Is it possible to write enough equations to correctly solve for everything simultaneously or do I need to select a particular parameter as a starting point are work outwards from there?
I need to preform a worst case design so it must meet the design criteria in the worst case scenarios. How can I identify what these worst case scenarios are?
I had to spend hours playing with an animated simulator (Falstad's), before I got a notion what is happening in these switched coil converters.
There are so many factors at work that it is hard to solve them all simultaneously.
These factors include:
* Coil henry value
* Coil's maximum current for inductive saturation, coil's maximum current for safe temperature
* operating frequency
* load resistance
* capacitor value
* supply voltage
etc.
Therefore when starting a design, you try one parameter and see what develops based on that. If you encounter something that won't work, then you may need to go back to the first parameter and change it.
Example of things that might not work: coil's henry value is too high or too low, coil is too expensive, coil is too bulky, etc.
The coil is the center of action. Looking at your equations, your next step was going to be to find the coil's henry value. This tends to be defined by the given operating frequency.
I made a user-interactive simulation of a buck converter which is oversimplified but should aid understanding.
It will demonstrate coil action as you press and release the On switch.
Click the link below. It will open falstad.com website, load my schematic, and run it on your computer. (Click Allow to load the Java applet.)
https://tinyurl.com/8zk6pjd
You will need to change some values. Right-click on a component to bring up an edit window.
This simulator won't do everything you need, but it will get you part of the way there.
Worst case scenarios include:
Will my design start up reliably, or does it need prodding?
What if I short circuit the output wires? Will components be destroyed?
What if I do not attach a load? Will anything bad happen?
Could my design ever expose the load to overvoltage? What about at power-up?
What if the load is momentarily disconnected? Will my design continue to operate?
What if the load resistance is suddenly changed?
What if the supply voltage drops? Will anything bad happen?
What if oscillations stall during switch-On cycle? Will anything bad happen?
If they stall during switch-Off cycle?
Etc.
There are IC's made to drive this type of supply. They may have a few safeguards built in, to prevent disasters.
After goofing around some more with BradtheRad's Falstad circuit, I tried simulating one of the rows of my tables in the previous post.
For, f = 20kHz, Vin = 100V, Vout = 25V I obtained a L = 118uH, C=200uF and R = 6.25Ω.
Throwing these into the falstad simulation with a duty cycle of 0.25 and a 20kHz switching frequency I obtained these results. (See figure attached)
As we can see I am obtaining ~19.7V at the output, which is close to the desired 20V.
We are also observing a current of 3.1A through the load resistor, giving an output power of 61.07W which is about a third less than our desired 100W.
How can I improve on my design?
Cheers
I see you put up your post while I was composing mine.
Good work with the simulator. (It appears Falstad's will not accept clock frequencies greater than 25 kHz. I suppose this is why you found it necessary to raise the coil value from your previous 95 uH figure, so the simulation could achieve your desired performance specs?)
Since you want greater output voltage, all you need to do is give proportionally more time for coil current to build. This can be done by increasing duty cycle, and/or changing operating frequency.
Hi Brad,
If you read and view the attachement in my previous post, (post #5) in this thread you will see how I chose the values that I did.
As mentioned previously, I made a table of values for L, C and R given a Vin, Vout, and f. (Contingent on a simple set of assumptions of course)
For my figure within the Falstad Simulator I injected a clock frequency of 20kHz with a duty cycle of 0.25.
How do I figure out my source ripple? Does it relate to the ripple in the inductor current?
As you can see from your graphs, the source current spikes from 0 to the minimum inductor current, and then climbs to peak current in the inductor and then falls back to 0. (It looks like a square with a triangle sitting on it)
So is the peak-peak ripple simply Imax then?
I'm sure I could throw a large cap after the source and before the switch to not allow the source to drop its current back to 0 every DT.
Do you know how to obtain the equation for source ripple if I had added a cap right after it?
As you could see, by starting with a few specs, the rest derives from that.
Your results were pretty consistent for coil and capacitor values which are key components. It shows your approach is sound.
I noticed one figure you had for 'average source current' of 1.11 A, which turned out to be questionable.
In real life you will usually obtain (or construct) a coil which is close to what you're looking for.
After you see how things are running, you will adjust frequency and/or duty cycle to get the desired output level. Or your control module will adjust it.
All I can picture is that source ripple refers to supply ripple. Because incoming supply current is either on or off with a single converter.
You can interleave two or more converters, which has the supply providing some current at all times.
That's right, as per coil current, when talking about non-CCM.
I'm not sure this would gain you anything, although it might stabilize supply voltage somewhat.
Hello again BradtheRad,
Thank you again for another informative reply. How would you recommend one suppress the source ripple then? Would a second inductor in series with the source resistance do the trick?
Apparently the strategy for this design is to identify the design requirements in terms of the duty cycle parameter, D. Once this has been achieved, one is to obtain a worst case D. We have also been told that as the designers we are allowed to simply select a frequency within our range, but not an input voltage. The input voltage will lie in the specified range, but it is not to be chosen by the designer.
Nonetheless, from the given range of our input/output voltages, we can easily determine a lower and upper bound on the range for D. That being said, the worst case D will either fall somewhere in this range, or at one of the bounds. (the lower bound if the worst case D is lower than our lower bound, or at the upper bound if the worst case D is larger than the upper bound)
We don't need to actually implement the control system that would be required to obtain a constant 100W of power over the range of input/output voltages, but we must show that we are indeed designing for the worst case D.
Later this weekend I am going to attempt to describe the constraints of the design in terms of D and attempt to identify the worst case D. Of course, I will post my findings or problems as they arise.
It seems as though this is a minimization problem.
If you have any tips/suggestions to further simplify my attempt at identifying and obtaining the worst case D, or perhaps another strategy to identify the worst case design, I would love to hear about it.
I'm interested to hear what your thoughts are.
Cheers!
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