Blog for students taking ECEN5817 Resonant and Soft Switching Techniques in Power Electronics, ECEE Department, University of Colorado at Boulder, Spring 2012
Sunday, February 12, 2012
Problem 19.9
Is anyone getting a Rcrit < 500 Ohms? I was going good until I hit part e and I think I may have done it all wrong..
Did you remember the n^2 term? I am thinking that it should be somewhat larger than 500 Ohms (just looking at part e) Otherwise, there wouldn't be much use doing the resonant converter because you'd never be less than Rcrit and wouldn't get ZVS. Thoughts?
Hmmm yeah I got a very small output voltage range. I'm not really sure what is going on, I thought I did account for the n^2 terms. What was your Voc? I got 153.9 V which I don't know if that is correct or not.
I like your Voc better than mine... I was getting 20.57 V, but I ended up getting Rcrit of 530 Ohms.
How did you do your Voc setup? I just had the tank with an open circuit output, which eliminated the transformer and the load R and used the voltage divider of Vs1*||Xp/(Xp+Xs)||
I'm not sure you can do that, since the voltage on the primary side of the transformer is different from the voltage on the other side of the transformer. I just reworked the problem up to part b. With my new way, I got Voc ~= 98 V, but I sc = 1636 A. I pushed the whole model of Vs1, and the tank circuit through the transformer, so I got Vs1_n = Vs1*n, and n^2 times all the tank elements. However, I got that in this case, the input impedance is always inductive, therefore Rcrit does not exist as zero voltage switching will always occur. I'm now scratching my head at this result.
I think that you should be in the area where R affects ZVS (between fo and finf) with 100 kHz switching.
The reasoning I took to using the tank was that I could push R through the transformer, giving the n^2 term on it, but also, if you short the load, you short the secondary on the transformer and if you open the load, the transformer is open on the secondary side.
I am certainly not saying that I am right in my model, but it does seem fairly logical.
I agree with Olga about opening and shorting the circuit, and solving for Voc.
This gave me an R_crit value greater than my R_matched value, but less than 500ohms. From problem 19.8, you should see that R_crit < R for ZVS, so the converter is not ZVS for matched load, but it is for the entire load range.
Are you sure that ZVS doesn't occur at R < R_crit? I was going off of Lecture 8, Slides 11-12 (annotated version). I do know that fo and finf swapped positions in Problem 19.8, does the relationship for R and R_crit do the same?
I thought the relationship was the same because the impedance is still capacitive between f_inf and fm and still inductive between fm and fo (I got the ||Zi|| plot to be essentially a mirror-image of Figure 19.37, and swapping Ls and Cs.
Luke, For Zoo, I pushed the resistor through the transformer (because it was easier) and looked at the output impedance of the tank when I shorted Vs1.
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10 comments:
Hmm I didn't get Rcrit to be that large, I got it to be around 1.77 Ohms.
Did you remember the n^2 term? I am thinking that it should be somewhat larger than 500 Ohms (just looking at part e) Otherwise, there wouldn't be much use doing the resonant converter because you'd never be less than Rcrit and wouldn't get ZVS. Thoughts?
Hmmm yeah I got a very small output voltage range. I'm not really sure what is going on, I thought I did account for the n^2 terms. What was your Voc? I got 153.9 V which I don't know if that is correct or not.
I like your Voc better than mine... I was getting 20.57 V, but I ended up getting Rcrit of 530 Ohms.
How did you do your Voc setup? I just had the tank with an open circuit output, which eliminated the transformer and the load R and used the voltage divider of Vs1*||Xp/(Xp+Xs)||
I'm not sure you can do that, since the voltage on the primary side of the transformer is different from the voltage on the other side of the transformer. I just reworked the problem up to part b. With my new way, I got Voc ~= 98 V, but I sc = 1636 A. I pushed the whole model of Vs1, and the tank circuit through the transformer, so I got Vs1_n = Vs1*n, and n^2 times all the tank elements. However, I got that in this case, the input impedance is always inductive, therefore Rcrit does not exist as zero voltage switching will always occur. I'm now scratching my head at this result.
Luke,
I think that you should be in the area where R affects ZVS (between fo and finf) with 100 kHz switching.
The reasoning I took to using the tank was that I could push R through the transformer, giving the n^2 term on it, but also, if you short the load, you short the secondary on the transformer and if you open the load, the transformer is open on the secondary side.
I am certainly not saying that I am right in my model, but it does seem fairly logical.
Hmm yeah I see what you are saying. How did you find Zo0? Did you push the tank circuit through the transformer?
I agree with Olga about opening and shorting the circuit, and solving for Voc.
This gave me an R_crit value greater than my R_matched value, but less than 500ohms. From problem 19.8, you should see that R_crit < R for ZVS, so the converter is not ZVS for matched load, but it is for the entire load range.
Carissa,
Are you sure that ZVS doesn't occur at R < R_crit? I was going off of Lecture 8, Slides 11-12 (annotated version). I do know that fo and finf swapped positions in Problem 19.8, does the relationship for R and R_crit do the same?
I thought the relationship was the same because the impedance is still capacitive between f_inf and fm and still inductive between fm and fo (I got the ||Zi|| plot to be essentially a mirror-image of Figure 19.37, and swapping Ls and Cs.
Luke,
For Zoo, I pushed the resistor through the transformer (because it was easier) and looked at the output impedance of the tank when I shorted Vs1.
Rcrit expression is the same for LCC and LLC. But for LLC, R>Rcrit is needed for ZVS. While for LCC, it is R<Rcrit.
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