We are going to make a high frequency ferrite core transformer based inverter circuit which can output 500 watt 220VAC at 50Hz with good efficiency. We will learn how a ferrite core inverter functions stage by stage extensively.
We will see:
- What is ferrite core transformer inverter?
- Why ferrite core transformer operated at high frequencies?
- Block diagram of ferrite core inverter circuit.
- Full circuit diagram ferrite core inverter circuit.
- Exploring different stages of inverter brief.
What is ferrite core transformer inverter?
Ferrite core inverter is a type of power inverter used for power backup, it uses ferrite core transformer to step-up the low voltage AC to high voltage AC at high frequencies derived from a DC source (battery).
Ferrite core transformer inverters are lite weight, high efficiency, low cost and have good portability than traditional bulky iron core transformer based inverter.
Ferrite core based inverter is often marketed as transformerless inverter because of their lite weight it feels like it doesn’t sport a transformer.
The ferrite core transformer can handle a lot of power in a much smaller dimension than iron core transformer because ferrite material has low energy loss at high frequencies.
Ferrite cores has high magnetic permeability, meaning it can hold lot of magnetic field around it, so less magnetic flux loss and because ferrite materials are bad conductor of electricity, at high frequencies very low eddy current is induced on the core.
Eddy Current: Eddy current is the current which is induced on a conductor due to alternating magnetic field which heats up the conductor thus energy is lost as heat.
Because of these properties we can fabricate the transformer in a smaller size; only thing we need to do is operate the transformer at high frequency (in KHz range).
Why we need to operate ferrite core transformer at high frequencies?
To know why ferrite core transformers are not suitable for low frequency operations you need to know a couple of terms “soft magnetic material” and “hard magnetic material”.
Soft magnetic material: The materials which get magnetized by external magnetic field and when external magnetic field is removed, the material get demagnetized.
Hard magnetic material: The materials which get magnetized by external magnetic field and will stay magnetized even after removal of external magnetic field.
Ferrite materials are referred as hard magnetic material. Now assume the core gets magnetized in a (north-south) polarity due to one half cycle of alternating current, in the next half cycle of AC the magnetic field is reversed and this magnetic field has to overcome previous magnetic field.
Due to the opposing magnetic field in every half cycle the secondary coil receive less magnetic flux and the opposing magnetic flux partially turns into waste heat. This is called hysteresis loss in a transformer.
At low frequencies, the magnetic field stays longer in one direction compare to magnetic field at high frequencies.
If we apply magnetic field much longer in one direction to the ferrite core, it will get magnetized strongly, Now when we reverse the magnetic field, it has to overcome the previously magnetized strong field and lot of energy is wasted as heat now.
Hysteresis loss can be reduced in ferrite core transformer by operating at high frequency. But in iron core transformer (which is soft magnetic material) hysteresis loss gets increased at higher frequency.
So that’s why it is very bad idea to operate ferrite core transformer at low frequencies.
Block diagram of ferrite core inverter:
A 12VDC source powers the inverter. The high frequency inverter block alone consists of a high frequency oscillator, a MOSFET driving stage to provide necessary current to drive the ferrite core transformer.
The transformer outputs 220VAC at several KHz which is not suitable for home appliances, so the high frequency must be converted to 50 Hz before feeding to a load.
The 220VAC high frequency is converted to 220VDC using a bridge rectifier and a smoothing capacitor. This high voltage is fed to H-bridge which will convert the 220VDC into 220VAC at 50Hz with the help of IC 555 oscillator which is tuned at 50Hz 50% duty cycle. The output of this inverter is 220VAC at 50Hz square wave.
Full Circuit Diagram of Ferrite Core Inverter:
Download high resolution circuit diagram, click here
The proposed ferrite core inverter has the following stages:
- High frequency inverter.
- 50Hz IC 555 Generators.
- Bridge Rectifier.
- H-bridge stage.
High Frequency Inverter:
From the block diagram we learned that we need to convert 12VDC to 220VAC at several KHz frequencies using a ferrite transformer. Well, dear readers let me tell you that you cannot purchase a ferrite core transformer from a local or online store like an iron core transformer that suits our circuit.
We may salvage a ferrite core transformer from some SMPS circuit and rewind the primary and secondary, according a complex calculation and we will end up making an inverter which is not giving sufficient output as we anticipated or sometimes not working at all.
Fortunately, we no need to do all these DIY transformers build. We can purchase a high frequency inverter circuit specifically made for homemade inverter build. Such a circuit is illustrated below:
The above circuit board converts 12VDC to 220VAC at high frequency, this circuit board alone cannot power our home appliances as the frequency is not suitable. We still need to build the rest of the inverter circuit. But you can power incandescent or CFL or some resistive loads directly from this circuit as these are frequency independent loads. The dimensions of the circuit are small enough to fit in your palm.
Backside of this inverter board:
On the flipside of the board we can see there are several outputs, 380V, 220V, 18V and 160V if you take output between 220V and 380V. We need to take output across 0 and 220V for this inverter build.
Am I purchasing an inverter board instead of building one? This question could arise in your mind now. Well no sir, let me tell you this again that this board is a not the whole inverter, this inverter kit eliminates the need for building a ferrite core transformer by yourself and increase the chance success rate for your build.
Okay, where I purchase one?
You can purchase this inverter board from any e-commerce sites like: Amazon or eBay or banggood or Alibaba, just search for “high frequency square wave inverter board”, several circuit boards will appear. There are several different boards with different power specifications.
Bridge Rectifier Stage:
The rectification stage consists of 4 diodes rated at 400V peak / 280V RMS at 6 ampere and this can handle frequency up to 4 MHz according to its datasheet. A 10uF capacitor rated at 400V smooth the high voltage and gives a constant 220VDC which is to be fed to H-bridge stage to get alternating current output.
IC 555 – 50Hz generator:
The IC 555 stage outputs signal at 50Hz and 50% duty cycle square wave which is to be fed to H-bridge stage, which will switch the high voltage DC to AC according to the frequency and duty cycle input by IC 555.
The frequency and duty cycle is determined by the RC network connected to IC 555, a diode across the pin #6 and #7 will makes the IC 555 to output frequency at 50% duty cycle.
Frequency Calculation of IC 555:
The IC 555 is connected with two 30K resistors and 0.47uF capacitor.
The frequency formula for IC 555 with a diode across pin # 6 and #7 is:
F = 1.44 / (R1 + R2) x C
F = 1.44 / (30 x 10^3 + 30 x 10^3) x 0.47 x 10^-6
F = 51.06 Hz
Let’s check by hooking an oscilloscope at pin #3 of IC555:
Not too shabby right? We are getting very close to 50Hz and 50% duty cycle.
Note: Do not omit the 0.01uF capacitor connected across pin #5 and GND, doing so you won’t get the calculated frequency.
This is the stage where the constant 220VDC is converted to 220VAC at 50Hz. The H-bridge consists of four IRF740 N-channel MOSFETs which are rated at 400V. A bootstrap circuit is connected to high side MOSFETs which switches the MOSFET with proper biasing.
Alternate H-bridge circuit:
A simple H-bridge circuit with two N-channel and two P-channel MOSFETs are illustrated above, this H-bridge doesn’t need a bootstrapping circuit. The P-channel MOSFET IXTP10P50P is rated at -500V to switch positive high voltage across the load.
We don’t recommend this circuit because of the higher resistance between drain and source on P-channel MOSFET which turns into waste heat a lot. But still this H-bridge configuration provides the intended high power output.
This concludes the construction of ferrite core inverter circuit.
If you have any questions regarding this project, ask us in the comment section, you can anticipate a guaranteed reply from us.