Sooo switched mode power supplies are amazing,right?I mean you simply plug them into mains voltageand then magically get granted access to alower DC voltage on their output which youcan use to charge your phone, your laptopor power pretty much anything in your modernhousehold.And they do this voltage conversion with apretty good efficiency which in this case wasaround 84%.Needless to say I am a big fan of them andI want to learn more about the components used in thesepower supplies.

One of the key components that plays a crucial rolein switched mode power supplies is the TL431.This IC is used for its ability to monitor a voltageand switch at a certain threshold, which makes it perfectfor monitoring circuits but also feedback circuits.A feedback circuit is essential in a power supply asit helps to regulate and stabilize the output voltage.

The TL431 works by adding a couple of passive componentsaround it. One resistor needs to pass 1mA for theTL431 and 1mA for the Optocoupler LED, whileanother resistor needs to create a voltage divider thatreaches 2.5V at the ref pin. The other resistor in thevoltage divider should be around 650ohm.

A capacitor is also needed in the feedback systemwhose purpose is to prevent oscillation. However, Iused a simple approach by just adding a 100nFcapacitor. A better version of this circuit would requiremore advanced knowledge of feedback control theory.

Now that we have the basic components and theirvalues, let's talk about how they are used in a switchedmode power supply. The TL431 is used for its ability tomonitor a voltage and switch at a certain threshold. Thisthreshold value determines when the output voltage shouldbe increased or decreased.

In my DIY version of a low voltage switched modepower supply, I used an optocoupler for the feedback system.I also had to do some programming in order to create arequired sawtooth signal with a frequency of 54kHz.The TL431 is a great component for this task as itcan amplify the error voltage and send it to the PWM circuit.

A basic flyback converter schematic was used as the foundationfor my power supply. The output voltage was set at 5V, andthe feedback system worked pretty much as expected. I builtmy test power supply partly on a perfboard and partly in the airwhich wasn't really pretty to look at.

One of the advantages of using a TL431 in a feedbacksystem is that it eliminates the need for an error amplifier.These components are often used in more complex systems, butthe TL431 can do this job without any problem. With its abilityto monitor a voltage and switch at a certain threshold, theTL431 is a great component for creating stable and accuratefeedback circuits.

Overall, I am very impressed with the performance of myDIY power supply using the TL431. It's amazing what this smallcomponent can do when used correctly. I hope that through this video,I have been able to explain how switched mode power supplieswork and provide some useful information about the componentsused in them.

WEBVTTKind: captionsLanguage: enSooo switched mode power supplies are amazing,right?I mean you simply plug them into mains voltageand then magically get granted access to alower DC voltage on their output which youcan use to charge your phone, your laptopor power pretty much anything in your modernhousehold.And they do this voltage conversion with apretty good effciency which in this case wasaround 84%.Needless to say I am a big fan of such powersupplies and so should you.So to honour them I did a video about themnot too long ago in which I investigated howthey function and while I was at it I alsocreated my own DIY version.The schematic for it looked like this andwhile I was pretty familiar with the majorityof the components there was one I never workedwith before and that was the TL431 which accordingto its datasheet is a Precise ProgrammableReference.That is why I started to do some researchon it and was immediately blown away by allthe applications for this little IC.So in this video I will tell you all abouthow the TL431 works and how to use it in lotsof different applications including how itis being used in a switched mode power supply.Let's get started!This video is sponsored by Skillshare whichis an online learning community for creativesthat offers thousands of inspiring classes.Topics include illustration, design, photography,video, freelancing and much more like forexample electronics.Last time I told you that I was watching theUltimate Electrical Machines for ElectricalEngineering class by Ahmed Mahdy and truthbe told I am still watching that because thereis a ton to learn.But luckily Skillshare always remembers whereI left off, so that I can always continuemy learning adventure seamlessly . And ifyou want to join me on this adventure thenI am happy to tell you that the first 1000subscribers to click the link in the descriptionwill get a 1 month free trial.First off let's take a look at the simplifiedschematic.The showcased diode symbol is that of a zenerdiode but why does it have a reference voltagepin?Well, deeper in the datasheet we can findthis functional block diagram which mightanswer my question.As you can see all the IC consists off isa comparator, a transistor, a diode and avoltage reference which is typically 2.495VBut the IC as a whole does only work whenpowered with a voltage at the cathode betweenVref and 36V which needs to supply a maximumof around 1mA of current.And while we are at the basics, the pinoutof the IC looks like this with the pins ref,anode and cathode.The cathode usually connects to a resistorthat is connected to a supply voltage andthe anode is connected to GND.The ref pin can be connected to pretty muchany voltage signal through a resistor, sofor a first example let's imagine a risingvoltage.As long as the ref voltage is lower than theVoltage reference, the output of the comparatorstays low, the transistor is off and the outputvoltage is being pulled high to the supplyvoltage.But as soon as the ref voltage is higher thanthe voltage reference, the output turns onand thus activates the transistor which nowpulls the output voltage low to around 2V.Thus in this open loop setup the IC basicallyacts like a comparator.I tried this circuit out on a breadboard andas you can see it works just the way we thoughtit would.And in case you are wondering what is a comparatorand why you should care about them or aboutzener diodes to which I will come in a second,then you can always go to my YouTube channeland simply search for those keywords; I betI already made a video about the topic.But anyway next let's simply connect the refpin to the cathode and thus create a closedloop circuit which is basically the simplifiedschematic we started with.Without thinking about what it could do Ibuilt it up on a breadboard with an inputresistor of 1kohm.And by powering the circuit with a slowlyincreasing DC voltage it seems like the voltageat the cathode stays at 2.5V no matter howmuch I increase the input voltage.The reason is of course the structure of theIC which opens up the transistor and let itsink current at voltage values of above 2.5Vwhich means the circuit is now a substitutefor a 2.5V zener diode.The calculations for it are pretty much thesame as with ordinary zener diodes and italso functions just like one, so you mightbe asking yourself why not just use a regularzener diode.The reason is that ordinary zeners featurequite a big drift voltage drift dependingon the input voltage, current and temperature.The TL431 on the other hand is way more stablebut make sure to not exceed the recommended100mA current flow.And it gets even more fun by adding a voltagedivider between the cathode, ref pin and anode.Since we know that 2.5V is once again ourthreshold, we can come up with this simplifiedformula that let's us create an adjustablezener diode.For example with two 10kohm resistors we cancreate a 5V zener diode.So with this technique we can make our ownsuper stable adjustable zener diode whichis quite useful because as you might knownot every zener voltage does exist with ordinarydiodes.Now this closed loop example might seem basicso far but it tell us all we need to knowabout this IC and that is that it can monitora voltage and switch at a certain thresholdwhich makes it perfect not only for monitoringcircuits but also feedback circuits.Don't believe me?Here is a schematic for a 12V battery overdischarge protection circuit that will cutthe current to your load when a certain adjustableundervotlage value is reached.Or here is schematic of a precision constantcurrent sink whose current we can fine adjustby utilizing a potentiometer.There are dozens of such handy circuits inthe datasheet or on the internet which youshould all understand now by remembering thebasic working principle of a TL431.And with that being said time to have a closerlook at switched mode power supplies whichoften come with a TL431 and an optocouplerfor their feedback system just like how Iused them in my DIY version.Before talking about this circuit though wehave to open the can of worms which is calledfeedback loop design.But don't worry I will keep it super simpleand not talk about things like for exampleLaplace transformation.So let's imagine we got a basic flyback converterschematic here whose output voltage we wantto keep at 5V.With a normal feedback loop system we firstlyneed to decrease the voltage with a voltagedivider and then compare it to a stable andknow reference voltage.This error amplifier, like the name implies,amplifies the error voltage between the twoinputs and sends it over to our PWM circuitthrough an optocoupler.The system works with negative feedback soif the output voltage increases, the analogerror voltage at the PWM circuit decreasesand thus the output PWM duty cycle decreasesas well which therefore lowers the outputvoltage.Of course this principle also works vice versaif the output voltage decreases.This all sounds pretty straightforward butyou do not often see pure error amplifiersin power supplies because the TL431 is kinghere and can do this job without a problemby adding a couple of passive components aroundit.So let's try to figure out their values.The voltage divider needs 2 500ohm resistorssince we once again need to reach 2.5V atthe ref pin and the output voltage shouldbe 5V.The other resistor needs to pass 1mA for theTL431 and 1mA for the Optocoupler LED andsince we know that the voltage drop acrossthe LED is 1.2V and across the TL431 is 2.5V,we can calculate a resistance of around 650ohm.Last but not least we got the capacitor whosepurpose and selection goes back to a wholelot of feedback control oscillation theorybut a 100nF one is a good start for tinkering.And keep in mind that this is only a barebone version of such a feedback system whichyou can definitely improve by learning moreabout the subject.But anyway, according to this final schematicI built up my test power supply partly ona perfboard and partly in the air which wasnot really pretty to look at.I also had to do a bit of teensy programmingin order to create the required sawtooth signalwith a frequency of 54kHz.And as you can see after doing a couple oftests the feedback system seems to be acceptablystable and works pretty much as expected.So feel free to build such a low voltage switchedmode converter by yourself and be amazed whatthis small TL431 component can all do.With that being said I hope you learned somethingthrough this video.If so don't forget to like, share, subscribeand hit the notification bell.Stay creative and I will see you next time.