In this post we are going to learn how to construct a simple transformerless inverter circuit which can power loads up to 1000 watt.
We will see:
- What is Transformerless inverter?
- Transformerless Inverter vs Transformer Based Inverter.
- Various Stages of Proposed Transformerless inverter.
- Circuit Diagram.
- Block Diagram.
- How to Test and Operate this Inverter Circuit.
- RMS calculation of this Inverter.
What is Transformerless Inverter?
As the name suggests, transformerless inverters are NOT equipped with a standard (iron-core) step-up transformer like traditional inverters utilize for converting low voltage AC to high voltage AC.
Transformerless inverters utilizes DC input from solar panels / battery bank which is inverted to standard 120VAC / 230VAC output using three main stages: oscillator, boost converter and H-bridge.
A boost converter stage converts a low voltage DC to a high voltage DC efficiently, the converted high voltage DC from boost converter is inverted to standard 50/60Hz AC using a H-bridge stage controlled by a 50/60Hz oscillator.
The high efficiency DC to DC boost converter is the key reason why we could eliminate the bulky transformer, it replaces the role of a traditional step-up transformer. The boost converter circuit doesn’t take much space and it is assembled on the main PCB it-self.
Since a traditional transformer doesn’t exist a efficiency greater than 97% is achievable, the only losses are from H-bridge and boost converter which is small compare to losses produced by a iron-core transformer.
Transformerless Inverter Vs Transformer Based Inverter:
|Parameters||Transformerless Inverter||Transformer based Inverter|
|Input / output isolation||No galvanic isolation exist between input and output.||Galvanic isolation exist between input and output due to the transformer.|
|Efficiency||Best efficiency, greater than 95%.||Reasonably (good) efficiency, above 85%.|
|Voltage step-up by?||DC to DC boost converter.||Iron core step-up transformer.|
|Power||Typically used where power demand is low, less than 10KVA.||Used where power demand is high 100KVA or more.|
|Size||Overall smaller dimension.||Bulkier than transformerless type.|
|Weight||Light weight.||Heavy & can get heavier if power requirement is high.|
|Cost||Cheaper than transformer based.||Expensive than transformerless type.|
|Applications||Commonly used in solar farms, solar roof installations for best efficiency and data-center's backup system where space is luxury.||Commonly used in line interactive backup UPS (uninterruptible power supply).|
What are High frequency Inverters / ferrite core inverter?
There is an another type of inverter closely related to transformerless type, it is called high frequency inverter / ferrite core inverter. This type of inverter is also marketed as transformerless which consist of a small ferrite core transformer, which steps-up the low voltage AC to high voltage AC efficiently and can handle significant amount of power in a smaller dimension, one such inverter is shown above.
A primary reason why ferrite core inverter exist beside transformerless design is because it can provide galvanic isolation between input and output. The real life efficiency, weight and other advantages are similar to a true transformerless inverter.
We have proposed a High Frequency Transformer Inverter / Ferrite Core Inverter Circuit with its working explanation here.
Stages of Proposed Transformer-less Inverter Circuit:
The proposed inverter is very simple, it consists of just three stages, we have simplified the inverter stages further by eliminating the boost converter stage which is a bit complex for a homemade transformerless inverter. Now the simplified inverter stages are:
- DC Power Source / Battery Bank.
- Oscillator / Multivibrator.
Full Circuit Diagram:
We have designed yet another best Transformerless inverter circuit which can output modified sine wave at 230VAC, click here for the circuit diagram.
Block Diagram of Transformerless Inverter Circuit:
DC power source:
The power source / battery bank consists of (12V / 7Ah) 19 batteries connected in series. A fully charged lead-acid battery reads 13V, the total DC voltage is: 13 x 19 = 247 VDC output.
The combined 19 batteries gives total power output of 12V x 7Ah x 19 = 1596 watt hour (Wh) of energy.
For those who have 120VAC as their country’s power supply, you can connect (12V / 7Ah) 10 batteries in series, which gives 13V x 10 = 130VDC. The combined 10 batteries give total power of 12V x 7Ah x 10 = 840 Watt hour (Wh).
The excess 10V gets drop due to the MOSFET and you will get nominal operating output voltage of 230V and 120VAC.
When the battery reaches 11.1V per battery you should consider it as low battery condition. The AC voltage output at low battery will be 11.1 x 19 = 210.9V or 11.1 x 10 = 111V.
So from full battery to low battery condition the output varies approximately 36V for 230V system and 18V for 120V system. The connected electronic appliances will work happily in those voltage ranges. You can also replace batteries with appropriate rated solar panels.
NOTE: There is a separate 12V battery for the oscillator.
The oscillator is the stage where appropriate waveform and frequency for the inverter’s output is generated. Here we are using a simple astable multivibrator using NPN transistors. The oscillator is powered separately by a 12V battery.
The circuit is tuned to generate 50Hz square wave output but due to the tolerance of the resistors and capacitors we may slightly undershoot or overshoot 50Hz frequency. For 60Hz you can replace R2 and R3 resistors with 25K ohm resistor.
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The frequency of this astable multivibrator can be calculated by:
F = 1 / 1.38 x R x C
F is the Frequency in Hz
C is capacitance in farad
R is resistance in ohm
Output of this Astable multivibrator circuit:
The H-bridge is the stage where the high voltage DC is inverted to high voltage AC, the oscillator switches the MOSFETs in H-bridge in a specific pattern to generate alternating current.
The H-bridge consists of four power MOSFETs: couple of N-channel MOSFETs (IRF740) and couple of P-channel MOSFETs (IXTP10P50P) which are rated for 400V 10A and -500V -10A respectively.
Now let’s see how an H-bridge functions:
The H-bridge changes the polarity across the load which invertes direct current to alternating current.
At the left hand side image above, S1 and S3 are closed, now the power flows from S1 to the load, through S3 and to -Ve, note the polarity across the load.
Now look at the right hand side image above, the S2 and S4 are closed and other two switchs are opened. Now the power flows from S4 to the load and completes the flow through S2, now look at the polarity across the load which is reversed from previous cycle.
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This back and forth cycle continues and generates alternating current in square-wave form. At no instant S1 and S2 should be ON simultaneously, if such instants occur it will lead to short circuit, this is same with S3 and S4.
The switches are replaced with MOSFETs as shown below:
At points “A” and “B” where the oscillator’s input is applied. A fuse is placed at +Ve rail of high voltage input to prevent unintentional short circuit.
Alternate H-bridge circuit:
You can also build the below given H-bridge circuit which works even more efficiently than the previous one because all four MOSFETs are N-channel type (IRF740). It may look a bit complicated because the upper (high-side) MOSFETs are bootstrapped so that it can switch +Ve supply to the load properly.
Specifications of MOSFETs:
|Parameters||IRF740 (N-Channel)||IXTP10P50P (P-Channel)|
|Voltage Drain to Source (Vds):||400V||-500V|
|Voltage Gate to Source (Vgs):||Nominal +/- 10V, Max +/- 20V||Nominal +/- 10V, Max +/- 20V|
|Continues Drain Current:||10A (Continuous)||-10A (Continuous)|
Note: If you couldn’t find exact MOSFETs you may substitute with a equivalent specification.
How to Test and Operate this Circuit:
You should test the inverter thoroughly before you connect the inverter to high voltage/high energy DC system. The testing is done at a lower and higher voltage using inexpensive batteries. Please follow the steps below:
Low voltage testing:
- Construct the circuit fully with all the mentioned fuse and switches.
- Connect a fully charged 12V lead-acid battery to oscillator and H-bridge.
- Turn ON the oscillator first and H-bridge second, not both simultaneously.
- Connect a 12V bulb across the H-bridge’s output terminal, the bulb should light immediately.
- Test the voltage across the load using a multimeter with AC voltmeter mode, it should read 11 to 12VAC.
- Once the above test is completed successfully, you may move to high voltage testing.
High voltage testing:
- Construct the circuit fully and it must have all the mentioned switches and fuse.
- Purchase 28 (fresh) 9V batteries: one for the oscillator circuit and 27 for the H-bridge.
- Connect the 27 (9V) batteries in series in the below shown schematic, it will give around 245VDC:
- Solder the output +Ve and –Ve terminals with thick wires. Now connect the battery bank’s output to a 40 Watt 230V tungsten bulb, It should glow at full brightness. If it glows, you have connected the batteries properly.
- Now connect the high voltage DC from the batteries to H-bridge and connect the separate 9V battery to the oscillator and also connect the 40 Watt bulb as load at H-bridge. Now, turn ON the oscillator by pushing/sliding “switch 1” first and then turn on the “switch 2” which will power the H-bridge.
- As soon as you turn ON the switch 2 the light bulb should glow immediately at full brightness.
- Now connect a multimeter with AC voltmeter mode (600V range) across the load carefully, it should read 220 to 240VAC.
- Now your transformerless inverter is ready and you may test it further with other AC loads.
Note: You should always turn ON the oscillator first i.e. switch 1 and then switch 2 for H-bridge.
RMS calculations for this Inverter:
What is RMS in alternating current?
The concept RMS in AC circuit is very important and it is one of the most crucial concept to be understood while designing an inverter circuit. It deals with the waveform of alternating current, its effective voltage, current and power.
To understand this concept better let’s consider a simple circuit with a light bulb as a resistive load and we will be applying fixed 9V with DC and 9V sine-wave AC and let’s see how it behaves.
On the left the bulb is glowing at full brightness on 9VDC and on the right the bulb is glow only as bright as it would when we apply 6.3VDC. The applied 9VAC is only as effective 6.3VDC, this is because even though we are applying 9V peak to peak, the effective voltage across the load bulb is less than 9V, this is because the waveform is rising and falling with time and not constant like DC. If we connect a 9V AC motor it will run only as effective as 6.3VDC.
To tackle such electrical measurement problem 20th century engineers introduced a concept called RMS or Root Mean Square in AC circuits. The RMS voltage of AC will be as effective as DC voltage.
Now let’s consider the above example again, to make the bulb glow at full brightness using AC, we have to apply 9V RMS not peak to peak. To get 9V RMS we have to apply 12.7VAC peak to peak. RMS for sine-wave AC can be calculated using the below given formula:
RMS = Voltage pk to pk / √2 | Example: 12.7V / √2 = ~9V RMS.
When we talk about AC voltage we should always talk about the RMS of the AC voltage for example when we say our home’s AC mains output is 230VAC we are talking about the RMS voltage i.e. the effective DC equivalent of 230VDC. The peak to peak of 230VAC RMS is 325V.
The RMS formula for different wave is different, you can find the RMS formula for common waves like, triangle, saw-tooth, pulse, modified sinewave and its related calculations here at Wikipedia resource page.
RMS calculation for square wave:
The proposed inverter’s waveform is square wave and thankfully its RMS calculation is very simple. The RMS of square wave AC is equal to peak to peak of the square wave AC.
RMS of square wave = Peak to peak of square wave
If our inverter outputs sinewave then we would have to apply 325VDC to get 230VAC RMS, since our inverter is just a basic square type 240VDC input is enough to output a 240VAC output RMS – loss.
Advantages and disadvantages of this inverter circuit:
|Simple circuit design with few components.||Square wave is not suitable many medical and sensitive electronics equipment.|
|Solar panel compatible.||Need huge battery bank / long solar panel array to meet the voltage needs.|
|High efficiency greater than 90%.||Need a separate battery for the oscillator.|
|No need for automatic voltage regulation for most part, as the AC load is directly placed on battery.||Battery discharge rate affects output AC voltage (240V to 210V)|
I am an electronic engineering student,
I have designed exactly the same inverter, i have supply of Solar panels 240 vdc, and i run 7 amperes load continuously successful,……………..
– Basit (Reader)