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  • 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.

  • Thank you for watching and if you enjoyed this video please check out the

  • video description section to see how you can support me.

  • Make sure you check out Patreon which is a way you can donate money per video

  • to keep the channel going.

Today I'm going to teach you about feedback resistors in DC to DC

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搭建一個2.5V到14V 3A可調電源! (Build a 2.5V to 14V 3A adjustable power supply!)

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    稲葉白兎 發佈於 2021 年 01 月 14 日
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