Probably the most used component nobody knows of! TL431 Guide! EB#50

Sooo switched mode power supplies are amazing, right? I mean you simply plug them into mains voltage and then magically get granted access to a lower DC voltage on their output which you can use to charge your phone, your laptop or power pretty much anything in your modern household. And they do this voltage conversion with a pretty good effciency which in this case was around 84%. Needless to say I am a big fan of such power supplies and so should you. So to honour them I did a video about them not too long ago in which I investigated how they function and while I was at it I also created my own DIY version. The schematic for it looked like this and while I was pretty familiar with the majority of the components there was one I never worked with before and that was the TL431 which according to its datasheet is a Precise Programmable Reference. That is why I started to do some research on it and was immediately blown away by all the applications for this little IC. So in this video I will tell you all about how the TL431 works and how to use it in lots of different applications including how it is being used in a switched mode power supply. Let's get started! This video is sponsored by Skillshare which is an online learning community for creatives that offers thousands of inspiring classes. Topics include illustration, design, photography, video, freelancing and much more like for example electronics. Last time I told you that I was watching the Ultimate Electrical Machines for Electrical Engineering class by Ahmed Mahdy and truth be told I am still watching that because there is a ton to learn. But luckily Skillshare always remembers where I left off, so that I can always continue my learning adventure seamlessly . And if you want to join me on this adventure then I am happy to tell you that the first 1000 subscribers to click the link in the description will get a 1 month free trial. First off let's take a look at the simplified schematic. The showcased diode symbol is that of a zener diode but why does it have a reference voltage pin? Well, deeper in the datasheet we can find this functional block diagram which might answer my question. As you can see all the IC consists off is a comparator, a transistor, a diode and a voltage reference which is typically 2.495V But the IC as a whole does only work when powered with a voltage at the cathode between Vref and 36V which needs to supply a maximum of around 1mA of current. And while we are at the basics, the pinout of the IC looks like this with the pins ref, anode and cathode. The cathode usually connects to a resistor that is connected to a supply voltage and the anode is connected to GND. The ref pin can be connected to pretty much any voltage signal through a resistor, so for a first example let's imagine a rising voltage. As long as the ref voltage is lower than the Voltage reference, the output of the comparator stays low, the transistor is off and the output voltage is being pulled high to the supply voltage. But as soon as the ref voltage is higher than the voltage reference, the output turns on and thus activates the transistor which now pulls the output voltage low to around 2V. Thus in this open loop setup the IC basically acts like a comparator. I tried this circuit out on a breadboard and as you can see it works just the way we thought it would. And in case you are wondering what is a comparator and why you should care about them or about zener diodes to which I will come in a second, then you can always go to my YouTube channel and simply search for those keywords; I bet I already made a video about the topic. But anyway next let's simply connect the ref pin to the cathode and thus create a closed loop circuit which is basically the simplified schematic we started with. Without thinking about what it could do I built it up on a breadboard with an input resistor of 1kohm. And by powering the circuit with a slowly increasing DC voltage it seems like the voltage at the cathode stays at 2.5V no matter how much I increase the input voltage. The reason is of course the structure of the IC which opens up the transistor and let it sink current at voltage values of above 2.5V which means the circuit is now a substitute for a 2.5V zener diode. The calculations for it are pretty much the same as with ordinary zener diodes and it also functions just like one, so you might be asking yourself why not just use a regular zener diode. The reason is that ordinary zeners feature quite a big drift voltage drift depending on the input voltage, current and temperature. The TL431 on the other hand is way more stable but make sure to not exceed the recommended 100mA current flow. And it gets even more fun by adding a voltage divider between the cathode, ref pin and anode. Since we know that 2.5V is once again our threshold, we can come up with this simplified formula that let's us create an adjustable zener diode. For example with two 10kohm resistors we can create a 5V zener diode. So with this technique we can make our own super stable adjustable zener diode which is quite useful because as you might know not every zener voltage does exist with ordinary diodes. Now this closed loop example might seem basic so far but it tell us all we need to know about this IC and that is that it can monitor a voltage and switch at a certain threshold which makes it perfect not only for monitoring circuits but also feedback circuits. Don't believe me? Here is a schematic for a 12V battery over discharge protection circuit that will cut the current to your load when a certain adjustable undervotlage value is reached. Or here is schematic of a precision constant current sink whose current we can fine adjust by utilizing a potentiometer. There are dozens of such handy circuits in the datasheet or on the internet which you should all understand now by remembering the basic working principle of a TL431. And with that being said time to have a closer look at switched mode power supplies which often come with a TL431 and an optocoupler for their feedback system just like how I used them in my DIY version. Before talking about this circuit though we have to open the can of worms which is called feedback loop design. But don't worry I will keep it super simple and not talk about things like for example Laplace transformation. So let's imagine we got a basic flyback converter schematic here whose output voltage we want to keep at 5V. With a normal feedback loop system we firstly need to decrease the voltage with a voltage divider and then compare it to a stable and know reference voltage. This error amplifier, like the name implies, amplifies the error voltage between the two inputs and sends it over to our PWM circuit through an optocoupler. The system works with negative feedback so if the output voltage increases, the analog error voltage at the PWM circuit decreases and thus the output PWM duty cycle decreases as well which therefore lowers the output voltage. Of course this principle also works vice versa if the output voltage decreases. This all sounds pretty straightforward but you do not often see pure error amplifiers in power supplies because the TL431 is king here and can do this job without a problem by adding a couple of passive components around it. So let's try to figure out their values. The voltage divider needs 2 500ohm resistors since we once again need to reach 2.5V at the ref pin and the output voltage should be 5V. The other resistor needs to pass 1mA for the TL431 and 1mA for the Optocoupler LED and since we know that the voltage drop across the 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 whose purpose and selection goes back to a whole lot of feedback control oscillation theory but a 100nF one is a good start for tinkering. And keep in mind that this is only a bare bone version of such a feedback system which you can definitely improve by learning more about the subject. But anyway, according to this final schematic I built up my test power supply partly on a perfboard and partly in the air which was not really pretty to look at. I also had to do a bit of teensy programming in order to create the required sawtooth signal with a frequency of 54kHz. And as you can see after doing a couple of tests the feedback system seems to be acceptably stable and works pretty much as expected. So feel free to build such a low voltage switched mode converter by yourself and be amazed what this small TL431 component can all do. With that being said I hope you learned something through this video. If so don't forget to like, share, subscribe and hit the notification bell. Stay creative and I will see you next time.

Channel: GreatScott!

Views: 61,211

Rating: 4.9567709 out of 5

Keywords: tl431, ic, component, integrated, circuit, tutorial, guide, make, project, diy, do, it, yourself, power, supply, switched, mode, feedback, loop, control, theory, king, comparator, voltage, current, reference, calculate, calculations, design, transistor, diode, zener, stable, adjustable, sink, source, monitor, switch, closed, open, schematic, greatscott, greatscott!, most, used, unknown, nobody, knows, basic, basics, electronics, electronic, transformer, error, amplifier, sawtooth, frequency, negative, mosfet, driver

Id: isutYMU2HHU

Length: 10min 59sec (659 seconds)

Published: Sun Oct 03 2021