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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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B1 US voltage output feedback power supply supply controller

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