Guru
Joined: 13/10/2014
Location: AustraliaPosts: 1845
| Posted: 08:30am 30 May 2023 |
Hi, the Inductance is calculated from the resonant frequency of the choke with a known value of parallel capacitance. In my case 4.08uF. I have a Vector analyzer and other RF equipment but I find this method gives me 100% consistent results for these types of chokes.
I think the way to select a Choke is to first find the value that gives you the lowest Idle current before it starts to drop off slowly, then test with something like a heating element with a Diode in series, find the Choke value that gives you the least Toriod noise with the least ringing. Lastly, the choke should not saturate until at least 300A for a small inverter that's not starting big induction motors or thinks like Drop Saws. For heavy loads IMHO, the Choke should still have some inductance past 600A.
Then, if idle current is higher than you would like with the best Choke value for noise and load handling, you obviously have to decide what you are willing to trade off. Sometimes you get a lucky combination where there is no real trade off to be made.
In any case, I would be uneasy going much below 25uH in big choke with very few turns, but that's more a personal feeling I have around some R-Loss and the Q of the Choke.
EDIT:
I was just looking back and found a test that Poida did:
Perhaps the minimum saturation point should be around 400A and not 300A, I believe Poida was measuring the Primary current. Which stands to reason as the FET Switching currents can be higher then DC input current depending on the Duty cycle.
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Edited 2023-05-30 19:35 by KeepIS