Something feels wrong with my analysis here - Page 1
Forum section: electronics / beginners

author: LoveLaika
author_rank: Frequent Contributor
timestamp: July 11, 2022, 10:43:10 pm
content: I'm having some trouble with a certain problem in Power Electronics, and I'm at a point where I need some advice on how to proceed. It's actually a long problem, but two parts of it confuse me.So, the problem starts out with a circuit, a buck-boost circuit, as shown in the first attached image. It attempts to use convert 1.5 volts to 5 volts given the values. The first part of this problem asks to derive an equivalent circuit that models the DC property of the converter including transistor and diode conduction loss. I've managed to work this out in the second attached image. By modeling the losses as resistors and as a voltage drop in the case of the diode, I came up with the top circuit using the ideal DC transformer. From that, I reflected the sources/loads at the transformers to get the equivalent circuit model shown below. (Sorry, now that I see it, the input should be D*Vin, not D'). However, after this first part, I get confused.You are given some constants as shown in the table in the image. The second part of this problem asks you to find the largest value you can for the inductor resistance (R_L) given a certain efficiency. Once you have that value, you are to use them and compute the power loss in each element. Now, the efficiency is 70%. The way I calculated it was by using MATLAB, playing around with the value of the inductor resistance and using approximate values to solve for the duty cycle. It feels a little cumbersome, but I was wondering if there was a smarter way to do this? (For what it is worth, I used the value of the load resistance and the transistor ON resistance to give me an idea of what order of magnitude the inductor resistance should be in order to have a noticeable effect).Moving on, with the calculations of the power resistances here (assuming you solved the previous part), I thought about this a bit, and I had some issues with the power losses at each element. Looking at the bottom circuit, since the dependent voltage source on the left is just a fraction of the input voltage, when you sum the power generated and the power absorbed by each element, it doesn't sum to 0. Given the numerical values, there's a 5-watt loss at the reflected load, and with the dependent source, it didn't make sense to me that the net power didn't sum to 0. However, it then hit me that the efficiency of the circuit is not 100%, but it's 70%. Furthermore, it's based on a ratio of power out to power in; so, I shouldn't use the dependent source as the basis for efficiency, but it should be the original Vin source. With an efficiency less than 100%, is that the reason why the sum of the powers (going by the second circuit at the bottom) doesn't sum to 0?EDIT: Oops....never mind, I'm dumb. I realized just now that given the numbers, I can just plug-and-chug the values if I know the output voltage/current and the input voltage. I can then relate the input/output values to the inductor current and the duty cycle to solve for the inductor resistance that way.....maybe...I'll have to work that out to make sure.EDIT 2: Whoops, looks like I used the wrong equation in my first run leading to more questions. But, at least I figured out what I was doing wrong. I was looking at just the efficiency and not the output voltage. That's what was screwing me up. All good now.