In this post we'll discuss how to convert any ordinary square wave H-bridge inverter into an almost pure sine wave inverter circuit.
Basic Concept
The idea is simple, just chop the low side MOSFET gates of the H-Bridge with reverse SPWM (Sinusoidal Pulse Width Modulation) waveform. This will force the low side 50 Hz, or the 60 Hz signal at the MOSFET gates to get chopped into the corresponding SPWM waveform.
This Waveform will be correspondingly replicated across the output load causing the load to operate with this SPWM waveform. Since the SPWM is equivalent to a sinusoidal waveform, the output of the inverter will be transformed into a sine waveform. A calculated high voltage capacitor added across the inverter output will further enhance the SPWM waveform into a real looking sinusoidal waveform.
If you do not want to read the whole article, you can watch this video instead:
Universal Design
The diagram below shows a universal H-bridge sine wave inverter layout which can be applied to convert any square wave H-bridge inverter into a sine wave H-bridge inverter.
On the right hand side we see the H-bridge MOSFET stage using 4 N-channel MOSFETs. However, virtually all variants will work as long as the low side devices are N-channel.
Since we are only interested in the low side MOSFETs, the upper high side MOSFETs are irrelevant and can be ignored.
Operating the upper MOSFETs with SPWM is not necessary and is not required, because in an H-bridge topology the high side and low side MOSFETs operate in series, so switching any one of these with the SPWM is sufficient for the intended results.
Moreover, in H-bridge designs, that have 4 N-channel MOSFETs, the high side MOSFETs are operated using bootstrapping mechanism, which makes feeding an SPWM too complicated, and also an overkill.
Coming back to our universal H-bridge sine wave inverter design, the left side consists of two IC stages: IC 555 astable stage and an op amp IC 741 comparator stage.
The comparator op amp 741 or any standard op amp performs the main function of an SPWM generator.
It is rigged as a comparator to compare a couple of triangle waves fed across its inverting input pin#2 and the non-inverting input pin#3.
The non-inverting input pin#3 is fed with slow triangle waves, while the inverting input pin#2 receives a relatively fast triangle wave supply.
The op amp compares the two input exponential waveforms which results in the generation of the corresponding sets of SPWM waveforms at its output pin#6.
The slow triangle waves are derived directly from the inverter's existing H-bridge oscillator IC, across its timing capacitor Ct. This Ct timing capacitor is available in all inverter designs regardless of the topology.
The slow triangle waveform across the Ct pin of the inverter IC actually determines the output 50 Hz or the 60 Hz frequency of the inverter. Therefore, by using this waveform as this slow triangle waves we get two advantages: firstly it helps to carve out the intended SPWMs, and secondly it perfectly synchronizes the SPWM with the main inverter frequency, enabling the inverter output sine waveform to be precisely in sync with the 50 Hz / 60 Hz ON/OFF timing.
The fast triangle waves are extracted from across the timing capacitor of the IC 555 astable C2. The fast triangle waves decide the frequency of each SPWM waveform, which must not exceed 300 Hz if the inverter transformer is an iron core transformer.
If the transformer is a ferrite cored transformer, then you can increase the fast triangle wave frequency to any higher limit, as suitable.
Higher the frequency of the SPWM, higher will be the efficiency of the output sine wave.
The presets P1, P2 can be used to tweak the SPWM so that it replicates an output sine waveform as close as possible to an original sinusoidal waveform.
The SPWM output waveform from the op amp IC 741 is finally applied to the gates of the low side H-Bridge MOSFETs via individual 1N4148 diodes.
You can see in the diagram, the 1N4148 diodes are reverse biased, meaning the chopping of the 50 Hz or the 60 Hz waveform at the MOSFET gates will be only through the negative pulses of the SPWM, the positive pulses have no effect.
This configuration will allow the low MOSFETs to switch exactly as per the switching of the SPWM waveform from the IC 741 output, generating the required sine wave AC output for the connected.
In this way any ordinary H-bridge inverter circuit can be transformed into a pure sine wave H-bridge inverter circuit.
A Practical Sine wave Inverter Design
The following diagram shows a practical example of how an simple IRS2453 H-bridge inverter circuit can be converted into a sine wave H-Bridge inverter circuit.
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