. sir why bc547 is used in the above circuit?

    ReplyDelete
    Replies
    1. you can use any general purpose NPN transistor in place of BC547

      Delete
  9. Hello Sir Swagatam,

    May i use this inverter circuit for input 48V dc to produce output 240V ac? and how about the current for this inverter?

    ReplyDelete
    Replies
    1. No, the above circuits are not suitable for 48V, you can try the following design with 48V:

      http://homemadecircuitsandschematics.blogspot.in/2012/09/mini-50-watt-mosfet-inverter-circuit.html

      use BC546 for T1 and T2.

      Delete
    2. Good Day sir,

      about the link u given, how can i increase the power of the inverter (around 2000w)? This is because my load is around 2000w and the voltage is 230 v ac. And what is the input current of the inverter?

      Thank You

      Delete
    3. Good day,

      Please make the basic design first and then we can gradually try upgrading it for more power.

      Delete
  10. Hello Sir Swagatam,

    How can i design an inverter with power 360w, input voltage 50Vdc, and input current 6.3A? (the output voltage is around 230v to 240v at frequency 50Hz)

    ReplyDelete
    Replies
    1. You can try the following basic model first and then go on upgrading it with more mosfets for getting more power:

      http://homemadecircuitsandschematics.blogspot.in/2012/09/mini-50-watt-mosfet-inverter-circuit.html

      T1/T2 must be replaced with BC546 for 50V supply.

      Delete
  11. hello sir
    do you have 3 ph inverter design, in 1 ph 220v 0ut 3 ph 220V 400Hz, with simple design
    thks
    reply

    ReplyDelete
    Replies
    1. I am sorry cnc, presently I do not have this design.

      Delete
  12. what is the use of bc547 or any transistor in this circuit

    ReplyDelete
  13. hi,i need 1000w circuit diagram of ups

    ReplyDelete
  14. Hi question = cncmesin but an investor to 220AC 220AC single phase of variable frequency from 50 to 400Hz you could make the sine wave generator integrated or L298 TB6560? and place suitable for 220V mosfet
    thank you very much

    ReplyDelete
    Replies
    1. Yes it's possible, thanks for the good suggestion!

      Delete
  15. Hello swagatam, will this circuit(link below) give me a modified sinewave?



    http://4.bp.blogspot.com/-Uepf26wgJO0/TpQGZ-0QbHI/AAAAAAAABA4/lb51Ejk9SAs/s1600/12+to+220V+modified+sinewave+inverter.jpg

    ReplyDelete
    Replies
    1. Hello ifeanyi,

      yes it will work and give you a modified sine wave,.

      Delete
  16. sir i also want to know all information about inverter

    ReplyDelete
  17. Hello Swagatam,

    i need to convert 12v dc to 120v ac with at least 5 amp load capacity.
    i need a suggestion what could use for my purpose.
    could u suggest me ???

    thank you

    ReplyDelete
    Replies
    1. Hello Rudra,
      you can try the circuit design shown in the following link:

      http://homemadecircuitsandschematics.blogspot.in/2012/09/mini-50-watt-mosfet-inverter-circuit.html

      Delete
  18. Hi dear Swagatam, I am ur fan & have built many projects for my personal use with success & had a lot of pleasure. God bless you. Now I intend to build a 1000 watt UPS with a different concept (inverter with high voltage input dc). I will use a battery bank of 18 to 20 sealed batteries in series each 12 volts/ 7 Ah to give a 220+ volts storage as input to a transformerless inverter. Can you suggest a simplest possible circuit for this concept which should include a battery charger + protection and auto switching by mains failure. Later I will include a solar power input too.

    ReplyDelete
    Replies
    1. Thank You Aquarius!

      I'll try to post it soon and let you know, most probably I would publish it within a couple of days.

      Delete
  19. Hi again, I hope you will soon be able to suggest a suitable circuit for my project requested in my last mail of March 17. .... Aquarius

    ReplyDelete
    Replies
    1. Aquarius, please refer to this post:

      http://homemadecircuitsandschematics.blogspot.in/2014/03/1000-watt-ups-circuit.html

      Delete
    2. My heartiest thanks for the circuit. i will build this one soon and let you know the result. Aquarius.

      Delete
  20. Can u plz name some alternatives to the mosfets IRF9Z30 and IRFZ40 coz they arent available in my area

    ReplyDelete
  21. would I need a gate driver if i am generating the PWM from a micro controller?

    ReplyDelete
  22. with 5v out, gate driver won't be required.

    ReplyDelete
    Replies
    1. MCUs don't have 12V out, but 12v would be also suitable for driving msfets directly.

      Delete
    2. Can etch and use this circuit with a load of 200W.
      Thanks a lot

      Delete

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