What is this circuit trying to accomplish? - Page 1
Forum section: electronics / beginners

author: LoveLaika
author_rank: Frequent Contributor
timestamp: July 13, 2022, 03:12:02 pm
content: This may seem a little mundane, but I was wondering if anyone could give me some insight on this circuit attached below? Is it a common, recognizable circuit? I'm studying power electronics, and one of the problems I'm working on involves this circuit. Running through what I've learned (Volt-Sec balance, Cap-Charge balance) and solving for the average voltage and current values, I came up with the following ratio of output to input voltage:M = Vout/Vin = (1-D) / ( D * (1-2D) ), where D is the duty cycle of the circuit. Note that SW1 and SW2 are never in the same state as the other (when one is open, the other is closed, and vice-versa).Running through basic values for D, I got some weird values for the output voltage, and that made me think that my derivation might be wrong. Since I don't know what this circuit is trying to accomplish given the output voltage values, I thought that perhaps if I knew what this circuit was used for, I could better understand it and see where I went wrong. At first, I thought it was a SEPIC or a CUK, but after looking at them, the components were in the wrong location, so I don't think it's either of them.

author: Benta
author_rank: Super Contributor
timestamp: July 13, 2022, 04:19:16 pm
content: It's a Cuk converter.https://en.wikipedia.org/wiki/Ćuk_converterEDIT: URL corrected. Hope that helps.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: July 13, 2022, 06:15:59 pm
content: Thanks for replying, but how so? Looking at the rough model on the Wikipedia page, though it's modeled differently, the input voltage source is present in both operating states of the switch. The middle capacitor has one node either connected to ground or the diode. Is the diode supposed to act as a SPST switch in this circuit? Sorry, I'm just confused on how to relate the circuit posted to the Cuk converter. If they're both the same, I should get the right ratio between output/input voltages, but I'm not seeing it with the way the switches in my circuit are positioned compared to the two different states in the Cuk wikipedia page. The first inductor and cap are never in series with the input voltage source. That's probably the main reason I don't see it as a Cuk.

author: Wolfram
author_rank: Frequent Contributor
timestamp: July 14, 2022, 12:31:05 am
content: It's a Zeta converter, related to the Cuk and SEPIC. In this case with synchronous rectification, it can also be seen as a SEPIC converter with the source and load ports switched.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: July 14, 2022, 01:31:30 pm
content: Thanks for the reply. Doing some research, I see the similarities now. Looking at thisapp note, I can see it (removing the input capacitor). Thanks a lot. I've never heard of this topology before, so this is really interesting. Though, with that being said, looking at their simple circuit and compared to my output/input voltage ratio, I'm clearly doing something very wrong if I'm not getting the right results. Thanks again, this really helps a lot.That being said, TI's app note feels a bit odd. Specifically at Figure 2, between the two switch states, the polarities of the components are swapped around. Is there a reason for doing this? When I did my analysis, I kept my polarities in the same position for each state, so that might be tripping up my analysis here.

author: LoveLaika
author_rank: Frequent Contributor
timestamp: July 15, 2022, 09:16:42 pm
content: Quote from: Wolfram on July 14, 2022, 12:31:05 amActually, if I may ask for your assistance, how do you solve for the output/input ratio for the Zeta converter? Looking at the app note, I see how they labeled the voltages, but I don't understand how they got to that point. If you look at my original model, if you didn't know what the voltage across each component, you would label them accordingly and solve for their average value (assuming small-ripple analysis).I've attached a picture showing the circuit in the two different operating states. Ensuring that the signs are correct in both states, and assuming small ripple analysis, you can solve for the volt-second balance in order to get a ratio of Vout to Vin. The way they have it though, because the app note makes reference to Vin and Vout for the component voltages, it's really causing some confusion. Ideally, I think I should get the same result as they did for the ratio, but I'm not. It appears to be due to the relationship between V_L1 and V_C1 in the second circuit. I'm confused as to where I'm going wrong.