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High Efficiency Sine Wave Inverter - Part 3 (Inverter Board)
To recap a little bit Ill briefly explain how the sine wave inverter works. Basically this inverter is a Class D audio power amplifier designed to work with high voltages. The inverter creates a square wave suitable for MOSFET switching with minimal power loss as heat because the MOSFETs will not be in the linear region. There is an on-board sine wave generator that is used as the input signal to the “amplifier”. After filtering out the high frequency square wave, the amplified input signal generated by the on-board oscillator remains. If you need further and more detailed explanation please read the first article of this series HERE.
Here is the schematic for the oscillator that generates the input signal:
To implement the PWM (pulse width modulation) circuit a ramp generator was used in conjunction with an op-amp configured as a comparator. The schematics of the ramp generator is illustrated above. I will not discuss in detail how this is accomplished, there are numerous 555 timer tutorials in the internet. The 555 is operated with a current limiter to charge a capacitor at constant current. One of the properties of the capacitors is that voltage can not change abruptly. Since we are charging it at constant current the voltage across the capacitor increases linearly. The 555 timer controls the time that the capacitor is going to be charged (it sets the PWM frequency, in this case 20kHz).
Now, here is where the magic takes place. The generated ramp is fed to a comparator as well as the sine wave generated by previous block. When the voltage of the ramp is greater than the voltage of the sine (reference voltage) it generates a perfectly square pulse lasting until the voltage of the ramp drop below the sine voltage.
In the illustration all the pulses are of equal length because the reference signal is a constant voltage but in the circuit the pulses will be of different length because the reference is a sine wave (variable voltage).
The output signal is then fed to a 4011 CMOS NAND gate. This chip squares up the wave even better and it also generates a wave that is 180 degrees out of phase. This makes possible current flow in the output. If both wires (power output of the inverter) are in phase then the voltages will be the same thus no current would be able to flow.
Now I think that this is the most important part of the circuit, the two half bridge drivers. There are two half bridge drivers in this circuit and they essentially do the same task. The only difference is that one side takes care of the original signal and the other side deals with the signal that have been shifted by 180 degrees. It is necessary to use a H bridge driver because the MOSFETs that are connected to the positive rail have their Source pin “floating” and thus they can not be fully turned off creating a very serious condition in the half bridge. When this happens both MOSFETs are conducting thus creating a short to ground. The IR21834 chip handles this by making a “fake ground” at the high side MOSFET's Source pin. Then, the gate is driven by a higher voltage to ensure that the MOSFET is fully on when a pulse from the signal comes.
Spike filtering is done with TVS diodes (transient voltage suppressor diodes) across each MOSFET and a very famous 0.1uF@1000V capacitor from the positive rail to ground. This capacitor absorbs large spikes and if any voltage remains above desired limits the TVSs take care of it by dissipating it as heat. TVS diodes work as zener diodes. When there is a voltage too high it conducts infinite amount of current (reverse biased) to ground.
As in any other driving circuit, care must be taken about ringing. All MOSFETs have a 5.6 Ohm resistor with a diode across it to avoid ringing. Zener diodes are used in the gates too in case that there is a spike that exceeds the Gate to Source voltage of the MOSFETs. This zener should dissipate that spike by clamping it. I used a 1k Ohm resistor from Gate to Source to make sure that the MOSFET's internal parasitic capacitor discharges entirely before the next pulse comes by.
The output from the MOSFETs is the power output, now all thats left is to filter the high frequency out and stay with the amplified sine wave. The filter is very simple, it is just two coils and one capacitor connected across the output of each half bridge. An illustration is shown below.
As a bonus here is a picture that I took of the oscilloscope's screen. It shows the output before and after filtering.
I want to give special thanks to Don Carroll and to Ross Wheeler for helping me out so much with this project. Without them this project would have been impossible.
WARNING: This information, all pictures shown here and all schematics are copyrighted material. The owner of this material, Argenis Bilbao (myself) prohibit the use of this information for anything other than personal use or educational purposes. The use of this information for commercial purposes without my authorization is a violation to copyrighted material and will be prosecuted by law.