I am primarily a digital engineer, but have some experience with the analog world. Heck, I started with electronics when digital was done with transistors, not on a chip, cans on a board. But I have not done much with switching power supplies. I am looking at using a TI Swift part for a very, very small footprint power supply module.
TI has some very nice software to support these designs even if it has a few rough edges. But one problem I am having is in the parts list. TI makes available a list of many caps and inductors with all the required information. However Murata makes some ceramic caps that are smaller for the values than anything in the TI parts list. I can add them to the list, but I am not clear on where to find the data.
The Swift designer parts list needs MinESR, MaxESR, ESL and Iripple. Murata provides a graph of temperature rise vs. Iripple, so that is covered. They also provide graphs of ESR and Impedance vs. frequency. This I am not clear on how to interpret.
First, I don't think that ESL is the same as Impedance. I expect that ESL is just Inductance. So I guess I need to analyze the graph to get the ESL value from the graph for impedance. I seem to remember the formulae Il = (2pi f L) and Ic = 1/(2pi f C)
So at the resonance point, where Ic and Il are equal,
2pi f L = 1/(2pi f C)
L = 1/(4(pi)^^2 f^^2 C)
Is this what is being requested in the TI Swift software?
The min and max ESR have me a bit stumped. I don't know if they want the ESR at the capacitor resonance point, the frequency of the switcher or some other point that corresponds to the loop analysis for the filter. I never got the classes for control theory. So I guess I need to sit down with a book or two someday to understand the whys and wherefores of this sort of control loop.
Anyone worked with this software before and know how to add new capacitors to the parts list?
--
Rick "rickman" Collins
rick.collins@XYarius.com
Ignore the reply address. To email me use the above address with the XY
removed.
Arius - A Signal Processing Solutions Company
Specializing in DSP and FPGA design URL http://www.arius.com
4 King Ave 301-682-7772 Voice
Frederick, MD 21701-3110 301-682-7666 FAX
I don't see a response after several days. You can probably get good responses if you ask this on sci.electronics.design, but I'll put in my two cents here.
I know other companies that make switchers have such software. Look at national.com - they've got an online simulator that runs spice - I've never used it, but I think they would give a similar parts list that might help you. Linear Technology has "Switcher CAD" for free, it's supposed to be a complete spice package. But I don't imagine either of these would help with TI switchers, other than as giving more clues about passive parts.
Let me check that from another direction: The formula for resonant frequency is: F = 1 / (2 pi sqrt (LC)) and solving for L gives what you got.
Yes, that sounds right to me.
Aww, just read Bob Pease's column titled "What's all this PID stuff anyhow" and that's all you need to know. :) But of course the idea of the design software for these chips is that you shouldn't have to know control theory, they supposedly have it all worked out, and if you use parts within the specs they give, it "should" work right off the bat. On the Murata parts, does the ESR really vary that much with frequency? It seems to me you should use the value at the switching frequency, as that's generally where the high currents happen that would be most affected by the voltage drop across the ESR. Looking at some typical Murata ceramic caps page, it looks like ESR DOES vary a lot with frequency:
formatting link
You say it wants min and max ESR - I'd use the switching frequency ESR for max, and ESR at self-resonance for min. Since the switching waveform is far from a sine wave, it will see all the ESR values in that range.
Have you asked someone at TI? They have an incentive to help designers with this - lots of others make switching regulator chips (National, Linear Technology, surely several others) and if TI wants your business they should help.
Thanks for the advice. That's a good idea. I'll look them up.
It has been awhile since I have used much from my AC circuits days. Although I did not take this in college, I did have a little bit in high school electronics.
I exchanged an email with TI support and they did not really answer the question. The one guy that always seems to answer my hard core switcher design questions is Ed Walker. He seems like a real good guy, but often he does not quite understand what a newbie like me is asking. Instead he espoused on how capacitor makers don't spec their parts very well.
You would think that, but there is no clear indication (that I can see) that a given design will work well as I vary the resistor that controls the output voltage. I believe that this resistor (or the output voltage itself) has very little impact on the control loop, but it is not zero impact. The tool provides the gain and phase curves vs. freq for the circuit, but then I am not familiar with how to analyze them other than the zero intercept for one gives you the analysis value for the other. Consider the gain when the phase is at zero and the phase when the gain is at 0. I can see the default values they chose for a limit so I guess I will be ok as long as I stat with those. But under some conditions I see the gain/phase curves get rather curvy and I am concerned that this might be a problem even if it does not go to zero.
Yes, the ESR in the graph is a "V" shape with capacitance dominating below SRF and inductance dominating above. The impedance curve has a sharp "V" at SFR in addition to this overall V shape while the ESR does not. They seem to reach the same value at SRF with the inductance about
10x ESR at other freqs. I don't have the URL for this data sheet and it is a bit large to attach in a non-binary group. But this seems to be a typical capacitor curve to me.
Yes, that is the link. cap04e is for the 22 uF in the 0805 package. I am actually finding some anomolous info in the TI library. All the work I have done with power decoupling showed me that smaller packages typically have lower min ESR/impedance with little variation with capacitance. But the values used in the Swift tool show higher ESR for the same value capacitance in smaller packages. I don't know if this is just a factor with these new, high value, very dense parts or if there is something wrong with the data.
Yes, like I said I have spoken or emailed with Ed Walker several times and he seems well intentioned, but it can be hard to get the answer to the question you are asking. My first try on this one did not work.
But while I was waiting for a reply from him I decided that I could try margin testing. Turns out you can set all your compensation component values and draw new curves with the tool. So I let the tool design the circuit using a set of caps that have a higher ESR. Then I lowered the ESR value in the curve drawing tool. That seemed to give workable results, but when I redesigned the circuit with caps that match the lower value, I get very different compensation components. So I repeated the test in the other direction. If you adjust the output cap ESR in this tool, it will not save the changes. But if you change the compensation components, you can save the changes. However, when I do that, the curves change slightly from what they were in the value editing tool (which lets you see the curves at any time).
So I think I have a solution although I don't have high confidence in the tool. It also has some problems with printing and saving results. I think TI needs a better windows programmer or two. :)
If anyone is more familiar with the tool and can talk switcher design to a novice, I would appriciate the help, or even just a sanity check.
--
Rick "rickman" Collins
rick.collins@XYarius.com
Ignore the reply address. To email me use the above address with the XY
removed.
Arius - A Signal Processing Solutions Company
Specializing in DSP and FPGA design URL http://www.arius.com
4 King Ave 301-682-7772 Voice
Frederick, MD 21701-3110 301-682-7666 FAX
In this datasheet, ESR declines monotonically. There is no 'V' to it. Impedance is a V, which is typical.
By the way, I'm a little suprised at the ESR curve. This seems like a highly non-ideal and non-linear capacitor. Also, if you look at graph 6, you will see that the capacitance falls off pretty dramatically with applied DC voltage, too. I don't know if this sheet is for the capacitors that the OP is planning to use. But if it is, he should make sure that there will be enough capacitance at the applied DC Voltage.
The switching controllers I've seen have all suggested tantalum or electrolytic filter cap's, FWIW.
[snip]
This sounds somewhat plausible to me, too, except I might go below switching frequency for the max, since we don't know what the manufacturing spread is like. The graph is presumably for a typical capacitor.
I read in sci.electronics.design that rickman wrote (in ) about 'TI Swift DCDC power supply design', on Sun, 14 Dec 2003:
That's not an ESR curve but an IMPEDANCE curve. the Equivalent Series Resistance can't be capacitive or inductive.
Hardly surprising, because the residual impedance at the Series Resonant Frequency IS the ESR.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
http://www.isce.org.uk
PLEASE do NOT copy news posts to me by E-MAIL!
The data sheet simply does not show enough to include the rising ESR above the SRF. If you look closely you will see that it does start to turn up above 600 MHz or so.
Hey, nothing in nature is perfect. This not one of the caps being considered. Seems they all have roughly the same curve so when used at
3 to 5 volts, there is not much loss of capacitance.
Yes, but they have a high ESR. Turns out that for the ripple spec the capacitance is not nearly as important as the ESR at the higher frequencies that are used in current switchers, ~1 MHz. The capacitance required is a function of the switching frequency and the current. The ESR typically limits the ripple. With ceramic caps this will be very low and for a 3 Amp supply with 700 kHz switching rate 4 x 22 uF (0805) works very well, at least on paper.
--
Rick "rickman" Collins
rick.collins@XYarius.com
Ignore the reply address. To email me use the above address with the XY
removed.
Arius - A Signal Processing Solutions Company
Specializing in DSP and FPGA design URL http://www.arius.com
4 King Ave 301-682-7772 Voice
Frederick, MD 21701-3110 301-682-7666 FAX
Did you look at the graph? They show an impedance curve as well as an ESR curve. Both have the V pattern with a sharper V at SRF for the impedance.
--
Rick "rickman" Collins
rick.collins@XYarius.com
Ignore the reply address. To email me use the above address with the XY
removed.
Arius - A Signal Processing Solutions Company
Specializing in DSP and FPGA design URL http://www.arius.com
4 King Ave 301-682-7772 Voice
Frederick, MD 21701-3110 301-682-7666 FAX
It wouldn't download when I looked before, but it does now. What I say is incorrect is this:
The ESR **cannot** be capacitive or inductive; it is by definition a pure resistance (but not necessarily independent of frequency, voltage, temperature, state of the tide etc.).
But the ESR curve is definitely weird. I guess that there is a lot of dielectric loss, which in a more realistic equivalent circuit appears as a resistance *in parallel* with the capacitor. Reducing the components to just a series R and C does produce behaviour like that.
If I had time, I'd Spice the impedance of a 1 uF with 1 ohm (? I don't remember the precise ESR values at the SRF) in series and 15 kohms in parallel. I think it might look a bit similar.
--
Regards, John Woodgate, OOO - Own Opinions Only. http://www.jmwa.demon.co.uk
Interested in professional sound reinforcement and distribution? Then go to
http://www.isce.org.uk
PLEASE do NOT copy news posts to me by E-MAIL!
And nothing made by man is perfect, either. Don't forget to check out the temperature coefficient.
The other thing that bothers me is the high ESR at DC. In between transitions, the capacitor really is at DC, in a sense. I think there may be a good reason that most switchers are designed with tantalum. Kemet has some very low ESR tantatlum caps (as low as 30 mOhm).
On the other hand, I went through the math with an ideal 88uF cap, and assuming you were stepping down from 5V to 3.3V, you get only 16mV of ripple. This is purely from the dV = Idt/C.
Then, with 3A current draining through a 0.03 Ohm tantalum cap, you get
90mV of droop. During the charge cycle, there will be even more current flowing, so you will probably have over 200 mV of ripple with 30 mOhms. In other words, I get what you are saying about most of the ripple being due to ESR, not capacitance. ;-)
If you are bound and determined to go ceramic, I would urge you to consider sticking with NPO/COG, or X7R dielectrics.
Thanks to all for the advice on the tantalum caps. But this module is very small and does not have the space for a D case tantalum. I have crammed two 0805 ceramics in that corner of the module and will live with the results for this design. I belive this will be a workable solution giving about 100 mV of I/Cdt ripple and somewhere around 10 mV of I*ESR ripple.
I am using a 10uF, 10V cap because I was taught to derate the caps for any typical design. I have also read about various modes of failure in SMPS due to underspecing the voltage rating, although this is normally a caution about tantalum caps. I could double the capacitance by using a
22uF, 6.3V cap, but for now I can live with the higher ripple since no logic will be running off the 5 volt rail directly. Everything lives at lower voltages with either a SMPS or an LDO providing conversion.
--
Rick "rickman" Collins
rick.collins@XYarius.com
Ignore the reply address. To email me use the above address with the XY
removed.
Arius - A Signal Processing Solutions Company
Specializing in DSP and FPGA design URL http://www.arius.com
4 King Ave 301-682-7772 Voice
Frederick, MD 21701-3110 301-682-7666 FAX
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.