**The Functional Principle of a Buck Converter**

A buck converter is a DC-to-DC converter that can step down voltages with high efficiency. In this video, we will delve into the functional principle of a buck converter and explore how to calculate the inductance value for such a converter.

To understand the functional principle of a buck converter, let's consider a simplified circuit diagram. We have a switch (like MOSFETs), diodes, a coil, and capacitors. When we connect 12 volts to the inputs and close the switch, current flows from the voltage source through the coil, through the loads, and back to the source. As soon as current starts flowing through the coil, it simultaneously builds up a magnetic field, which induces a voltage into the coil itself and creates an opposing current.

This opposing current is what allows us to step down the voltage at the outputs of the converter. However, we cannot keep the switch closed forever because the stored energy of the coil in the form of magnetic field reaches its limit and the converter becomes inefficient.

**The Importance of Coil Selection**

When selecting a coil for a buck converter, there are several factors to consider beyond just the inductance value. For instance, the choice of core material can be more important than the actual inductance value. The inductance of a coil will decrease drastically at a certain DC current value when the core material reaches its magnetic saturation region.

Let's take an example from the learning kit boards we have been using. One coil features an inductance of 100 microHenries, while another coil has an inductance of 68 microHenries. Even though the 100 microHenry coil should be able to store more energy, its current flow is limited to 1.2 mA, whereas the second coil with a lower inductance can handle higher currents.

**Temperature and Coil Resistance**

Another crucial factor to consider when selecting a coil is its resistance and how it affects the temperature of the coil. A smaller coil with the same core material will reach its magnetic saturation earlier than a larger coil and will also produce more heat due to its higher resistance.

In our case, the smaller coil with an inductance of 10 microHenries starts causing problems at higher output currents before the bigger coil does. This is because it reaches its magnetic saturation earlier and thus its inductance gets decreased. Furthermore, the temperature of the small coil increases up to 80 degrees Celsius when a current flow of 4 mA is applied, whereas the bigger coil would need 13 amps to reach the same temperature.

**Conclusion**

In conclusion, while calculating the inductance value for a buck converter is essential, it's not the only important property. The choice of core material and its magnetic saturation region, as well as the resistance of the coil and how it affects the temperature of the coil, must also be considered when selecting a suitable coil.

For those interested in learning more about coils, I highly recommend checking out the experiment book that came with the learning kit boards or using tools like the "Red Expert" tool to simulate different inductances in a circuit.

WEBVTTKind: captionsLanguage: enI think we are all familiar with such a commercial park inverter circuits asAn example you could simply supply 12 volts to its inputsConnect the multimeter to its outputs and then use the given potentiometer to adjust in the output voltage to 5 voltstherefore you could say that about converter is a DC to DC converter, which can step down voltages andBest of all, it can do that job with a high efficiency, which was around 79% in my example hereIf we now take a look at the simplified circuit diagram of such a converterthen we can see that it only requires a switch like MOSFETs adiodes a coil and capacitor to billetsThat is why I also created one during a previous video of mine with the help of the lm2577Switch IC from Texas Instruments. Iused this ICEA because it combines a MOSFET switch with the required feedback control system andThus building up the buck converter circuits only required for additional passive componentsFor mere leak oil selection was the hardest part thoughbecause there exists 100 micron recoils with different shapes sizes and with different core materials andI was never 100% sure which coil would be the best for the jobTo solve this problem the brought electronic ISOs group sent me a buck converter learning kitsWhich utilizes different kinds of coils?So in this video, we will not only find out how to calculate the inductance for such a buck converterBut we will also find out how for example the size core material and temperature influence our utilized coilsLet's get startedThis video is sponsored by the broad electronic Isis groupBefore supplying the learning kit board with currentWe have to figure out what kind of function D coil fulfills in the buck converter circuitsTo do that we firstly hook up 12 volts to our simplified super diagram and close the switch nowCurrent flows from our voltage source through the coil through the loads and back to the sourceBut as soon as current started flowing through the coil it simultaneously built up a magnetic fieldsWhich in turn induced a voltage into the coil itself and thus created an opposing currentWhich is the reason why the original current through the coil can only rise slowly?While the current is now slowly rising in a relatively linear fashionThe voltage across the coil is corresponding to this equationPretty much constant and also opposites in comparison to the input voltageThat means the output voltage equals the input voltage reduced by the voltage across the coilwhich therefore means that we successfully step down the voltage at the outputs ofCourse, we cannot keep the switch closed foreverBecause the stored energy of the coil in the form of the magnetic fields will at some point reach its maximumThis state is called magnetic saturation in which a constant current flowsthat is only limited by the resistance of the windings of the coil ifWe neglect the load resistance or rather nearly short. It'sthere also does no longer exist anoticeable voltage across the coil and thus the output voltage does more or less equal the input voltage toGet the desired output voltage though. We have to open the switch at the right timeWhich would lead to an abrupt breakdown of the current if there wouldn't exist the inductive properties of the coilSo during that switch opening momentThe coil uses its stored energy in order to push a current through the loads and the diodesWhich once again leads to relatively linear this and decrease of the current value?because of this current we got a voltage across the coil with reverse polarity in comparison to beforeWhich reduce by the diode voltage?equals in the output voltageNow the current through the coil drops longer or shorterBefore the switch gets once again closeddepending on how much energy is required at the outputs andBecause of the continuous repeating of the to switch States we create such a square wave signal for the switchThe proportion between the on time of the switch and the cycle durationequals the so called duty cycle andThe outer ring of the duty cycle depending on the output currents is called pulse width modulationWhich can reach a maximum of 1 so 100%The higher this value becomes the more energy is being stored in the coil andAlso, the output voltage gets closer to the input voltage. IThink due to this explanation of the functional principle of a buck converterit should be clear that the inductance value depends on a whole lots of different factors likethe input voltage the occurringResistances the load current the output voltage the duty cycle the switch frequency and even the ripple currentSo you can either use this relatively precise formula to calculate the inductance valueOr make your life easier by simply following the recommendation of the ICS datasheetsAt this point some of you might think wellI will just use a big inductance because it can store more energyand thus the converter should be able to output more power andWhen we have a look at different commercial buck converters, then it seems like this FIRREA is somehow correctsBut in order to learn more about how to properly select a coil, let's have a look at the learning kit boardsWhich uses three different coils for its first our converter circuits?One of them features an inductance of 100 micro Andrea and two of them an inductance of 68. Micro HenrySo after hooking up 12 volt power to the boards and connecting a constant load circuits to its outputsI had a look at the voltage and current wave of each coil on the oscilloscope andEven though the 100 micro Henry coil should be able to store the most energyI did not like its current flow with an output current above 1.2 m/s incomparison to the current flow of the second coilThe reason for that is that the 100 micro NB coil uses ferrite as a core materialwhile the second coil uses ayran powderThe consequence is that the inductance of the first coil will drastically decrease at a certain DC current valueBecause the material reaches its magnetic saturation regionWhile the inductance of the second coil will only decrease relatively constant across a wider current rangeTherefore you can see that at a similar coil size the choice of the core material can be more important than the actualInductance value and you should always have a look at the magnetic saturation current and not exceededWith that being said we can move on to the second our converter circuits which coincidentallyUses a pig and a small coil with the exact same inductance value of 10. Micro HenryBoth of them fulfill the job in the back when brooder circuits without a problemBut it is not a surprise that the physical smaller coilstarts causing problems at higher output currents before the bigger coil doesOne of the reasons is that the smaller coil reaches its magnetic saturation earlier and thus its inductance gets decreasedBut more important in this case, is that the temperature of the small coil?Increases up to 80 degrees Celsius and a current flow of 4 msWhile the bigger coil would need 13 amps to reach the same temperatureThe reason is simple in order to achieve a certain inductance value. You need a certain number of windingsthis number is of course bigger for smaller coil in comparison to a bigger coil boots with the same core material andTo achieve this number of windings for the smaller coil. We also have to use a wave in our enamel copper wireWell, the bigger coil can use a thicker wirePractically speakingthat means that our smaller coil features an almost nine times bigger resistance in comparison to the bigger coil andThus it produces nine times more power losses which need to get dissipated as heatBut because the smaller coil also features less surface area to dissipate the heatswe can easily slip into a region in which the winding isolation can melts and thus destroy the coil ofCourse, there are also other factors which influence the coil selection and if you're interested in learning more about themThen I would highly recommend checking out the experiment book that came with the learning hitsWhich tells you pretty much everything you need to know about coilsAnother assistance tool to select the right coil would be the red expert toolWhich can conduct the simulation of different inductances in a circuitsbut nevertheless I hope that I was able to give you a small insight into the world of coils andThat you now understand that the inductance value is not the only important propertythere is for example also the coil resistance and the magnetic saturation current andmost of the time a small look into the datasheet of a coil can be very helpful ofcourse, you can also visit the broad electronic Isis group websites to get more information about coils or toOrder them since they come with very detailed data sheetsWhich for example eBay sellers do not offer in 99% of the cases?As always thanks for watching. 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