Today I'm going to teach you about feedback resistors in DC to DC
converters.
And I'm going to show you how to use this knowledge to build an adjustable power
supply
with an output voltage between 2.5 volts and 14 volts.
So in a previous video I showed you how to make a 5 ampere
buck converter with a five volt output. I told you that these resistors
configure the LM2678
to have a 5 volt output. Let's talk more about how this works.
Most DC to DC converter controller chips have a pin
called the feedback pin. This is the part of the chip that's used to monitor the
output voltage.
The controller basically looks at the voltage on the feedback pin,
and if the voltage is too high or too low, it adjusts the pulse width
of the switching waveform, which then gets filtered, and the correct output voltage gets
restored.
In this example I have a 10 volt input going to the supply
and the load is changing between 0.5 amperes and 5 amperes.
The feedback mechanism takes care of this, adjusts the duty cycle,
and a perfect 5 volt output gets maintained. Let's talk more about how
this works,
and how we can design our own feedback resistor network. DC to DC converter
controllers usually have a precise
internal reference voltage called "VFB". The exact value will depend on the chip
you're using
but it will always be given in the datasheet. And it's usually around
1.2 volts.
For our LM2678 it's 1.21 volts.
If we removed the feedback resistors
and connected the output of the supply directly to the feedback pin,
the controller would look at the output voltage, compare it to 1.21 volts,
and then do whatever it has to do to ensure that the output stays at
1.21 volts.
But that's not very useful is it? Why would you want to use a 1.21 volt supply?
Okay let's add a 10 to 1 voltage divider here,
so whatever the output voltage is, it gets divided by 10,
and that's what the feedback pin on the controller is receiving. This effectively
multiplies the output voltage by 10
and you get 12.1 volts on the output. So... we're dividing... but we're
multiplying... which is a little weird... but check this out.
Let's say the output of the supply was 12.0 volts.
This gets divided by 10, and the controller would see 1.20 volts on the feedback pin.
The controller would then say, "Hey! This is too low! We need to increase the
output voltage!"
So it increases the pulse width and raises the voltage to 12.1 volts again.
The controller sees 1.21 volts on the feedback pin,
and now it's happy. Now let's say there's a sudden drop in the output current,
and the output voltage shoots up to 12.2 volts.
The controller would see 1.22 volts.
The negative feedback control loop inside the chip
would then reduce the duty cycle, restoring the desired output voltage
of 12.1 volts. By changing the values of the resistors in the feedback
resistor network here,
we can set the output voltage to be almost anything we want...
assuming all the components can handle the extra voltage! You can use these
formulas
to set the output voltage to whatever you want it to be, within the limits of
what the controller chip is capable of.
You can also have a little bit of fun. If you make these resistors fixed,
and also add a variable resistor, you can create a variable output voltage power
supply.
Now you have a step down power supply that can output
2.5 volts to 14 volts DC. Right now I have my power supply set to 13.8 volts
and I am using it to charge a 12 volt lead-acid battery.
I can use the supply to dim LEDs,
power amplifiers, or just see how much voltage something can handle.
Now if you remember my video about voltage dividers
I talked about how it's the ratio of resistance values that determine the
voltage.
If that's the case, why not just use these resistor values?
If you think about it this would reduce the power consumption of the circuit.
But there is a trade-off! Our switch mode power supply
is switching high currents at high frequencies. Whenever you do this
your circuit will put out some electromagnetic interference.
You can see this for yourself with a cheap AM radio.
The electromagnetic interference is inducing a small current
into the antenna of my radio and it's getting picked up as unwanted noise.
Now there's a difference between how electric and magnetic fields affect
things
but I'm just trying to keep things simple here. Things get really
interesting
when you realize that the switch mode power supply can actually interfere with
itself!
Let's say some interference from the inductor reaches the feedback resistors.
This will induce a tiny unwanted current in the resistors.
When you have current flowing through a resistor, a difference in voltage gets
created.
Because volts = current multiplied by resistance,
the higher the resistance, the higher the unwanted voltage you get in the form of
noise.
And this can affect the controller's ability to regulate the output voltage.
In general you want to keep the total resistance of your feedback resistors
somewhere between a few kiloohms but under 1 megaohm.
This will minimize the amount of noise in your power supply that's created by
interference.
This is also why I like to work on high powered electronics
with my oscilloscope probe set to X1 attenuation.
The lower resistance makes them less susceptible to interference.
Alright, now you know what feedback resistors are, and you can use this
knowledge to change the output voltage of almost
any dc-dc converter! Just make sure you double check the voltage limits of your
capacitors,
diodes, MOSFETs etc. according to the design guidelines of your
controller's datasheet.
Sometimes overclockers use this trick on their video cards and motherboards
to change the power supply voltages to achieve higher clock speeds.
Or if you want to save power you can run things at lower voltages.
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