Hello guys, in the last post I have explained the basics of inverters along with its types and also the inverters topology in other words working of inverters. Then we discussed the main components essential for an inverter. Now in this post I am gonna explain the modified sine wave inverter and how to create it. I have used AVR microcontroller int his project. The reason I am using random microcontrollers is that so you guys get a taste of each one. Before starting on sine wave inverter read this article again and again as I have also mentioned the problem i got while making it.I have done this project with one of my friend and really we enjoyed a lot.
I tried my best to keep it simple but still if you guys got stuck at any point ask in comments and I will remove your query. I have divided it in the following four parts :
- Basics of Inverters With Topology (Part 1)
- Major Components of Inverters (Part 2)
- Modified Sine Wave Design With Code (Part 4)
Modified Sine Wave Inverter
- Modified sine wave inverter gives an output which is intermediate between the square wave and pure sine wave. Is has much lower efficiency than the pure sine wave.
- The circuit diagram of a modified sine wave inverter is shown below :
- For a modified sine wave inverter we need two inverted square waves signal to switch the MOSFETs. Two generate these signals we use HEF4047 multi-vibrator IC.
- We use it in a-stable mode. It gives a buffered output so there is no need for impedance matching between TTL based ICs and CMOS based ICs.
- In a-stable mode it generates square wave of exactly 50% duty cycle at its pin 10 and gives its inverted signal on pin 11.
- It also generates frequency double to that of pin 10 at pin 13. It can be done by using 0.2μF capacitor and a 7.6KΩ resistor as mentioned in its datasheet to generate frequency of 500Hz at pin 13.
- From pin 13 we give a signal to HCF4017 IC as a clock. HCF 4017 is a decade counter with 12 outputs. It is used to generate a 50Hz signal with controlled delay at both positive and negative edge. Pins 1, 5, 6, 9 and pins 2, 4, 7 are used as output.
- All other outputs are grounded as CMOS ICs are very sensitive and even a small stray signal can burn them out.
- The output of HCF4047 is about 1v, which cannot be used to drive MOSFETs so the signal is amplified by using BC-547 as an amplifier (connected in common emitter biased configuration.
- This signal is again amplified and inverted. The signal obtained is a controlled PWM which we now give at the MOSFET gates.
- The block diagram of the modified sine wave inverter module is given below :
- We gave a PWM signal to the pair of MOSFET connected in a push pull configuration.
- When M1 MOSFET is turned on by a high input (Q), the M2 MOSFET turns off at that time because on its input we had given an inverted signal (Q ) with some delay.
- This delay is used so that one of the MOSFETs gets time to turn off before second one turns on.
- In this mode, the current flows from source to MOSFET M1 as shown in figure.
- When Q1 goes low Q becomes high and M2 turns on, resulting in a current flowing from source as shown in diagram by I2.
- Thus, we get a bipolar high voltage output at the transformers secondary.
- The 22kΩ resistor from Gate to the source as shown in the diagram is important because when the input signal goes to zero, the MOSFETs may not completely turn off because of the capacitance between gate and source so this resistance makes sure that signal is fully grounded.
- The threshold voltage to turn on a MOSFET is approximately 4V. We are driving MOSFETs with 12v signal so that MOSFETs is completely turned on otherwise it will result in power dissipation.
- In power inverter, shoot through current is a major problem and needs to be solved.
- It is a short circuit current, and as described in the previous topic, occurs when both MOSFETs are on. This happens for a very short time i.e. for some Nano-seconds.
- But eventually it results in a short circuit current, which causes loading and thus it may damage the MOSFETs.
- This situation can be avoided by introducing a dead time between the two signals both at rise and fall edge.
- If the dead time is increased too much, the output voltages drop because MOSFETs are turned ON for a very short time.
- Finally impedance matching is an important factor. Transformers output impedance (Secondary) should be low so that minimum voltage drop occurs when we connect any load to with it.