I’d like to know: why in many PCB designs, there are several small capacitors placed in parallel instead of using one bigger capacitor ?
For instance, this “SparkFun FTDI Basic Breakout”, can’t you replace C2 and C5 with one 10.1uF capacitor ?
http://i.imgur.com/lUAaiMv.pngSmaller capacitors respond to higher frequencies better. Larger capacitors respond to lower frequencies better.** To get broader coverage sometimes you combine a smaller capacitor and a larger capacitor.
** Smaller or larger isn’t just the capacitance value or the physical size. It is also the construction materials of the capacitor. However the best construction materials for high frequency response generally don’t make the larger capacitance values so the correlation above serves. A tantalum capacitor is an example of good frequency response materials that also yield larger capacitance values, but they are also much more expensive. So again, using two cheap capacitors can be a better (for some definition of better) solution.
You will also see many small “bypass” capacitors scattered around the PCB. In this case it is not the aggregate capacitance that is important. Instead what is important is that the capacitance is really close to the portion of the circuit (typically an IC) that the capacitor is serving. Again, this is about frequency response, but here the critical factor is the length of the conductor between the capacitor and the IC. Keeping that conductor length short allows the capacitor to do its job supplying instantaneous current where it is needed and suppressing noise.
Yep, and you can also Google, “decoupling capacitors.”
And where would you get a 10.1uf capacitor? lol
Thanks for reply.
I have googled “ESI” “ESR”, learned some interesting characteristics capacitor exhibits at various frequency.
In PCB designs, using several small capacitors in parallel instead of one larger capacitor offers several advantages:
Decoupling Capacitance: Different values of capacitors cover a wider range of frequencies, providing effective decoupling across a broader spectrum. This is crucial for reducing noise and stabilizing voltage levels in various parts of the circuit.
Impedance Matching: Multiple capacitors with different values can better match the impedance of the power supply and the load across different frequencies, ensuring more stable operation.
Space and Layout Constraints: Sometimes, using several smaller capacitors allows for better placement on the PCB, reducing trace lengths and improving signal integrity.
Improved Filtering: Different capacitor values can work together to filter out noise and provide smoother power delivery, especially in circuits sensitive to voltage fluctuations.
Regarding the specific example of the “SparkFun FTDI Basic Breakout” and C2/C5, replacing them with one larger capacitor (like a single 10.1uF capacitor) might not provide the same level of decoupling across frequencies as the combination of C2 (0.1uF) and C5 (1uF) does. These two capacitors likely serve different frequency ranges, with C2 handling higher frequencies and C5 providing stability across a broader range.
Therefore, the choice to use multiple capacitors is often a deliberate design decision to optimize performance across different frequencies and to manage noise and stability effectively in electronic circuits.
It is also important to choose the right supplier. I am currently studying in Shanghai, China. Last week I went to the Munich Electronics Show in Shanghai. I usually buy samples from these companies to complete my PCB design project. I have collected the top ten famous capacitor brands in China.
