Isolated dc-dc converters - understanding the flow of energy

I have been creating videos on dc-dc converters for close to year. The entire playlist of videos can be found at:

https://www.youtube.com/playlist?list=PL-_jTul4we2TUk7zANWQhGNc6kkJ4hg5J

Typically, when learning power electronics as either an undergraduate or in the early stages of a master's, dc-dc converters are presented one after the other with very little talk about possible links between them or how one could progress from one to the other. During the course of this lecture series, I have been trying to do exactly that - focus on how the converters differ from one another in terms of how the energy flows.

As I have been teaching power electronics, I find that using equations alone to describe the operation of a converter is quite inadequate. Power electronic converters are nonlinear and therefore, equations rarely help to understand how they behave. Equations and analysis can help in designing and optimizing their performance but to understand how they work, it is necessary to understand how energy flows between the different states of the converter and how that fits in with the purpose of the converter.

In the last few weeks, I have been gradually approaching the flyback converter topology. The reason I say I have been gradually approaching this topology is because rather than just present the topology and simulate it, I felt it is necessary to describe how this topology came into being. So, instead of using the opposing winding dot polarity that is characteristic and fundamental in a flyback converter, I use the normal transformer where both winding have a dot polarity at the same (upper) terminal.

The simulation shows what happens when the magnetizing current in the transformer is broken. I show how it is possible to connect a LC filter at the secondary winding to produce a filtered output voltage and additionally how a freewheel diode can be used to ensure the continuity of current through the filter inductor, the main problem arises with the magnetic energy in the transformer having nowhere to go during one stage of the converter operation.

With almost every topology of isolated converters, the main question arises about how to ensure the energy stored in a magnetic component (inductor or transformer) increases and decreases during a switching cycle. If the energy continuously increases which implies the peak current keeps increasing, eventually the inductor or transformer will saturate as the flux corresponding to the peak will be greater than the knee-point of the B-H curve. Additionally, the changes need to happen cyclically and gradually - an increase in current should be following by a decrease.

In general, this is what makes power electronics fascinating - how one can condition power without moving parts but just by ensuring a cyclical flow of power. This is true for any power electronic converter.

Fourth Udemy course and second book

 It has been a very long time since I blogged. Quite sadly, my project of writing automatic tests for the circuit simulator has not progressed for several months.

I am in the process of creating my fourth online course for Udemy. This course will cover the basics of control and operation of grid converters. It will start with basic grid topologies, computations performed on grids - peak voltage computation, RMS value computation, frequency estimation. A basic overview of the PWM strategy backed by Fourier series harmonic content of the inverter output voltages will be covered. In terms of control, there will be a coverage of the principle of designing a converter, frequency response plots and how to use these to design controllers. A basic current controller will be designed using this basic technique. The current controller will be a simple PI controller in synchronous reference frame.

My second book will soon be in print. This book is titled Digital Filter Design for Power Engineering Applications using Python: An open source guide. The book is being published by Springer International. Hopefully, the online version will be available in December.

There are a few changes long pending with the circuit simulator. These might be done sometime early in 2021.

Third Udemy course and the road ahead

It has been a while since I blogged. Quite a lot has happened in the meantime. I created three online courses that are available on the MOOC website Udemy. They are available in the following links:
Simulating Power Electronic Circuits using Python:
https://www.udemy.com/course/simulating-power-electronic-circuits-using-python/?couponCode=A8E5404EE6DD73D31530

Basics of Digital Signal Processing for Power Engineers:
https://www.udemy.com/course/basics-of-digital-signal-processing-for-power-engineers/?couponCode=0DEFA2CAD031AB8DEEEA

Simulation of Magnetics for Power Electronics using Python:
https://www.udemy.com/course/simulation-of-magnetics-for-power-electronics-using-python/?couponCode=F3803A3E81906B554A6A

I am currently writing a book based on the course material of my second course on digital signal processing. I hope the first draft of the book to be ready by the end of the month.

The next steps will be courses on different topics in power electronics. To begin with, there will be a course on control of single-phase grid connected converters. In this I will go into complete details of how to control the current injected by a single-phase converter that could potentially be used for interfacing a PV panel or a battery or for any other purpose.

I have several courses planned for the future. The eventual objective is to establish an online power electronics university with continuously expanding courses that tackle the latest breakthroughs in power electronics. The objective is to take complex research topics and break them up into courses with simulations for students to be able to learn on their own pace.

A few tentative topics will be as follows. Since, my Master's thesis was on active filters, one course could be on shunt and series active filters in distribution systems. The course will cover techniques of injection and also provide simulation cases with converter topologies and closed loop control strategies. Another potential course will be on UPS and the applications in distributed systems. Going further, I will examine parallel-connected UPS systems to form a microgrid.

Once, I tackle the topic of microgrids, I will then examine integration of renewable energy sources such as a solar PV and wind turbines. I will examine how co-ordinated control of renewable energy sources can result in a smart grid.

To get quick updates on this project, like or follow my Facebook page:
https://www.facebook.com/pythonpowerelectronics
Or follow me on Twitter:
https://twitter.com/pythonpoweretrx

Next course on simulation of magnetics

It has been a while since I blogged. Nowadays I am not doing much dev with respect to the circuit simulator. Most of my time is spent in writing my next book on using Python for digital filtering for power engineering and in creating my next course on simulation of transformers using Python Power Electronics. So this blog will be about my writing and my next video course.

The book on digital signal processing using Python is based on the Udemy course Basics of Digital Signal Processing for Power Engineers:
https://www.udemy.com/course/basics-of-digital-signal-processing-for-power-engineers/

While creating the course, I put together the course material (what I would be talking about) for every lecture as just a text file. The idea was to build a story for the course - lecture by lecture, section by section. At the end of the course, I found that the text file was close to 100 pages. This was without any diagrams, simulation results or additional. It seemed a waste to this let this course material be as is as the course itself has become a bestseller on Udemy. So, I started putting this text file together as a book and so far have written four chapters and 117 pages. Another two chapters are remaining after which I will write the introduction and conclusion. And then the hunt for a publisher.

My main objective behind publishing is to write books that are easy to read and break hard-core engineering down into simple understandable text. Another major feature of the book is that all references will be open links - primarily Wikipedia. I am a supported of Wikipedia and use Wikipedia links heavily in my online teaching as well.

The next online course is based on magnetics primarily simulating transformers. Though it is a well reported topic, my reason for digging into it was that in terms of implementation, transformers and magnetics in general are tackled fairly heuristically - mainly by just trial and error. The objective of the course is to bring this entire topic down to basic physics - Faraday's Law, Lenz's, Ampere's Law. In the future, my advanced courses will use magnetics particularly multi-winding transformers and therefore, this course is an introduction to that.

Gradually, the process of creating video courses is becoming far easier and quite enjoyable. The initial agony in filtering out noise from videos is now a thing I laugh about. A decent noise canceling mic is fairly important for recording. My very first course was recorded with a mobile phone headset. I had to wrap myself in a blanket to shield myself from noise as it picked up everything not just in the apartment but in the entire neighbourhood. I had Audacity setup for amplifying audio and removing whatever little noise that remains. I still end up with a minor background hum but I would not worry about that.

With that said, I would like to see how long it takes to record my next course. I have recorded and uploaded 3 hours and 48 minutes of video lectures already in 5 days. At this rate, if all goes well, I should be done by the end of May.

Philosophy behind the video lectures

It has been a while since I last blogged. Since I am way too busy right now with creating my next Udemy course, I haven't had time to blog about code. So I thought I would blog about the video series that I have been now consistently adding to on a weekly basis. In case you haven't seen any of them, this is the YouTube channel.
https://www.youtube.com/channel/UCxVbKNK18A_a9Ohd0Kb7kNA

The idea for this came from listening to a few podcasts. Initially, I thought of creating a podcast for power electronics, but then realized that I wouldn't be able to convey much just through audio as I usually like to code along. So, I decided with a weekly video on some topic in power electronics. Initially, I used to spend a lot of time preparing them, and for that reason, could only create a new video once in several weeks. But then, I watered it down a bit, and just recorded myself as I casually simulated and now am able to keep my weekly routine.

The main reason for these lectures was to provide a consistent flow of content for someone who wants to learn power electronics but is too busy. The target audience is someone who works as an electrical engineer, would like to level up with their skills to move on to being a design engineer but just can't find the time and due to financial and/or family constraints can't take a few years off to go back and do another degree.

For this reason, these lectures have a bit of theory but the main aspect is that someone who is interested can just code along. All software are free and open source and you could install every software in any operating system that you use. So, the idea is that a student could learn more than just equations from these video lectures.

A part of my philosophy behind creating these videos is similar to Richard Feynman's philosophy of teaching. Pick a topic. Teach it as if you were teaching it to a child who was hearing about it for the first time. Look for where your message didn't go through. Go back and simplify it further.

I have taken many courses on Udemy. One of the problems with adult education is that adults already have responsibilities. The traditional approach to giving homework and expecting them to work on it on their own time will not have much effect as time is what is lacking. So now that I have launched my own courses, I realize the need to make things explicit. And this goes back to Feynman's approach to teaching.

Making these videos has given me to opportunity to get better gradually. And I am enjoying creating videos as it also helps me to relearn what I might have conveniently forgotten.

Testing node and branch methods

I have tested the methods in determining nodes and branches. What is left is the main method that calls these methods. The code can be found on:
https://bitbucket.org/shivkiyer/ppe_simulator/src/testing/
https://sourceforge.net/p/pythonpowerelec/code/ci/testing/tree/

And if you would like a full length course on simulating power electronics using Python, check out:
https://www.udemy.com/course/simulating-power-electronic-circuits-using-python/

Here is the code for the tests in command line and web app:



The tests work ok except for a few discrepancies. There is a case in the branch_advance method where an invalid jump does not throw an exception or even return an indication of an error, but rather returns the correct element. For example, while advancing to the right on a branch, the method checks if there is an element to the right of the current element and if that element has already been added to the temporary branch. Legally the advance should occur if the element on the right exists. There could be no jump executed in which case it is simply an element that exists to the right of the last element in the temporary branch. If a jump is executed, the jump direction should be to the right.

However, if the jump executed has a direction of up, down or right, there will be no exception and the element to the right of the current element will be returned. The only time an exception will be generated will be when the jump direction executed will be to the left. The reason this did not cause a major issue was because the methods related to the jump are fairly ok and do not produce wrong jump directions. But in principle, the code is wrong.

The check should be, according to the jump direction, is there an element in that direction? If there is no jump, we could follow some sequence of directions while searching for the next element.

Now that the methods have been tested, the next issue is to test the outer method determine_nodes_branches which calls these methods. At this point, the only was to test this outer method is manually by inputting different circuits. And that doesn't seem like a good way to test as the testing is not at all automatic and how many circuits can I come up with?

This brings me to the next crossroad. I need an automatic circuit builder. This is because things will get more complicated at the next step where I will have to test loops and loop manipulations. I need to generate loops automatically while also introducing errors and bugs in the loops automatically. So, the circuit builder will need to generate circuit topologies only while testing at every stage. For example, I need to start with a simple two node, three branch, two loop circuit and verify that that is generated. Then continuously add nodes and branches automatically to form new loops. The structure should not matter as long as there are a few random jump labels inserted every now and then.

This circuit builder seems very interesting and challenging. The next few blog posts will be dedicated to this.

Testing jump labels

I finally wrote the first bunch of serious tests with respect to the way the simulator processes circuits. I grouped together a few tests that test the jump labels in a circuit under a class. The code can be found in my bitbucket and sourceforge repos:
https://bitbucket.org/shivkiyer/ppe_simulator/src/testing/
https://sourceforge.net/p/pythonpowerelec/code/ci/testing/tree/

Also, to get a full length course on power electronics simulations, check out my online course:
https://www.udemy.com/course/simulating-power-electronic-circuits-using-python/

The class in the command line interface is:



The functionalities that I am testing for specifically are:

1. Check whether an element is a component (could also be a wire), a jump label or no connection at all.
2. A jump must be the extreme element in a branch segment. If it has components on more than one side, that is a violation that throws an exception.
3. Two jump labels cannot be adjacent to each other - that is a violation that throws an exception.
4. A jump label cannot be next to a node - a violation that throws an exception.

Most of the tests could be accomplished by a simple assert statement. However, to check if a method throws an exception, I found this block to be most useful:

with pytest.raises(SystemExit):
    ...method...

So, the method or the block of code must throw an error or else the test fails. SystemExit could be replaced by other errors, but in this case, all I do is exit with an error code of 1 and so a SystemExit is all I can check for.

Conversely, if a method or block of code should not throw and error, the test can be written as:

try:
    ...method...
except:
    pytest.fail('This should haved worked')

The class for the web app is a bit different:



The primary reason is that the web application does not exit with an error code when a violation takes place but merely displays the errors to the user on the web browser. So the check is for the result of the methods to be a non null list that would be an error message.

One major advantage of testing was that I found myself investigating code and thinking of possible ways that it could fail. It is a completely different form of coding as opposed to regular development where you are just trying to get the code to work. In testing, you are thinking of ways to break the code. This is a totally different aspect to development that can be fun if you look at it the right way.