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
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