Name: 搭建一個2.5V到14V 3A可調電源! (Build a 2.5V to 14V 3A adjustable power supply!)
Uploaded: 2021-01-14T05:10:33.000Z
Duration: 6 min 18 s
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Today I'm going to teach you about feedback resistors in DC to DC

And I'm going to show you how to use this knowledge to build an adjustable power

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

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

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

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

and how we can design our own feedback resistor network. DC to DC converter

internal reference voltage called "VFB". The exact value will depend on the chip

but it will always be given in the datasheet. And it's usually around

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

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

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

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 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

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

to set the output voltage to whatever you want it to be, within the limits 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

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.

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

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

but I'm just trying to keep things simple here. Things get really

when you realize that the switch mode power supply can actually interfere with

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

Because volts = current multiplied by resistance,

the higher the resistance, the higher the unwanted voltage you get in the form of

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

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