Today I'm going to teach you about "buck
converters", and show you how to make a switch mode power supply that can work with input voltages
between 8 and 30V, and it steps the output down to 5 volts.
It can supply three amperes continuously and can handle peak currents of up to
5 amps for several minutes. Let's start with why we need switch mode
power supplies in the first place. In previous videos I talked about linear
voltage regulators like the LM7805 and LM317. These are really easy to use, but they're
very inefficient at high voltages. For example, if you try to power a linear
voltage regulator with 28 volts, and had by 5 volts and 3 amps on the output,
you'd end up with 69 watts of heat produced. And that might cause a few problems with
your circuit. For high powered projects you want to be using what is called a
switch mode power supply. There are many different types of switch
mode power supplies that can get you from one voltage to
another. But today we're going to talk about buck converters which is a type of supply that can
step higher voltages down to lower voltages. Let's start with an input voltage of 10V
and let's put a switch in series with it. It doesn't matter what the switch is... it
could be a bipolar transistor, a MOSFET, even a crazy person pushing a
mechanical switch. For efficiency reasons you should use a MOSFET but but let's just use a generic switch
symbol for now. Next, let's control the switch with a high
frequency pulse width modulated signal with a duty cycle of fifty percent. This
will give us a square wave that's 10V half the time and 0V half the time. Now let's add an LC low-pass filter. The inductor resists sudden changes in current,
and the capacitor resists sudden changes in voltage. The combined effect is that our LC low-pass
filter averages out the square wave and we get
5 volts of relatively steady DC on the output. Now unfortunately, if you build this in
real life, this will happen. But why? Well let's say the switch is
closed... and our power supply is delivering some
current. This means that current is flowing through this inductor. Now let's open the switch. Since current in an inductor cannot instantly change
this means that current is still flowing through the inductor. But this side of the inductor isn't connected
to anything. So you get this huge mass of electrons building up here, creating a massive negative voltage
spike. This voltage spike can reach hundreds or even thousands of volts, enough to blow up any switch you connect
here. If you want more detail about this phenomenon, check out my video on inductive spiking. In
that video, you will learn that the solution to the
problem is to add a diode. With the diode in place, now whenever
you open the switch, current can flow in a nice complete path
and the voltage after the switch barely goes below zero. This is the classic buck converter configuration. And you can use this basic circuit
to step high DC voltages down to lower DC voltages in a much more efficient way than linear
regulators. Now in school they might tell you that this
formula will give you the duty cycle you need to get the output voltage you want.
Unfortunately in the real world this is garbage. As soon as you start drawing a few amps
from your power supply, the various non-ideal parts of the
circuit will complicate things. You'll get power losses in your switch,
your diode, and your inductor, and even your wires. It's also highly unlikely that your
output current will stay exactly the same. The more current your load draws, the more the voltage will drop. So what
we need is a system that can continually monitor the output voltage and adjust the pulse width accordingly. If
you draw more current on the output, and the output voltage drops too low,
increase the pulse width. If the output voltage gets too high, decrease the pulse width. And we have to do
this within a fraction of a second otherwise we could fry the thing we're
trying to power! In other words we need a closed-loop control scheme with negative
feedback. We can accomplish this by adding a
feedback resistor network at the output, a ramp oscillator and an error amplifier.
We're also going to need a precision voltage reference and a suitable slope comp- (SHUT UP!) Okay, how about we make things easy? Texas
Instruments has a portfolio of products called "simple switchers". All the simple switcher products are
practically idiot proof and all you have to do is add a diode, an
inductor and some capacitors. They take care of the complicated
control electronics inside the chip. Let's use the adjustable version of the
LM2678. And here's the circuit diagram. On the
input we have a large electrolytic capacitor in parallel with a ceramic
capacitor and this is necessary to ensure that the LM2678 can easily switch current from the input
at very high speeds. If you don't have sufficient capacitance
on the input, the parasitic inductance of your input
power wires will limit the amount of current you can
switch in every switching cycle and the regulator just won't work. For
the diode, when you're designing switch mode power
supplies you almost always want to use schottky diodes. These have a lower forward voltage than
regular silicon diodes so they produce less heat, which is
something you always have to worry about when you're working with high currents. I'll put links to the components in the
video description section. This is a bootstrap capacitor. It is used to help drive the switching
transistor inside the LM2678. It can 10nF to 100nF and it
should be a ceramic capacitor with at least a 50V rating.
On the output we have a combination of capacitors that will smooth out the high frequency
content of the switching waveform, leaving you with relatively clean and
stable DC. These resistors configure the LM2678 to give you a 5 volt output. Try to
use 1% resistors if you want an accurate 5 volt output. Alright let's build this thing! You
don't want to use a breadboard for this. Use perfboard, or make your own PCB. Solder in
in the LM2678 first leaving a large amount of space around
it to fit the other components. Solder in the input electrolytic
capacitor within a centimeter or two of the controller. And use short, thick lines of solder to
make the connections. Do the same with the diode and the
output capacitor. Keep the component leads short as
possible. When you solder the feedback resistors,
try to keep the wire going back to the chip as short as possible. The layout on the underside of the board
is even more important than the topside! Notice how my ground is one continuous
straight line, and I arranged all the components on the
top side around this. I also soldered on a ground wire for
oscilloscope probing later. And look how I soldered the ceramic
input and boost capacitors directly across the pins of the LM2678. If these capacitors are even a few millimeters
away from the chip, everything will perform a lot worse. Okay,
now that everything soldered up, I'm going to power my buck converter with
10 volts, and I'm going to use my programmable electronic load to see how it performs delivering
different amounts of current. If you are doing this at home, you can just
use 5 ohm 10 watt power resistors as a dummy load. First, let's check the output voltage is what we want it to be. And it is! Perfect
5 volts DC! Excellent. Now let's take a look at this
node in the circuit which is called the switching node. This is before the LC low-pass filter.
You can see our familiar 0 to 10 volt square wave, and the switching
frequency is 250 kHz. But you can see the duty cycle is about
52 percent instead of the theoretical 50 percent.
This is with a 0.5 ampere load on the supply. If I increase the load to 3 amps, the duty cycle increases to 56%. And at five amps, the power losses are significant enough
that the controller had to change the duty cycle to 61% to maintain the regulated
5 volt output. Remember when I said we were getting a
perfect 5 volts? I lied! Let's change the coupling on
the oscilloscope to AC coupling and zoom in. You can see that there's a
small AC component on the output because our low pass filter is not
perfect. We call this the output ripple of the power supply because it looks like little wave
ripples. We have about 20 millivolts of ripple and noise with a 3 ampere load. If I increase the load to 5 amperes, things
get noisier. If I increase the input voltage to 28
volts, the ripple waveform gets bigger, and it changes shape. Ideally we want this ripple to be as
small as possible. For most applications, under 100
millivolts peak to peak will be fine. But in general you don't want to use
switch mode power supplies to power sensitive circuits like radio receivers. If you want to learn more about
measuring power supply ripple, enable video annotations and check out Dave
Jones's excellent video on the subject. Now let's
measure the efficiency of our supply and compare it to a linear voltage
regulator. From a 28 volt input my bench power supply is supplying 0.61 amperes
to the DC to DC converter. My multimeter says the output of the
converter is 5.019 volts. and I have the load set to exactly 3 amperes. If you're doing this at home with
resistors as a load, make sure you use a multimeter to accurately measure the
output current. Here's the equation for power supply
efficiency. Plugging in the values we measured earlier, we find that our power supply is around
88 percent efficient which is pretty good! This is why people
usually use switch mode power supplies for currents higher than 1 ampere. Alright, now you know how to make a high
current power supply, and knowing is half the battle. Thank you for watching, and if you liked this video please check out the video description section
to see how you can support me. Make sure you check out Patreon, which is
kind of like an ongoing Kickstarter campaign
to help fund the channel!
Cool! Does anyone know some good videos explaining the other types of switching-type powersupplies? (I'd really like to know how the boost type works)
Nice!
I like this. Very easily understandable and informative.-
damn, I just posted this http://www.reddit.com/r/electronics/comments/2dxj25/dcdc_switch_mode_power_supply_tutorial_very/4 hours before you did...