We focused on the LCC resonant tank and found that if R > Rcritical, we are in ZCS mode. If R < Rcritical we are in ZVS mode.
(A) I'm not sure exactly why this is true?
(B) Is this a general statement for any converter....i.e., if R> Rcritical its in ZCS mode?
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Brandon,
The way I understood this is as follows.....let me answer B first.....I think this applies only to LCC case because fig 19.37 is a plot of Zi_0 & Zi_inf specifically for the LCC case.
For f < f_0, Zi is definitely capacitive, i lags v, and we have ZCS.
For f > f_inf, Zi is definitly inductive, i leads v, and we have ZVS.
Uncertainty creeps in for f_0<f<f_inf. Here we cannot say definitly what is phase of i w.r.t v. That is why we introduce the concept of Rcrit which is then derived in Theorem 2. Also the 4 points above Thm 2 kind of say the same thing.
Hope this helps and hope I have myself understood this. I would welcome any constructive criticism to what I said above.
I totally agree with what you said (this is the depth of what I got out of the book).....but not sure for all converters.
I think I figured it out.......
Take the LCC network and put a load, R, on it....find the equivalent impedance of that network (Zi). Zi = sL + 1/sC_s + R/(R(sC_p)+1)
As R becomes larger (towards infinite), the parallel combination of Cp and R is going to dominate that expression. Zi = R/(R(sC_p)+1)
With R huge, that little terms becomes Zi = 1/sC_p ---> implies ZCS (since capacitor). So when R > Rcritical, you get ZCS because the input impedance sees only the parallel capacitor.......Figure 19.38 shows this.
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