3 Best Transformerless Inverter Circuits

As the name suggests, an inverter circuit that converts a DC input into AC without depending on an inductor or a transformer is called a transformerless inverter.

Since an inductor based transformer is not employed, the input DC is normally equal to the peak value of the AC generated at the output of the inverter.

The post helps us to understand 3 inverter circuits designed to work without using a transformer, and using a full bridge IC network and a SPWM generator circuit.

Transformerless Inverter using IC 4047

Let's begin with an H-Bridge topology that's probably the simplest in its form. However, technically it's not the ideal one, and not recommended, since It is designed using p/n-channel mosfets. P-channel mosfets are used as the high side mosfets, and n-channel as the low side.

Since, p-channel mosfets are used on the high side, the bootstrapping becomes unnecessary, and this simplifies the design a lot. This also means this design does not have to depend on special driver ICs.

Although the design looks cool and enticing, it has a few underlying disadvantages. And that's exactly why this topology is avoided in professional and commercial units.

That said, if it's built correctly may serve the purpose for low frequency applications.

Here's the complete circuit using IC 4047 as the astable totem pole frequency generator

Parts List

All resistors are 1/4 watt 5%

R1 = 56k
C1 = 0.1uF / PPC
IC pin10/11 resistor = 330 ohms - 2nos
MOSFET gate resistors = 100k - 2nos
Opto-couplers = 4N25 - 2 nos
Upper P-channel MOSFETs = FQP4P40 - 2nos
Lower N-Channel MOSFETs = IRF740 = 2nos
Zener diodes = 12V, 1/2 watt - 2 nos

The next idea is also an h-bridge circuit but this one uses the recommended n-channel mosfets. The circuit was requested by Mr. Ralph Wiechert

Main Specifications

Greetings from Saint Louis, Missouri.
Would you be willing to collaborate on an inverter project? I would pay you for a design and/or your time, if you'd like.

I have a 2012 & 2013 Prius, and my mother has a 2007 Prius. The Prius is unique in that it has a 200 VDC (nominal) high-voltage battery pack. Prius owners in the past have tapped into this battery pack with off-the-shelf inverters to output their native voltages and run tools and appliances. (Here in the USA, 60 Hz, 120 & 240 VAC, as I'm sure you know). The problem is those inverters are no-longer made, but the Prius is still is.

Here are a couple inverters that were used in the past for this purpose:

1) PWRI2000S240VDC (See attachment) No longer manufactured!

2)Emerson Liebert Upstation S (This is actually a UPS, but you remove the battery pack, which was 192 VDC nominal.) (See attachment.) No longer manufactured!

Ideally, I'm looking to design a 3000 Watt continuous inverter, pure sine wave, output 60 Hz, 120 VAC (with 240 VAC split phase, if possible), and transformer-less. Perhaps 4000-5000 Watts peak. Input: 180-240 VDC. Quite a wish-list, I know.

I am a mechanical engineer, with some experience building circuits, as well as programming Picaxe micro-controllers. I just don't have much experience designing circuits from scratch. I'm willing to try & to fail, if needed!

The Design

In this blog I have already discussed more than 100 inverter designs and concepts, the above request can be easily accomplished by modifying one of my existing designs, and tried for the given application.

For any transformerless design there has to be a couple of basic things included for the implementation: 1) The inverter must be a full bridge inverter using a full bridge driver and 2) the fed input DC supply must be equal to the required output peak voltage level.

Incorporating the above two factors, a basic 3000 watt inverter design can be witnessed in the following diagram, which has a pure sinewave output waveform feature.

transformerless 3kva sinewave inverters full bridge circuit

The functioning details of the inverter can be understood with the help of the following points:

The basic or the standard full bridge inverter configuration is formed by the full bridge driver IC IRS2453 and the associated mosfet network.

Calculating the Inverter Frequency

The function of this stage is to oscillate the connected load between the mosfets at a given frequency rate as determined by the values of the Rt/Ct network.

The values of these timing RC components can be set by the formula: f = 1/1.453 x Rt x Ct where Rt is in Ohms and Ct in Farads. It should be set for achieving 60Hz for complementing the specified 120V output, alternatively for 220V specs this could be changed to 50Hz.

This may be also achieved through some practical trial and error, by assessing the frequency range with a digital frequency meter.

For achieving a pure sinewave outcome, the low-side mosfets gates are disconnected from their respective IC feeds, and are applied the same through a BJT buffer stage, configured to operate through an SPWM input.

Generating SPWM

The SPWM which stands for sinewave pulse width modulation is configured around an opamp IC and a single IC 555 PWM geneartor.

Although the IC 555 are configured as PWM, the PWM output from its pin#3 is never used, rather the triangle waves generated across its timing capacitor is utilized for the carving of the SPWMs. Here one of the triangle wave samples is supposed to be much slower in frequency, and synchronized with the main IC's frequency, while the other needs to be faster triangle waves, whose frequency essentially determines the number of pillars the SPWM may have.

The opamp is configured like a comparator and is fed with triangle wave samples for processing out the required SPWMs. One triangle wave which is the slower one is extracted from the Ct pinout of the main IC IRS2453

The processing is done by the opamp IC by comparing the two triangle waves at its input pinouts, and the generated SPWM is applied to the bases of the BJT buffer stage.

The BJTs buffers switch according to the SPWM pulses and make sure that the low side mosfets are also switched at the same pattern.

The above switching enables the output AC also to switch with an SPWM pattern for both the cycles of the AC frequecny waveform.

Selecting the mosfets

Since a 3kva transformerless inverter is specified, the mosfets need to be rated appropriately for handling this load.

The mosfet number 2SK 4124 indicated in the diagram will actually not be able to sustain a 3kva load because these are rated to handle a maximum of 2kva.

Some research on the net allows us to find the mosfet: IRFB4137PBF-ND which looks good for operating over 3kva loads, due to its massive power rating at 300V/38amps.

Since it is a transformerless 3kva inverter, the question of selecting transformer is eliminated, however the batteries must be appropriately rated to produce a minimum of 160V while moderately charged, and around 190V when fully charged.

Automatic Voltage Correction.

An automatic correction can be achieved by hooking up a feedback network between the output terminals and the Ct pinout, but this may be actually not required because the IC 555 pots can be effectively used for fixing the RMS of the output voltage, and once set the output voltage can be expected to be absolutely fixed and constant regardless of the load conditions, but only as long as the load does not exceed the maximum power capacity of the inverter.

2) Transformerless Inverter with Battery Charger and Feedback Control

The second circuit diagram of a compact transformeress inverter without incorporating bulky iron transformer is discussed below. Instead of an heavy iron transformer it uses a ferrite core inductor as shown in the following article. The schematic is not designed by me, it was provided to me by one of the avid readers of this blog Mr. Ritesh.

The design is a full fledged configuration with includes most of the features such as ferrite transformer winding details, low voltage indicator stage, output voltage regulation facility etc.

transformerless ferrite core inverter circuit with low battery cut off, automatic feedback control

The explanation for the above design hasn't been updated yet, I will try to update it soon, in the meantime you can refer the diagram and get your doubts clarified through comment, if any.

200 watt Compact Transformerless Inverter Design#3

A third design below shows a 200 watt inverter circuit without a transformer (transformerless) using a 310V DC input. It is a sine wave compatible design.

Introduction

Inverters as we know are devices which convert or rather invert a low voltage DC source to a high voltage AC output.

The produced high voltage AC output is generally in the order of the local mains voltage levels. However the conversion process from a low voltage to high voltage invariably necessitates the inclusion of hefty and bulky transformers. Do we have an option to avoid these and make a transformerless inverter circuit?

Yes there is a rather very simple way of implementing a transformerless inverter design.

Basically inverter utilizing low DC voltage battery require to boost them to the intended higher AC voltage which in turn makes the inclusion of a transformer imperative.

That means if we could just replace the input low voltage DC with a DC level equal to the intended output AC level, the need of a transformer could be simply eliminated.

The circuit diagram incorporates a high voltage DC input for operating a simple mosfet inverter circuit and we can clearly see that there's no transformer involved.

Circuit Operation

The high voltage DC equal to the required output AC derived by arranging 18 small, 12 volt batteries in series.

The gate N1 is from the IC 4093, N1 has been configured as the oscillator here.

Since the IC requires a strict operating voltage between 5 and 15 volts,the required input is taken from one of the 12 volt batteries and applied to the relevant IC pin outs.

The entire configuration thus becomes very simple and efficient and completely eliminates the need of a bulky and heavy transformer.

The batteries are all 12 volt, 4 AH rated which are quite small and even when connected together does not seem to cover too much of space.They may stacked tightly to form a compact unit.

The output will be 110 V AC at 200 watts.