Modified sine wave inverter analysis

I would like to analyse this circuit, but I do not understand how this circuit works.

J10 connector is connected a three wire, center tapped tranformer.

What should be the input of the transformer to produce 50 Hz, 220V modified sine wave?

How do these transformers work?

Second question: Why do we use resistor R19 in this diagram?

Third Question: Why do we use mosfets in this diagram?

• J10 probably connects to a center-tapped transformer. The transistors alternately pull down on the ends of the primary, producing alternating polarity pulses on the secondary. Commented Mar 26 at 20:33
• What transformer? There is no transformer on that schematic.
– vir
Commented Mar 26 at 20:33
• I think, 3 Wire Transformer, Centre Tapped Transformer. I couldn't find anything in the video. I guess I don't know how this transformer works. Commented Mar 26 at 20:51
• @SerkanKaya For the working principles (I haven't wanted the youtube link you provided), you may wish to watch this video on 1950's switching supply. The author is quite thorough on explaining the concept and details. He will be repairing a vibrator that used to be used in automobile radios (I used to repair them as a kid) and Ham radio equipment used in cars and trucks. The key here is that Carlson goes through the schematic with great care. So you should be able to follow the idea, I think. How it may apply here? Not sure, but it may help. Commented Mar 26 at 20:58
• @periblepsis OK, thanks. Commented Mar 26 at 21:03

There's nothing magic about the MOSFETS. If Q12 is turned on, then J10 pin 1 is pulled down to ground. Otherwise, it's floating. The same for Q13 and J10 pin 3.

J10 pin 2 is always connected to the positive supply.

The transformer connected to J10 would be centre-tapped. The two ends would be connected to J10 pins 1 and 3, and the centre tap would be connected to J10 pin 2.

The circuit alternates between connecting the supply voltage to one half of the primary winding and then the other half of the winding. This creates an alternating magnetic field in the transformer, which creates AC on the secondary output.

simulate this circuit – Schematic created using CircuitLab

Notice how current flowing from 2 to 1 runs in the opposite direction, relative to the transformer core, than current flowing from 2 to 3.

It's worth saying that a "modified sine wave" looks nothing like a real sine wave. It's marketing speak.