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.
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