AC-DC Inverter Circuit Principles

[Haggis and Neeps]

Can anyone offer some guidance to a newbie?

Q- i'm trying to develop an 12v DC-230v AC 50Hz inverter.
I'm staring with a 555ic as a simple base(no doubt I will have many unsuccefful attempts hence the "affordable(cheap)" nature of the circuit).
Here's what I was thinking:

Voltage Regulator->555ic -> Low Power NPN and PNP transistor driving larger parralled High Power Mosfets -. transient suppressors/filters-> 12v-230v step up Transformer -> transient suppressors/filters

This I hope would give a basic inverter(square wave).

I'm really looking to develop a quasi sinewave by having multiple square waves with different duty cycles overlayed upon each other on the carrier frequency (50Hz).
What would the easiest way of doing this?
I know from experience that the commercial quasi sinewave inverters use a 556ic to generate two square waves on the carrier frequency.
Would it be possible to harmoinies multiple 555 ics in one circuit so that I could effectively piggyback multiple inverter circuits to give a relatively smoth sinewave?
So many Q's?

Many thanks in advance if you can help.

Thanks.

 

[audioguru2]

A modified sine-wave is really a modified square-wave. The wave goes to a positive peak then to 0V for a pause then to a negative peak then to 0V for a pause again over and over. The wave is produced with pulse-width-modulation so that voltage regulation can be achieved.

One group of Mosfets produce the positive peak and another group of Mosfets produce the negative peak.

Your idea needs too many groups of Mosfets. It also needs a huge, heavy and expensive low frequency transformer.

A pure sine-wave inverter uses a 25kHz power oscillator driving a small inexpensive ferrite-core transformer that is rectified and filtered to make a high peak voltage for the Mosfets. Then two or four groups of Mosfets switch at 25kHz and are PWM modulated by a 50Hz or 60Hz sine-wave. The output is filtered by a small inexpensive LC filter into a 50Hz or 60hz pure sine-wave.

View attachment 40877

 

[Haggis and Neeps]

Your idea needs too many groups of Mosfets. It also needs a huge, heavy and expensive low frequency transformer.

A pure sine-wave inverter uses a 25kHz power oscillator driving a small inexpensive ferrite-core transformer that is rectified and filtered to make a high peak voltage for the Mosfets. Then two or four groups of Mosfets switch at 25kHz and are PWM modulated by a 50Hz or 60Hz sine-wave. The output is filtered by a small inexpensive LC filter into a 50Hz or 60hz pure sine-wave.

Many thanks for the advice.
I have had several of the inexpensive(cheap) AC-DC inverters and they have all failed when used with multiple loads/ and or inductive loads(motors, etc).
Even when the inverter rating exceeds twice that of the total loads i.e 1Kw load, 2Kw Inverter(4Kw Surge Capacity).
Generally it is the Mosfets that fail(12volt side).This then cases a cascading burn out of the Mosfets which can be very expensive to repair.
I think this is priimarily caused by the small size of the inverter coils.
The phsical size of the coils appear to be around the 12VA capacity(PCB mounted).Even with 8 coils in a 2Kw inverter then this effectively only gives a 96VA capacity.
My thinking for the design is that the power handling is a dirrect function of the transformers/s mass.
Would it be feasible to chop the DC in my inverter design at 50Hz through parralleled high power Mosfets. Some of these high power Mosfets can handle 300Amps(pulsed).
Would the current  phase be in unison with the voltage phase in my proposal?
I'm assuming that the voltage would drop with load so I would have to design a voltage correction element to the circuit?
Am I flogging a dead horse with this idea?

Many thanks









/I maybe asking pointless questions so I apologise in advance.

 

[audioguru2]

A 25kHz ferrite transformer is inexpensive, small and light weight. Your 50Hz iron transformer will be expensive, huge and heavy.

The 25kHz circuit produces a pure 50Hz sine-wave. Your 50Hz circuit produces a square-wave or a modified square-wave.

 

[Haggis and Neeps]

A 25kHz ferrite transformer is inexpensive, small and light weight. Your 50Hz iron transformer will be expensive, huge and heavy.

The 25kHz circuit produces a pure 50Hz sine-wave. Your 50Hz circuit produces a square-wave or a modified square-wave.

I'll bin my initial idea.
Seems PWM is the way to go. How could I provide a "sensing" circuit for the 555 oscillator so that I can vary the pulse widths on the back of a reference sine wave.
I think I can do the circuitry at the 555 side?
Would I use some kind of function generator/ sinewave generator? If so, is it possible to get this circuit to communicate with the 555?
Is there a risk that the PWM in an inverter when compensating for high loads may just produce a monolithic waveform(semi-square)?
Ultimately I'm after reliability, ruggedness and simplicity(i'm sure that's every amateur's dream).
Any thoughts, as I'm a real novice(but keen to learn).

 

[audioguru2]

A 555 uses a pot to adjust its duty-cycle. Pin 5 can change the duty-cycle only a little.
Usually a PWM controller IC (TL494 etc) is used because it has all the features needed to make a high frequency PWM pure-sine-wave inverter. Make the mains-frequency sine-wave any way you want.

 

[Haggis and Neeps]

Thanks for the advice.
Seems like a TL494 is the way to go as it appears to have all of the functions.
My revised circuit thoughts are:

Voltage reg - sinewave generator(50Hz)-TL494-op amp/s- power mosfets-centre tapped HF transformers.

I'm hoping I've finally figured the basics out?

I've done a little bit of research and it appears that instead of PWM it is possible to utilise a variable DC source. Any thoughts on this?

Thanks in advance.

 

[audioguru2]

PWM has a high frequency square-wave that has its duty-cycle changed by the 50Hz or 60Hz sine-wave. A square-wave is used because the output transistors switch completely on and completely off quickly. When they are switched on, the voltage across them is very low so their power dissipation (heat caused by voltage times current) is low and when they are switched off they have no current so again their power dissipation is low. But the output power can be high.

The duty-cycle of the high frequency square-wave makes a high amount of power when the duty-cycle is high because the transistors are turned on for most of the time and makes a low amount of power when the duty-cycle is low. The result is a stepped sine-wave and the high frequency steps are removed with a small high frequency ferrite filter.

I don't know how a variable DC source can make a pure sine-wave inverter.
Maybe you mean to use a linear audio amplifier to amplify the 50Hz or 60Hz mains frequency. But a linear audio amplifier wastes almost as much power as the load uses by making heat. Lots of heat. You don't want an inverter to throw away nearly half the power of the battery and the severe cooling problem.