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.
I wouldn't use the term "kinda shitty". That sounds like they are low quality.
They are just a different part. Compared to ceramic caps, they have higher ESR, and a limited life that is aggravated by high temperature.
However, aluminum electrolytics offer capacitance * voltage that can't be matched by other caps. I guarantee, the power supply in your desktop has aluminum electrolytics for the output caps.
If you want to be a power supply guru, you don't just refuse to use electrolytic caps; you understand when they are necessary, and you learn how to use them properly.
This was very informative
This channel always has great brain scratching material.
Really good video. Couple of side notes: aluminum polymer capacitors have the benefits of traditional aluminum caps while having better esr/esl figures although being a bit more expensive. Decoupling caps are fine ceramic, but some supply chips are picky with output/input capacitance and esr, read the datasheet! Most ceramics lose effective capacitance with dc voltage applied; capacitor manufacturers provide a lookup for that info (with X7R/X5R it gets really nasty above 1uF @ 0603
Great video! Really makes clear why we use the capacitor model including series resistance and inductance. I’ll recommend this video to my EMC professor
I was worried it was going to be clickbait from the thumb but it was actually a great little video. Thank you
Gotta check your data sheets. Good video