The post helps us to design a relatively simple transformerless 3kva inverter circuit using a full bridge IC network and a SPWM generator circuit. The idea was requested by Mr. Ralph Wiechert
How can we Make a Transformerless Inverter Circuit
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! Anyways, let me know if you're interested.
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.
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.
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 3kave 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.