wiseguy Guru Joined: 21/06/2018 Location: AustraliaPosts: 1206
Posted: 01:41pm 21 Jan 2020
I havent stirred the pot for a while so here goes. PS Happy New Year All
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