What is the 12 volts supplying the voltage reg or is it a direct output signal or bot

[JAMES IRVINE]

I'll begin by saying I'm an electrical engineering student doing an electronics project. I was wondering if anyone can explain to me what is going on with the mosfets in this circuit **broken link removed** What is puzzling me is that I initiially thought that ic1 creating a square wave pulse was the template for the output waveform but then i learned that the mosfet gate driver changes this square wave shape to something completely different.
As far as the action of the mosfets what does this pulse from the gater driver make the mosfet do?
What it sounds like is happening is the 12 volts from the supply is feeding the voltage regulator and is bypassing all components to form the output signal for the mosfet at the same time. Is this correct?

 

[dknguyen]

IC1 is just using the TR1 and R4 (or TR2 and R6) as a level shifter (and gate driver). THe level shifter lets IC1 switch the MOSFET on and off when it does not output the right voltage levels to do so.

What MOSFET gate driver are you talking about? I see no MOSFET gate driver. The only MOSFETs are TR3-TR6 and those are being driven directly by the transistor-pull-up resistor level shifter (if you realy want to call that a gate driver). THe level shifter is there because the MOSFETs require 10-12V gate voltage to turn on, but IC1 can only output 5V.

A MOSFET has a gate capacitance that needs to be charged up before the gate voltage exceeds the level needed for the MOSFET to turn on. A gate driver is supposed to provide a current PULSE to charge up this capacitor as quickly as possible to allow the MOSFET to switch on and off quickly.

I do not understand what you mean by the gate driver (the pull-up resistor and pull-down transistor in this case) completely changing the square waveform. It looks pretty obvious to me what it's doing. I don't see any changing of the waveform anywhere.

The 12V is not feeding the voltage regulator, but it is NOT bypassing all the components to form the output signal for the MOSFET. It is being USED by the MOSFETs to form the output signal. It is NOT bypassing the MOSFETs. I don't know if it's just because English is not your first language or not that you worded it that way or if it was because you misunderstood something. It's exactly the same as the voltage regulator's 5V. 5V is not bypassing IC1 to make a 5V signal- IC1 is using the 5V to make a 5V signal.

 

[JAMES IRVINE]

Thanks very much for your reply. Out of all the things I've asked about you are the first person who has given me a decent answer and who actually read the question properly. Maybe you could help me with my previous questions. Only joking you have done enough already you have been great.
What I said about the output of IC1 becoming a wave shape other than a square wave I deduced from this page of the circuit explantion **broken link removed** as for why I'm confused about the operation of the mosfet has come from this page **broken link removed** I am assuming that the current being drawn from the 12v supply in this diagram is merely being directed in clockwise and anti clockwise directions across the primary of the transformer in order to induce an ac wave form in the secondary. you said "The only MOSFETs are TR3-TR6 and those are being driven directly by the transistor-pull-up resistor level shifter..." where do mosfets tr4 and tr5 come into things. What is the advantage of using this C-MOS FET circuits explained in the first link. Why would I choose this configuration instead of using two single mosfets or two transistors.
Also the gate drivers I mentioned well I just assumed that the Fet drive circuit was a gate driver thanks for explaining that because I would have looked stupid by saying it was when I hand in my report.
As for my english, I'm scottish so may'be that explains it.

 

[colin mac]

The mosfets act like the switches in this attachment.
Sw1 and sw4 are closed so current flows through the load.
If they are then opened and sw2 and sw3 are closed, current flows through the load in the other direction.
The outputs of TR1 and TR2 in that circuit are opposite levels of one another and a mixture of P-channel and N-channel mosfets are used to accomplish it.

 

[Joe McGivern]

12Volt DC to AC Converter.

Congratulations James.
It looks a nice little project. I looked at the wiring diagram and convinced myself that I could understand the principle of operation. (My knowledge of electronics is limited, but I keep trying to learn a bit more.) Would it have been possible to use a transformer with a 230 Volt output. If so, it would be better suited to run electrical appliances in the UK

Regards JMcG

 

[audioguru]

Look at the performance in the project's article.
Without a load, yours will have an output voltage of 230VAC.
With a load of 140W, yours will have an output voltage of only 188V.

Its output is a square-wave. Many electronic products and variable speed motors won't work.

 

[JAMES IRVINE]

A good anology colin I need to get this dictionary out from my mouth but why mosfets and not just transistors? do you know, possibly the high current capability do you think.

 

[JAMES IRVINE]

Im not so concerned about the performance uncle scrooge but maybe you could help me with this question. It probably will seem to be such a stupid question but its not so clear to me.
When the 12v of the dc supply feeds directly to the output of the mosfets, well it looks like that to me from all the information. When it does this it also supplies the regulator ic2 so the current must be divided between these two destinations. I would guess that this may be ok as far as the regulator is concerned but I just cant see this being what is actually happening at the outputs of the mosfets. So if it is then the current being driven through the primary winding in alternate directions is only part of the supply current the other part must feed the regulator. Am I correct, what is going on?

 

[ericgibbs]

When the 12v of the dc supply feeds directly to the output of the mosfets, well it looks like that to me from all the information.
Thats correct

When it does this it also supplies the regulator ic2 so the current must be divided between these two destinations.
Thats NOT correct, the current thats flowing into ic2 continues to flow, the 12V power supply is designed to supply the extra current required by the FET switches. There is NO division of curren as such, just additional current.

I would guess that this may be ok as far as the regulator is concerned but I just cant see this being what is actually happening at the outputs of the mosfets.
Look at my attached drawing, the FET's can be considered as switches when used in this way.

So if it is then the current being driven through the primary winding in alternate directions is only part of the supply current the other part must feed the regulator.
No, the power supply feeds both ic2 and the FET switches.
Am I correct, what is going on?

Hi James,
Consider the 12V psu is designed to supply upto a maximum of say 10Amps.

Without the FET's connected, the current drawn from the psu could be just a few mAmps, say 50mA for example.

When the FET's and transformer are connected to the ic2 output, the FET's will switch a heavy current thru the winding, say 5Amps.

This 5Amp FET current will not starve the ic2 of current, because the 12V psu is designed to give upto 10A.

Does this explain it any better.?

 

[JAMES IRVINE]

Well its roughly the same things as current division by the sounds of it but at least now I can say that the mosfets draw as much current as they need and the remainder will supply the regulator. I guess it doesnt matter how much the regulator takes because its going to reduce it anyway. Well I can perfectly imagine what you are saying perfectly and it seems pretty logical.
Yeah thanks a lot at least now I can seem like I know what I'm talking about cheers Eric.

 

[ericgibbs]

Well its roughly the same things as current division by the sounds of it but at least now I can say that the mosfets draw as much current as they need and the remainder will supply the regulator. I guess it doesnt matter how much the regulator takes because its going to reduce it anyway. Well I can perfectly imagine what you are saying perfectly and it seems pretty logical.
Yeah thanks a lot at least now I can seem like I know what I'm talking about cheers Eric.

Hi James,
Almost right.
Use this analogy:
Consider your house mains electrical wiring.
If you have the kitchen light on and you switch on the radio, the light dosn't dim.?
The voltage of the mains supply is fairly constant, it dosnt fall to any extent when you you have the light and the radio on.
The mains supply is designed to supply a fairly large current at a steady voltage. This is like the 12Vpsu in the invertor.

IF the mains supply was designed to provide a constant current, then as you switched on lights and radio's etc, they would share/divide the current and the mains voltage would fall.

Hope that helps.

 

[JAMES IRVINE]

I see so I just explain that extra current is drawn to compensate. Thats good enough for me.
I've come across another problem while looking at the operation of the square wave oscillator the 4069UB. The problem I have is that it shows two different types of operation within the circuit explanation **broken link removed** this one for the square wave oscillator 2 and this one which is said to use the schmitt inverter **broken link removed** is this two different types of function which can be made capable by using different gates of the IC1. Everything else I understand this is the only place I'm stuck now it's kind of something that I should know otherwise how could I build it. I feel I'm asking too many questions now but this one may be time consuming if not not impossible to find an answer to in the time I have left. Any suggestions on how to find out the answer to that?

 

[ericgibbs]

hi,
The 4069 osc waveform 'D' is the square wave that drives the FET gates, if you compare it with the 'OUT' waveform of the 74HC14, you will see they are both square waves.

There are a number of ways of generating a square wave, I prefer the 74HC14.

For ref only , if you scoped the signal at the junction of R and C on the 74HC14 it would be quite different from the 'OUT' square wave.

The square wave drive to the FET's is important in order to switch them ON/OFF rapidly at the optimum frequency.

 

[audioguru]

This inverter uses 4 Mosfets because the transformer does not have a center-tap. If the transformer has a center-tap then only 2 Mosfets are nedded to drive each end of the center-tapped winding.
That website has an inverter that uses a transformer with a center-tap.