In previous videos I've shown schematics with two capacitors in parallel.
It's always a bigger electrolytic capacitor in parallel with a relatively tiny
ceramic capacitor.
One of the most common questions I get is why do you do this?
It's such an insignificant amount of extra capacitance.
Why not just use a single larger capacitor?
This video is going to show you in detail the effects that choosing the wrong type of
capacitor can have.
The short answer is that different types of capacitors behave completely differently at
different frequencies.
This is not something they teach you in school and if you don't know it you're going to get
nowhere with electronics.
When you first hear about capacitors you learn that they allow alternating current to pass
through them.
The problem is this ideal capacitor doesn't actually exist in real life.
Actual capacitors will have unwanted resistance and inductance that comes from
all the different materials that are used to put them together.
Even the leads going to the capacitor itself have a non trivial amount of resistance and
inductance.
So because of how capacitors are made, a real world capacitor behaves a lot more like this.
There are other capacitor models but this is the one
most relevant to this video.
In the middle we have the actual capacitance.
In series with it we have two non ideal parameters called
ESL and ESR.
This is the equivalent series inductance and equivalent series resistance created by all
the different physical materials used to make
a capacitor.
As you already know, resistors resist current at all frequencies, and inductors will impede
alternating current.
The higher the frequency, the higher the impedance.
So the ESR and ESL limit the capacitor's ability to act as a high pass filter and pass
high frequency current.
Both these capacitors are 10uF.
Intuitively, which capacitor do you think has the higher ESR and ESL?
The 10uF electrolytic capacitor with the 1 inch
wire leads, or the tiny 10uF ceramic capacitor?
The tiny ceramic will have lower ESR and ESL and that is why
even though both these capacitors are rated at 10uF the
ceramic one will perform much better at high frequencies in real world applications.
We can quantify this difference with a graph called an impedance curve.
You can see that at low frequencies, under 1kHz, all capacitors perform
pretty much the same.
As the frequency of the alternating current increases, the impedance decreases
linearly as you'd expect.
But above a few kHz different capacitors start behaving really differently.
You can see that this electrolytic capacitor is basically useless
above 3kHz.
But the ceramic capacitor has a linearly decreasing impedance all the way into the MHz range.
Now this is interesting... because the ceramic capacitor has a certain ESL, once you get
into really high frequencies, the inductance really starts
affecting things, and impedance actually goes up!
So after a certain point you start picking out capacitors
not for their capacitance value, but based on their equivalent
series inductance and equivalent series resistance!
Okay, enough theory.
I want to show you a practical example.
Here I have a simple buck converter evaluation board.
It's designed by Texas Instruments themselves so it's reasonable to assume that the
component choices and PCB layout is going to be good.
I'm giving it 10 volts on the input and it's spitting
out 3.3 volts on the output, and there's a light 650mA load on it.
I'm going to probe the output voltage with my oscilloscope and see how clean the
output of this supply is.
You can see that we've got about 40mVp-p of AC ripple and noise on top of the
3.3 volt DC output.
Not bad - that's about 1.2% of the output voltage.
If we take a look at the schematic, we can see that they used two 47uF ceramic capacitors
in parallel on the output to achieve this.
A total of 94uF.
So here's an idea... let's increase the capacitance on the output,
and that should provide more filtering, and the circuit will perform even better!
Let's get rid of those ceramic capacitors and replace them with a
single 220uF electrolytic capacitor.
That's more than double the capacitance, so we should get less than
half the noise!
Riiight?
Wow... so yeah that's definitely more than 40mV...
The output has 330mVp-p of ripple, which is abysmal.
That's 10% of the DC output of 3.3V and nothing that
runs off of 3.3 volts would be happy with that.
So you get the idea.
Electrolytic capacitors are basically useless at filtering out high frequencies,
and that's why in switch mode power supplies they either use
ceramic or tantalum capacitors, or sometimes designers put electrolytic and ceramic in
parallel to cheaply get a combination of high capacitance and
low ESR and low ESL.
I chose to do an example with a power supply because the effects are really obvious.
But you can get problems in other circuits too.
Sometimes amplifiers can become unstable and start randomly oscillating if
you don't add a ceramic bypass capacitor.
And digital designs need ceramic caps too.
If a processor has a 32MHz clock frequency, you've got millions
of little transistors in there being switched at 32MHz, and a big aluminum electrolytic
capacitor with a ton of series inductance is going to be completely
useless at keeping the voltage stable.
And if the voltage going to the microcontroller is unstable,
things can get glitchy, your analog measurements can be wrong,
it just creates a ton of problems.
Here are some more examples from the official Raspberry Pi schematic.
Notice how it always seems to be this very particular value... 100nF.
Why 0.1uF?
What is it about 100nF that is so damn special?
Shouldn't you be choosing caps according to the frequency of what you are trying to filter?
Well, yes, in theory you should.
And for some applications you have to.
If you are designing a 5GHz wifi system you might find that the right capacitor
to use is 100 picofarads, because even a 100nF ceramic
capacitor will have too much inductance!
But let's assume you aren't designing RF circuitry for a second.
For low powered things like op amps, microcontrollers, and digital logic chips, 0.1uF is going to
work just fine 99.9% of the time.
It'll filter out the high frequency garbage, and it might end up being overkill, but
given that these things literally cost a penny or two, it's not worth spending even 5 minutes
trying to figure out whether 33nF or 47nF is the theoretically
optimal value.
So every designer just ends up throwing 100nF onto the low powered chips in their design.
Thank you for watching.
Now you should have a better idea of what type of capacitors to use in different
situations.