another inverter project


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wiseguy

Guru

Joined: 21/06/2018
Location: Australia
Posts: 485
Posted: 01:41pm 21 Jan 2020      

I havent stirred the pot for a while so here goes.  PS Happy New Year All

  Warpspeed said  
Here is a further thought.

The choke is directly in series with the primary of the inverter transformer, and both carry the exact same identical current.  Why is it that the choke must have a ferrite core, while the main inverter transformer can get away with a steel core ?  
Remember the current, and the magnetic effects produced by that current in the respective windings and cores will be the same in both.
.


If I had to describe what is going on, first an observation. On a Buck converter operating in the 10's (100s?)of kilohertz, ferrite or powdered iron performs brilliantly whereas iron sucks (technical term).

I see the choke and transformer performing quite different jobs even though they share the same series current, the choke is essentially integrating the HF energy into a LF fundamental content of the PWM. The transformer then changes the LF voltage level to the desired voltage. I see the PWM driving squarewaves into the choke resulting in a sawtooth triangular current ramp (in the choke) as energy is briefly being stored and then returning that stored energy to the series choke/transformer circuit. The same 20kHz waveform applied to the transformer meets the reflected very low impedance of the tuning capacitor (resembling a short at 20kHz) so the resultant HF energy dissipation & storage at 20khz of the toroid is very low.

I said it a year ago that for my modelling purposes the inverter really resembles an overgrown synchronous bipolar (2 quadrant) buck converter.  I dont agree that the magnetic effects of both will be the same though, the transformer has a tuned secondary (50/75Hz) with reflected impedance back to the primary, and the choke is just a choke.  The transformer is dealing with the resultant integrated HF energy ( a 50Hz fundamental) the choke is doing the integration work of the 20kHz HF PWM waveform.

I sense Tony was setting out to wind someone up and I bit  
If at first you dont succeed, I suggest you avoid sky diving....
Cheers Mike