Hello, I am a newbie on this platform, but wish I could get some help! I am working on a project and I need to convert 24DC voltage in 12V. I am using a TPS62933DRLR (TPS6293x 3.8-V to 30-V, 2-A, 3-A Synchronous Buck Converters in a SOT583 Package) and this is the link for the datasheet : here)
I know I have to use capacitors to filter my signal but to be honest, I am facing few questions.
First, I have calculated the power of the circuit, effm = Poutmax/Pin, where Poutmax = VoutIoutmax = 123 = 36W. eff is around 0.9 so I_in = 36/(0.912) = 3.3A. Which means I lose 0.3A and 12V so Ploss = 120.3 = 3.6W.
1-How can I easely, on a SMD component, dissipate the Power loss while using SMD components?
2-If my calculation is correct, I have to use capacitors to filter the signal with at least 25VDC, but how about the ripple current limitation? I am using an inductance of 10µH for I_out = 3A (see formula in datasheet page 30), then Iripple_max = 1.2A. If I_out = 0.7A i.e, then, L = 42µH and Iripple = 0.3A. The question : Do I need capacitors to filtrate the signal with at least 1.2A for I_max? Can I use SMD capacitors? What should I use the most?
To be honest I only need around 1.5, maybe 2A but I want to design everything so my PCB can deal with other devices in the futur.
I don't really know if SMD capacitors can be an option for a converter circuit and if I may be limitated to dissipate the temperature.
I have to work with 15µF and 22µF (for other circuits) and I know it's expensive to buy these kind of components so I'd like not miss my shot.
Through proper heat sink design of the PCB, and possibly, additional heat sink components.
This is hardly a beginner project, especially since the market is saturated with high quality, very efficient DC-DC converters designed, tested and manufactured by professional engineers. I strongly recommend Pololu's offerings, as they actively support their products.
You can't
The best Rϴ from the IC to ambient through the PCB you will be able to achieve with convection cooling will be around 40°C/W
If we limit the junction temperature to 140° and assume a max ambient temperature of 27°,
then the max power dissipation will be around (140-27)/40 = 2.83W
So without a big heatsink and fans you won't be able the dissipate your 3.6W.
So this kind of IC isn't the best idea to convert 24V to 12V?
I didn't knew about Polulu products and that's clearly something that could solve my issue... However; I want to do it myself, learning the technical part and being the "mastermind" behind my PCB, and not just use what's already existing. Idk if there is something as we say in France but "Je ne vais pas réinventer la roue". I wil check the technical details of these products!
Thankfully, I am designing everything to be able to support 3A in the maximum. I want to powering an ESP8266 (around 500mA at the maximum of the capacity), a MCP41010 (a few µA) and 9 power LEDS (3 serial/parallel, which leads to 700mA. I will have 0.7+0.5 = 1.2A + few mA for all the converters IC but to achieve this kind of voltages and current, I have 3 blocs. One for 24 to 12V, another on 12 to 5 and LDO for 5 to 3.3V.
Knowing that, do you think I have better chances to dissipate the temperature by using trad components only for one circuit ( i.e the 5-3.3V LDO)? and letting the 15µF and 22µF capacitors being SMD components on my PCB?
What you are asking me to do would require me to do a design and a thermal analysis, it's a tall order so to speak.
My best advice at this point is to use compoments that have extra and/or large thermal pads and devices that use external MOSFETs for switching. That way the heat will be distributed among several components.
Many switching controllers have a big thermal pad underneath and MOSFETs in TO252, Dpak and D2Pak have a large pad for thermal dissapation
No I don't ask you to design anything. I am a beginner in PCB design and I don't have enough experience to judge that my work is 80% sure. I know some tricks to design PCB with these kind of IC but for the capacitors, is it better to use shielded SMD components or Trad ones?
Because I can't figure out if it can change something or not, except wasting space..
In essence you are. For me to verify that your designs are electrically and thermally correct, I would have to read the datasheets for all the parts you are using, do my own design and see if your design is different in any way.
but for the capacitors, is it better to use shielded SMD components or Trad ones?
What do you mean by shielded components?
What do you mean by Trad components?
SMD and THT(Through hole) can both be Trad.
I may have misspoken, sorry. I'm not asking you for anything more than advice, so I can find the answers on my own sorry.
I've shared my work with you so that you can understand the different components of the project. There are several values of filter capacitors. I can't predict the heating of my control systems, including the capacitors. That's why I'm wondering what type of capacitor I should use. For example, MLCC - SMD multilayer ceramic capacitors, I have no information on the minimum rejection current required to avoid malfunctioning (cf : that capacitor), and as I can't predict the overall temperature rise, I'm having trouble knowing what minimum junction temperature to choose. My capacitors will heat up, yes, but to how many °C? Is there a formula that can tell me without using the Ro? I'm wondering what type of capacitor to choose...
For aluminum electrolytic capacitors, there is information on the rejection current, for example on this datasheet : here
However, I don't know if this might be the best option...
How do I choose a type of filter capacitor? What are the criteria in a voltage regulation system?
Paul_KD7HB shared a tip I didn't know about, that of looking at the capacitor's internal parasitic resistance, but I don't think that's the only criterion.
Is there any other parameter to check before chosing one capacitor (f.e ripple current limite?, max VDC voltage at least + 10%/20% of the maximum voltage in the circuit?)
For the ESR, sometimes it is just mentionned "very low ESR"... not very precise. Should I be less fussy ?
If I am working with high value of ESR, how can I avoiding failure on my circuit? Parallel resistor?
Again, you can't make generalizations about types of capacitors to use. For example, when designing a circuit with an an LDO you need to read the datasheet and follow the manufacturer's recommendations. Some LDOs will not work with ceramic, some need capacitors with ESRs in a specific range, some don't need capacitors at all, some only on the output.
Then again the type you use will also depend on what is connected to the input and output. You also need to consider tolerance and temperature stability. Will a 10uF 20% work or will I need a 10%. A Z5U ceramic can have a capacitance variation of +22/-56% over a temperature range of 10 to 85 degrees C, is that good enough? Ripple current ratings usually are not a concern with LDOs
The same basically applies to switch mode designs, follow the manufacturers recommendations for the type, quantity, and ratings to use. If this information is not provided then I would look for another IC. If a reference design is provided, copy the design, look at the datasheets for the capacitors used.
So I don't have to choose my capacitors according to their maximum ripple current value? When can this be a constraint?
I've seen details on the datasheet about the type of capacitor to use, for example for the TPS56339 cf : datasheet page 17, under the tab.
The COUT is the sum of nominal output capacitance. Two 22-uF, 0805, 16V capacitors are recommended for VOUT ≤ 5V, three 22-uF, 0805, 25VDC capacitors are recommended for VOUT > 5V.
But here again, no precise details on the maximum temperature, on the type (ceramic, aluminum electrolytic capacitors...). I was actually pretty reassured, but when I got to the suppliers' websites, I wasn't sure where to look and what to choose.
I'll try to reread the datasheets and find the best choice for my project.
If you read ALL of page 17, everything would become clear. They even give a recommended part number.
Download the TPS56339EVM evaluation module schematic and study it
I might spend several days even a week reading the datasheets, appnotes and reference designs before I do a design. TI also has the webbench simulator that I have also used in switch mode designs.
I used Webbench to find the best IC for my application, and also compared several solutions. I'm somehow losing myself in the datasheet, crossing information and forgetting the most important...
Sorry for that but still thank for you help, I think this subject can be closed.
It's not a trival task, especially for a beginner. At least you are asking the right questions and most beginners don't even consider the thermal aspects.
Good luck with the design.
Have a nice day!
What test equipment do you have to evaluate your finished design to make sure it is stable under all values of current draw.
This is not a beginners project.
I used to have specialist experience professional engineers in my teams. None of them could develop a satisfactory circuit in less than three PCB iterations.
It is not something that you can just learn from data sheets.
When I am saying beginner it is for the PCB design and schematics according respect technical rules and not something for a "personnal project".
I am using Altium since 3 years but I want to challenge me doing new things, trying my best to have to most accurate schematic, then PCB desgin to reduce at the maximum the potential errors.
I don't think this is something I want to achieve in 1 PCB, I don't even have the material to test all the current values BUT I can still do it by my own
