In a previous video I showed you how to
make a dc-dc converter that can step down voltage. Today I'm going to show you a new circuit
that can step up voltage. It's kind of like a joule thief except a
million times better! It's called a boost converter and this
one can take input voltages as low as 4 volts and step it up to 30 volts. And you can adjust the output to
whatever you want. This video builds upon material that I covered in my buck
converter and inductor tutorials so make sure you watch those first if
you have any questions. Let's begin with the classic boost converter schematic. Normally you'd use a transistor for the
switch here but let's keep it simple with a switch symbol. If we start with the switch open, current
from our 10 volt source will flow through the inductor, through the diode, and charge up the
output capacitor. There'll be a small voltage drop across the diode (let's say half a volt) so the output
capacitor gets charged to 9.5 volts. So far we are dropping voltage but that's
going to change soon. Next let's close the switch. The diode
prevents the capacitor from discharging so the output stays at 9.5 volts. But now
we have a current path from the input through the inductor straight to ground.
If we keep the switch closed like this we'd basically be short-circuiting the
input through the inductor. Inductors don't like it when you do
that. So we're not going to do that! We're only going to close the switch for a
fraction of a second. We're going to close the switch just long
enough for some current to start flowing through the inductor. When we do this we're storing energy in
the inductor in the form of a magnetic field. Now that we have some current flowing
through the inductor, and therefore we have some energy stored
in the inductor, we're going to quickly open the switch. Since current in inductors can't instantly
change current has to flow through the inductor,
through the diode, and into the output capacitor. So energy gets transferred from the
inductor to the output capacitor and the voltage increases! Our boost
converter has now boosted the voltage. Now as I said earlier we don't want to
be closing the switch for very long. we want to be controlling the switch
with a high frequency pulse width modulated square wave. And from watching previous videos you
can guess that by increasing the pulse width we can increase the output voltage. But
how do you predict what the output voltage will be? Well in textbooks you will often see
this equation. But this equation will only work for
very low currents. In the real world the output voltage you
get will be a function of your duty cycle,
your inductance value, your switching frequency, your output current and many
more things. So just like with our buck converter, if
we want to build a real-life boost converter we need a controller
chip that will adjust the pulse width to make sure we have a constantly
regulated output voltage. Let's use the LT1370HV from Linear
Technology. And as usual I'll put links in the video
description section. The input and output of the supply have
a combination of electrolytic and ceramic capacitance to help deal with
the high-frequency currents. You're going to see this in nearly every
switch mode power supply. We're going to use a schottky diode with
as low forward voltage as possible to keep things efficient. And here we've
got our feedback resistor network that I calculated to give you guys a
maximum output of around 30 volts. Depending on the tolerances of your
resistors and potentiometer you may get a slightly different maximum.
Now here's something new... these components are used for control
loop frequency compensation. Basically they help the controller
adapt to small changes in the output voltage, giving you a cleaner DC output. Learning
how to design compensation circuits requires a lot of complicated math and knowledge of control theory which
is beyond the scope of this tutorial. But here are a few links to some
information. The good news is that most of the time you can just use the
recommended values in your controller's datasheet and your circuit will work. Alright
let's build this thing! Start by soldering the heatsinked
controller chip onto some perfboard or make your own PCB. Leave plenty of
space for the other components. Keep component leads short and your solder
traces thick. Add the inductor, the diode, and the
capacitors. Next add the feedback resistors keeping
things as short as possible. Finally solder the frequency
compensation components near the input and leave pins 3 and 6 unconnected. Let's look at the bottom side of the
board. Notice how I arranged the components so that I could have a simple
continuous ground from the input to the output. And I have
soldered the ceramic input capacitor directly across the controller's input
pins. And the output ceramic is in parallel with the output electrolytic. Now if we power it up we can take our
input voltage and step it up to 30 volts! Now let's talk about the limitations of
this design. It can only deliver about 50 watts
before things get uncomfortably hot. You should also be
aware that it has an input current limit of 5 amps.
Notice how I said input current limit, not output current
limit. Check this out. Let's say our converter has an output
voltage of 12 volts and output current of 1 ampere. So we're
delivering 12 watts of power. When I power it from a 5 volt source
it's drawing 2.78 amperes. So the input current is higher than the
output current and this is because we can't just create power out of nowhere. In order to get the extra voltage on the
output we're drawing more current on the input. Overall our boost converter is roughly
86% efficient. Finally, BE CAREFUL! For safety reasons
I've limited the design of the circuit to a 30 volt output and even just 30 volts
from a large capacitor can do some damage. If you build a boost converter
incorrectly, or you handle it carelessly, you could electrocute yourself! Don't do
that! With great power electronics comes great
responsibility. Thanks for watching and if you enjoyed
this video please check out the video description section to see how you can support the channel!
I wonder why there are so many (largely) incomprehensible articles on buck/boost converters on the net when two videos from this guy explain everything perfectly in 6 minutes. God how I love that guy.
In the schematic he shows at 0:30, wouldn't the output capacitor get charged to the full 10 V when there is no load? Does he implicitly assume that there is a load, even though he has omitted it from the schematic?