Driving MOSFETs is a crucial topic in electronics, especially when it comes to power supply design. In this article, we will explore the world of MOSFET driving and discuss various aspects that are essential for designing an efficient and reliable power supply.

One of the key challenges in driving MOSFETs is ensuring that they turn on quickly enough to handle the required switching frequencies. Let's consider a simple example where we want to drive an LED with a microcontroller. We start by connecting the LED to the gate of the MOSFET, which means we need to ensure that the gate voltage is high enough to turn on the MOSFET. If the gate voltage is not sufficient, the MOSFET will not turn on, and the circuit will not function correctly.

To overcome this challenge, we can use a charging method to raise the gate voltage quickly. In our example, we chose a charging time of 850 nanoseconds, which means that the gate capacitance needs to be charged up in just 850 nanoseconds to reach the required voltage. This is quite a fast charging process, and it requires careful consideration of the gate resistor and capacitor values.

The chosen charge time resulted in an average current of 56mA, which is relatively low compared to other switching frequencies. However, as we move on to more complex applications such as switched mode power supplies, the required charging times become even shorter, and the gate capacitance needs to be charged up in a fraction of a nanosecond.

To overcome this challenge, we can use a MOSFET driver circuit that is designed specifically for high-frequency switching. One common solution is to use a transistor circuit known as an MOSFET driver, which feeds a large amount of current quickly into the gate while drawing only a small amount from the microcontroller. This approach has several advantages, including reduced power losses and improved efficiency.

However, designing such a circuit can be complex, and it often requires specialized components and expertise. As a result, many engineers prefer to use a MOSFET driver IC, which is designed specifically for this purpose. The LMG1210 from Texas Instruments is an example of such an IC, which comes with a bootstrapping feature that simplifies the design process.

The only disadvantage of using a MOSFET driver IC is that it usually comes as an SMD component, making it more difficult to work with than a traditional transistor circuit. To overcome this challenge, we can use a breakout PCB and solder the driver IC onto it using a reflow oven. This approach allows us to design a custom PCB that meets our specific requirements.

Once we have designed the PCB, we need to add screw terminals to connect the driver IC between the microcontroller and the MOSFET. We also need to add decoupling capacitors to the supply voltage to ensure stable operation of the circuit.

The final result is a MOSFET driver circuit that can turn on and off extremely quickly, making it suitable for high-frequency switching applications. However, as we explore further, we realize that there are additional challenges that need to be addressed when designing a power supply.

One such challenge is bootstrapping, which refers to the process of creating a higher gate voltage for the upper MOSFET in a half-bridge circuit. In this scenario, the lower MOSFET turns on and charges up the capacitor through the diode, while the gate of the upper MOSFET gets charged up through the supply voltage plus the capacitor voltage.

To achieve this, we need to choose a suitable bootstrapping diode and capacitor that can handle the required voltages and currents. The datasheet provides guidance on selecting these components, including choosing high-quality capacitors from the Würth Elektronik eiSos group.

In conclusion, driving MOSFETs requires careful consideration of several factors, including charging times, gate resistor and capacitor values, and bootstrapping techniques. By understanding these aspects and using specialized components and expertise, we can design efficient and reliable power supplies that meet the demands of modern electronics applications.

WEBVTTKind: captionsLanguage: enNowadays the field of power electronics isruled by so called MOSFETs also known as MetalOxide Semiconductor Field Effect Transistors.The reasons are easy to understand: They onlyrequire a certain threshold voltage at theirgate in order to switch high voltages andcurrents with high frequencies and low powerlosses.That makes them the perfect component forbuilding efficient switched mode power supplies,motor speed controllers or just to turn onand off a couple of LEDs really fast and thereforedimming them.The only problem is that while driving suchMOSFETs sounds super simple; there are infact lots of hidden pitfalls which I moreor less skipped over during my previous MOFETvideo.And since I constantly receive comments whyI sometimes use MOSFET driver ICs and sometimesI don’t; I thought it is finally time toexplain in detail how to drive MOSFETs, whatthe term bootstrapping has to do with it andwhy you sometimes need a Gate-Drive Transformer.Let’s get started!This video is sponsored by the Würth ElektronikeiSos group.First off we need a practical example.For that I chose the IRLZ44N N-channel MOSFETfrom Infineon.The first place to go for information aboutthis component is of course its datasheet.There we can find out that the MOSFET canhandle a maximum of 55V across its Drain Sourcepath, it can withstand a maximum of 47A andit comes with 3 pins which are called Gate,Drain and Source.Now for my first practical example circuitI want to switch this blue 1W high power LEDwith the MOSFET.And what I have on hand is a supply voltageof 5V.The LED draws a current of 320mA at a forwardvoltage of 3.12V, which means that I willneed a 6Ω resistor that can handle a powerof 0.62W.This resistor LED combination gets hookedup to the drain of the MOSFET while the sourcegets directly connected to ground.All that is now left to add is a suitablegate voltage which will either put the MOSFETinto its conductive or non-conductive state.For that we can find a graph in the datasheet,which tells us at what gate source voltage,we can achieve what drain source voltage andwhat current values.So let’s imagine we directly connect thesupply voltage of 5V to our gate.According to the graph we have to approximatelyfollow this line.If there would be no MOSFET then the maximumcurrent of 320mA would flow, which would evenlay underneath the beginning of our observedline.That basically means that our drain sourcepath will come with a minimal voltage drop.This is of course beneficial since the drainsource voltage multiplied by the flowing currentequals our power losses and we want to keepthose as small as possible.So let’s just connect our supply voltageto the gate and as you can see the LED lightsup without any problems.The drain source voltage is around 5.8mV ata current flow of 330mA which equals a resistanceof 17.6mΩ and a power loss of 1.9mW.It is noteworthy that if my load would drawa current of for example 20A, then the drainsource voltage would according to the graphof the datasheet rise to 0.3V.This might only be a rather small voltagedrop, but due to the 20A it creates a powerloss of 6W which needs to get dissipated asheat through a heatsink.If that does not happen correctly then thetemperature of the MOSFET can for exampleincrease to a value of 175 degree Celsius,which means that according to the next graphthe drain source voltage increases to 0.8Vwhich makes the whole situation even worseand can lead to the destruction of the MOSFET.But anyway I can turn on and off the LED inmy first example circuit by either applying5V or the GND voltage to the gate.Of course you usually do not use a MOSFETfor such a simple switching task, which iswhy we should continue with a more realisticexample.So let’s take this Arduino microcontrollerwhose pin 9 I connect with the gate of theMOSFET.As the first programming example I will turnthe Arduino pin on, wait a second, turn itoff, wait another second and repeat this loop.And this is how the practical circuit lookslike and as you can see the MOSFET still switcheswithout any problems and the Arduino alsosurvives the switching events.But to get more into detail let’s have alook at the gate source and drain source voltageon the oscilloscope.During the turn on event we can see how thegate voltage slowly rises while the drainvoltage drops and therefore the MOSFET becomesconductive.Looks pretty ordinary at first sight, butit is important to mention that this turnon event example took around 300ns.If we compare that with the example wherewe connected the supply voltage to the gate,then we can observe that this turn on eventonly took around 150ns.The reason for that is the big issue thatcomes with all MOSFET driving tasks and thoseare the parasitic capacitances between gatedrain, gate source and drain source.That means that even though you often hearthat you only need a voltage at the MOSFETgate in order to switch it, you also needa not insignificant current which constantlycharges up and discharges the gate capacitance.You sometimes find this input capacitancevalue in the datasheet of the MOSFET, butyou usually use the total gate charge forcalculations.So for our datasheet example we have to feed48nC into and out of the gate in order toswitch the MOSFET.Since we know that the average current equalsthe derivation of the charge above the time,we can simply insert the values for the microcontrollerand supply voltage example and therefore geta current of 160mA and 320mA.Here we can clearly see that the lab benchpower supply can switch the MOSFET fasterbecause it can provide more current in a shorteramount of time.The microcontroller on the other hand evenexceeds its maximum output current per pinand therefore runs the risk of getting destroyedduring the switching event.If we now go one step further and reprogramthe microcontroller in order to output a pulsewidth modulated signal which dims the brightnessof the LED then the microcontroller can getdestroyed faster since now it gets overloadedconstantly with a frequency of 490Hz.A solution for such a low frequency microcontrollerMOSFET driving task is the simple insertionof a resistor like in this case with a valueof 200ΩIf we now measure the charge times then wecan determine that they got stretched outto 850ns and thus the average current gotlowered to 56mA.For an ideal microcontroller setup you canalso add a pull down resistor in order tomake sure that the gate gets discharged inan undefined output state event and also aZener diode that gets rid of occurring overvoltagesat the pin.This circuit is more than enough for sucha simple LED example, but let’s imaginethis would be a switched mode power supplyand we would have to switch the MOSFET witha much higher frequency.I chose an example frequency of 250kHz forwhich the programming of the microcontrollerwas not really hard to do by simply utilizingits timers but if we leave the circuit likebefore then we can see that through the slowcharging up of the gate capacitance, the MOSFETbarely has any time left to be conductive.That basically means we need a more powerfulcurrent source to charge up the gate fasterwhich also has the advantage that we go throughthe more power loss-making operating areasof MOSFET faster and thus obviously reducepower losses.For those reasons we can use such a transistorcircuit which is often called a MOSFET driver.Its job is to basically feed lots of currentquickly into the gate while only drawing asmall amount of current from the microcontroller.But since such a circuit requires quite afew components to build up, we can also simplyuse a MOSFET driver IC.As a practical example I chose the LMG1210from Texas Instruments, which is not onlyrelatively modern, but it also comes witha bootstrapping feature which I will talkabout later.The only disadvantage of this IC is that itonly comes as an SMD component.That is why I designed such a breakout PCB,onto which I soldered the driver IC throughthe help of solder paste and my reflow oven.After I then added screw terminals to thePCB, I also added decoupling capacitors tothe supply voltage and connected the driverbetween the microcontroller and the MOSFET.As you can see the driver shortens the switchingtimes to merely 70ns.But if we have a look at the voltages acrossthe decoupling capacitors, then we easilynotice that those are neither correctly positionednor do they feature the optimal capacitancevalues.To avoid such a problem you usually followthe reference design of the datasheet in whichwe are of course pointed to the high qualitycomponents from the Würth Elektronik eiSosgroup.But anyway, it should now be pretty clearwhy I sometimes use MOSFET drivers and whysometimes not.Now the term bootstrapping becomes importantwhen you want to for example develop suchan inverter which connects a load to the supplyvoltage or GND.With such a half bridge schematic you caneasily find out that we need at least 2 MOSFETs.The problem however is that the upper MOSFETcomes with a floating GND.That means even though I apply 5V to its gate,the voltage across the load can for examplealso be 5V and thus the resulting voltageat the gate equals 0V and therefore the MOSFETdoes not turn on.To solve this problem we can build up sucha bootstrapping circuit whose goal it is tocreate a higher gate voltage for the upperMOSFET.If the input is high, the lower MOSFET turnson while the capacitor gets charged up throughthe diode.When the input becomes low, the lower MOSFETbecomes nonconductive, while the gate of theupper MOSFET gets charged up through the supplyvoltage plus the capacitor voltage and thusit can turn on without any problems.This only does work however if the MOSFETsget turned on alternatingly because the capacitorneeds to get charged up constantly.So next I looked through the datasheet ofmy MOSFET driver IC to not only choose a suitablebootstrapping diode and capacitor but alsoto find a typical schematic for such a bootstrappingapplication.And as soon as I built up my own bootstrappingcircuit with the IC, I could use my oscilloscopein order to find out that everything worksjust like I planned it.And with that being said you already knowquite a lot about driving MOSFETs, but ofcourse you do not know everything yet if weconsider that driving MOSFETs potential-freeis also rather popular when it comes to forexample creating a tesla coil.But for that we would require a Gate-Drivetransformer and that is a subject for Part2 of this MOSFET video series.As always thanks for watching.Don’t forget to like, share, subscribe andhit the notification bell.Stay creative and I will see you next time.