12 V DC to a 220V AC Inverter AMplfier Design

[audioguru2]

If your inverter is efficient then it doesn't waste much power as heat because its Mosfets switch nearly completely on and off and do not operate linear.

Power= Volts x Amps. Amps= Power/Volts.
Input power from the battery = output power to the load plus heating power.

Example again:
500W inverter with a 500W load. 100W of heating. Total power from the battery= 600W. Total current from a 12V battery = 50A. Current in the Mosfets on each side= 50A max.

 

[kachew]

dear Au,

means that the mosfet will draw 50A from the battery and each of the mosfet can withstand a current of 23A, so the current from battery will 50A divided by 3 mosfet so each of them draw 16.7A from the battry? is this what u mean?  about the heating power ....how u know i will dissipate 100w? 1 more question, as i know the amps value also must take into consideration when wan to buy transformer....how can i know the amps value?

 

[audioguru2]

means that the mosfet will draw 50A from the battery and each of the mosfet can withstand a current of 23A, so the current from battery will 50A divided by 3 mosfet so each of them draw 16.7A from the battry? is this what u mean?

The transformer has a center-tap that is connected to the battery. The winding has a 12V terminal that connects to Mosfets on one side and another 12V terminal that connects to Mosfets on the other side. If the inverter is for 500W and has 100W of heating then the Mosfets on each side conduct a total of 50A from a 12V battery. You need only 2 Mosfets on each side.

about the heating power ....how u know i will dissipate 100w?

I guessed. If you use IRF540 Mosfets that are old and cheap then their typical on-resistance is 44milli-ohms when cool and 66 milli-ohms when warm. 25A through 66 milli-ohms is a power dissipation of 41.3W for 1 Mosfet and 82.6W for 2 Mosfets. The Mosfets on each side of the transformer winding alternate so there is always a power dissipation of 82.6W when the load is 500W. More of these cheap Mosfets or better ones will reduce their heating. The transformer will heat with about 17.4W so the total heating is 100W.
 

1 more question, as i know the amps value also must take into consideration when wan to buy transformer....how can i know the amps value?

Use a 600VA transformer. It will be your mains voltage to 24V center-tapped. Each half of the center-tapped winding must be able to pass 50A.

 

[kachew]

Dear Au,
thanks for your explaination, can i buy transformer with lower amps? such as 12va or 30va for this circuit?

 

[audioguru2]

can i buy transformer with lower amps? such as 12va or 30va for this circuit?

A 12VA transformer has a power of only 12W. A 30VA transformer has a power of only 30W. What do you have that uses such a small amount of power from the mains?
Stores sell a tiny cheap inverter with an output of 75W. I don't know what it is used to power.

 

[kachew]

dear Au,
i know the transformer with small amps couldnt power anything....but the transformer with higer amps is very expensive in my country.....mayb i will use it for small applications such as charging my handphone or something else which need small amps.....do u think its ok?
thanks

 

[audioguru2]

This simple inverter has a square-wave output. Many electronic devices need a sine-wave because they charge their power supply capacitor to the much higher peak voltage of the sine-wave. Some transformers overheat when driven with a square-wave.

 

[kachew]

dear Au,

can i know how much the current will flow to turn on the mosfet for switchin? and if i wan to get an output for 150W but not 600W but using the same circuit can it make it? as i know if i buy a smaller transformer as i asked the shopkeeper, the battery which supply 50A to the transformer which is 10A will blow the transformer off....can i know wat is the amps input will go to the transfomer? how can i calculate the output current of the transformer....i still cant get what u mean by previous post....
your help is highly appreciated.

 

[audioguru2]

Can i know how much the current will flow to turn on the mosfet for switchin?

The gate of a Mosfet is a capacitor. It needs current to charge and discharge quickly. For the low mains frequency the peak gate current of a few milliamps will be fine.

if i wan to get an output for 150W but not 600W but using the same circuit can it make it? as i know if i buy a smaller transformer as i asked the shopkeeper, the battery which supply 50A to the transformer which is 10A will blow the transformer off....can i know wat is the amps input will go to the transfomer?

The battery supplies 50A to the circuit only if the load on the inverter is 500W. The current from the battery and through the Mosfets and transformer is less when the load draws less power. Without a load then the current is only one or two amps.

A 150W load on an inverter will use about 188W from the battery. Then the current from a 12V battery through the Mosfets and into the transformer is 188/12= 15.7A.
A 10A transformer will be overloaded.

How can i calculate the output current of the transformer.

Calculate the transformer's output current I= P/V. I= 150W/230V. It is only 0.65A.

...i still cant get what u mean by previous post....

1) This simple inverter has a square-wave output. It is not a smooth sine-wave like the mains.
2) Many electronic devices need a sine-wave input. A sine-wave has a peak voltage that is 1.414 times higher than a square-wave that has the same average power. The power supply of an electronic device charges its main filter capacitor to the peak voltage that is higher with a sine-wave. Then the electronic device won't work properly with the lower peak voltage from a square-wave.
3) Transformers are designed for a sine-wave input. A square-wave has many high frequency harmonics that create extra heat in a transformer.

 

[kachew]

Dear AU,

the surge capability is the amount of the voltage needed to start an appliance....Multiply: WATTS X 2 = Starting Load, for inverter of output has 150W need to have a surge capabilty of 300W. how can i archieve that? by increasing the transfomer higher rating amps? (buy transformer that have more amps?).... i saw the inverter sold in the market which has rating of surge 300W but have overloading protection of 150W....so what is this means? i though is the inverter has the rating of 300W, it's aleady higher than the 150W how can the overloading protection can work appropiately?
thanks

 

[audioguru2]

Hi Kachew,
A simple inverter uses a fuse from the battery for its overload protection. If you need 300W of surge power then use a fuse for a little over 300W, a 200W transformer, a circuit that has a 300W output and heatsinking for 200W. Then a surge can draw 300W for one minute and the inverter will continuously power a 200W load.
If this simple inverter is overloaded continuously supplying a power of between 200W to 300W then it will overheat and might be destroyed.

Every power transformer that I have seen has only a continuous power rating. They don't have a surge rating and don't have overload protection.

 

[kachew]

dear Au,

A 150W load on an inverter will use about 188W from the battery. Then the current from a 12V battery through the Mosfets and into the transformer is 188/12= 15.7A.

so the transformer i need is the center tapped transformer(12-0-12 to 220V) with 15.7A multiply 2=31.4 or higher amps step up transformer for which to have continous power of 150W and surge capability of 300W with the overloading protection fuse (300W) to meet all my requirement?
thanks again

 

[audioguru2]

Hi Kachew,
You described a power transformer for a 300W continuous power inverter.

 

[kachew]

hi Au,
yeah that's is what i want....can i implement it in the circuit with the cd4047 and cd4001?
as i know the peak voltage of the sine wave is square root of 2 larger than the square wave....so which transformer i need to buy? (10-0-10) or the (12-0-12)?
thanks

 

[audioguru2]

Where is the schematic of a 300W modified sine-wave inverter with a CD4047 and a CD4001?
Does it use Mosfets?

 

[kachew]

deat au,
yeah...wait i reattach the picture of the schematic to you
pls give some comment and guide to me.... ;D ;D
thanks

 

[audioguru2]

Your 300W modified sine-wave inverter schematic is very fuzzy because you saved it as a JPG file type instead of saving it as a very clear GIF or PNG file type. I can hardly read it.

The transformer should be 8.5V-0V-8.5V/230V for the output peak voltage to be root-2 of the voltage of a square-wave inverter. It will need to be custom-made.
The transformer will have the same average 376W as before but the peak current in the Mosfets is 2-root-2 times 31.4A= 88.8A.

 

[kachew]

dear au,

means that the output of the modified square wave will be a Vrms value? from the we multiply by square root of 2 to get the peak value? means 230 from the output of the transformer will multiply with square root 2 to get the peak value?

The transformer will have the same average 376W as before but the peak current in the Mosfets is 2-root-2 times 31.4A= 88.8A.

what do u mean by this statement?

how do you know that i must choose 8.5-8.5 instead of 10-0-10V transformer? can give me more explaination about this??

do i need to reduce the mosfet if i am making the 150w instead of 500w? as i know the mosfet is use for switching only....i think its doesnt affect much on the output value...??

thanks agian

 

[audioguru2]

means that the output of the modified square wave will be a Vrms value?

Every inverter has an RMS power which is its average power to the load.

Yes, just like with a sine-wave.


The Mosfets are turned off for half of the time so their current must be doubled for the inverter to produce the same average output power as a square-wave inverter.


You want the peak output voltage to be root-2 times the mains voltage. 12V/8.5V= 1.412 which is nearly root-2.


The current in the Mosfets on each side for a square-wave 150W inverter is about 15.7A.
The current for a modified sine-wave 150W inverter is doubled to 31.4A. You can buy modern Mosfets that can conduct 31.4A or use two on each side in parallel.

 

[kachew]

dear au,

sorry for my mistake, actually i need a transformer with 150W continous output,
the rating for my transfomer is (8.5 to 8.5V) how about the amps i should buy for the transformer? izzit still the same 31.4amps?
thanks  ;D ;D