DESIGN OF SINE WAVE POWER INVERTER
Introduction
There are two types of electrical power in use. DC (Direct current) and AC (Alternating Current). Dc power is not varying with time and AC power oscillates between two voltage (or current) values at a specific frequency. The most common, economical, and easily produced AC power is sinusoidal AC Power. Also. When transmission of electrical power, AC power is more economical than DC power. So most of the types of equipment, and types of machinery are designed for work with AC power. But since AC power couldn’t store on the battery, DC to AC power converters (Inverters) must be employed to store and use electric power.
The power inverters are the equipment that converts DC Electrical power (Voltage or Current) to AC. In market mainly there are two types of power inverters known as ‘modified sine wave inverters’ and ‘pure sine wave inverters. Modified sine wave inverters are produced nearly square wave output than sine wave. They are much cheaper than sine wave inverters. But some devices like motors, Laptops, and sensitive devices couldn’t operate under this type of power. Therefore, there are a lot of applications in sine wave inverters, such as wind energy/photovoltaic applications, Space shuttle energy systems, high voltage power transmission systems, motors, and model digital systems.
During this project, I try to design a pure sine wave inverter circuit, efficient, and inexpensive as it satisfied the given specifications and with minimum noise. To achieve this, I used high frequency transformers, First I converted Battery DC volage to high frequency PWM voltage signal. Then it step up to achive high voltages via DC transformer and finally I obtain sine wave with 230V rms with help of a 4th order LC ladder filter
Design Specifications
- Input – 12V DC
- Output voltage – 230V AC
- Output frequency - 50Hz ± 1𝐻𝑧
- Max output power – 250W
- Efficiency > 90%
- Power factor = 1
- Output waveform - Sinusoidal Output harmonic distortion < 3%
Methodology
Making a sine wave power inverter is a tedious and complex task when thought of as a whole. When broken up into blocks and sections it much easier to manage and design. In this design, I mainly use sine wave PWM signal to transfer power from the battery and create sinusoidal power. I used H-bridge inverter that made from MOSFETS as switching devices and a filter to get smooth sine wave output. Following sections are describe the main functionalities of each sub system of the whole circuit.
There are main 5 sub blocks in my design. Basically, H-Bridge produce sine wave PWM signal with both positive and negative polarities. 5V regulator, PWM generator and MOSFET driver used to produce the control signals that required to H-Bridge inverter. The output PWM signal will be a high frequency, low voltage signal. So it step up using a DC transformer. Finally This high voltage PWM signal converter to smooth sinusoidal wave with help of Filter circuit. Also, The PWM generator creates with few other sib circuits as showing below.
Let’s discuss the operations, specifications, used component and schematics of each block in the following section.
Voltage Regulator
For the produce sine wave PWM signal, it required to middle voltage between 0V and 12V. In here I used LM7805 voltage regulator IC to produce 5V output. Also to complete the regulator circuit I used two mylar capacitors as they have high isolation resistance and high-temperature coefficient.
Sine wave Generator
To create a accurate PWM signal, it required to create a precise sine wave with stable required frequency (In this case 50 Hz). For this I used bubba oscillator circuit. It uses 4 filters with 4 Opamp’s oscillator circuit. In here I used regulated 5V as the reference level. The most valuable advantage of this circuit is, the output has very low distortion.
To Implement the design, I used LM348 Quad Bipolar Op Amp IC.
Calculations for capacitor and resistor values for get 50Hz.
Although the calculation values not exactly generated desired output frequency in practically and also in simulations. So adept two variable resistors to here to adjust the frequencies.
Carrier wave Generator
For create high frequency PWM signal, it also required high frequency carrier signal also. There are two types of carrier waves are uses for this purpose. Sawtooth or triangular signal. In this design I used triangular signal with 40 kHz frequency. First, I create high frequency square wave and then I obtained triangular wave from the integrator.
The frequencies that can be generated by this circuit depend greatly on the slew rate of the Op Amp. Using a TL-084, output waves with frequencies of up to 40KHz can be generated.
Comparators
PWM signal easily can create by using comparator, triangular and a sine wave. Also, I used comparators for make proper control signals that required to H-bridge. That’s mean it required a square wave with 50hz and it inverted signal. Also, it required PWM signals and it inverted signal. The triangular wave is simple to create by utilizing an Opamp’s driver. It must then be modified such that it switched between a mid-to-high triangular wave, to a mid-to-low triangular wave. This is accomplished by generating a triangular wave at roughly half the amplitude of reference sin, centered at the same voltage. For the comporator, I used MC3302D Quad voltage comparator IC.
H-Bridge Inverter
In inverter, output should oscillate between both positive and negative voltages as sinusoidal. Since sinusoidal power injected to PWM signal, output waveform has to be PWM shape with both positive and negative waveform. To achieve this from a single source, I used H-Bridge configuration. Also, to minimize the power losses with higher switching speed. It uses N-Channel MOSFETs. In this design I used IRFZ34N MOSFETs. Summarized specs of selected MOSFET as follows,
Step Up Transformer and Filter
The last but not least sub system is transformer and filter. After creating high power PWM waveform with 12V p-p, it required to create sinusoidal wave with 230V rms. Since H-bridge output is a PWM signal, it varies high frequency. Therefore, to this design I used 400W 12V DC transformer to step up H-Bridge output. After stepping up I attached 4th order LC ladder filter to remove the noises and smooth the output as sinusoidal.
Design a filter is tedious work as it have to consider lot of factors. Selected topology as below. It transfer all the PWM signals that below 4kHZ
Simulation Results
Implemented Design and Lab Testing
Future Improvements
- Add a control system to maintain voltage regulation, Frequency.
- Designed circuit there are no any safety methods, therefore improve this circuit by adding device protection circuit, and surge protection system.
References
