Electronic Basics #23: Transistor (MOSFET) as a Switch

in my previous electronic basics video I showed you that bipolar junction transistors can easily be used as a switch In order to turn on and off your load slowly or even rapidly if you want to for example Dim the brightness of your favorite led efficiently But as soon as I try to control the bigger loads the transistor start to heat up quite a bit Which is mainly due to the energy loss of the collector and emitter path This means our circuits efficiency can still be improved and for that They likely exist another popular and more suitable transistor type these so called Mosfets By creating a similar circuits which can basically do the same as before The Mosfet only as an energy loss of 0.6 watts across as equivalent collector emitter path and thus increases the overall efficiency of the circuits up to 97% Not bad so in this video I will show you at first how easy it is to control such a mosfet with an Arduino And then how difficult it can actually get when you want to use them in more demanding applications Let's get started There exists two types of Mosfets n-Channel ones and P-Channel ones But more commonly used are n-channel types like this IRLZ44N Which has three pins called gate drain and source which is the equivalent to the base Collector and emitter of a BJT but instead of utilizing Currents that flows through the base of a bJt in order to switch on the loads the mosfet only requires a high enough voltage At the gates no current this voltage needs to be higher than the threshold voltage mentioned in the datasheets But lower than the maximum rated gate source voltage So with the 5 volts of the Arduino we should easily be able to control around 5 amps of current while maintaining the lowest possible drain to source voltage The region we use here in the output Characteristic curves is called the linear region in which the resistance of the drain to source path is almost constant But before going too much into the theory let's build up the circuits by connecting the source directly to ground the cathode of my LED to the drain and the anode to the supply voltage But one problem that was immediately noticeable was that even electrostatic voltages of my body can turn on the loads Even big ones like this light bulb, so it is always a good idea to place a 10k ohm pulldown resistor between gate and source in order to prevent that and after directly connecting the Pwm signal of the Arduino to the gates the circuit was complete and does work the way it is supposed to so let's inspect the voltages on the oscilloscope While the Arduino voltage goes high the drain to source voltage goes low and the other way around Perfect and by adding a potentiometer as analog inputs and tweaking the code a bit We just created an led dimmer, but let's say you are below it that is tied around this time applying 5 volts to the gate of the Mosfet Does very little to nothing because you need to add the voltage of your loads in order to turn on the switch A common way to do this is called bootstrapping for which exists various Ics But a much easier solution would be to use a P-channel mosfets The only difference is that we would need a pull-up resistor instead of a pulldown Because this time +5 volts turn the mosfet off and zero volts turn it on. Now that was the easy part But let's kick it up a notch by connecting a bigger load Everything still seems to work Just fine But when we have a look at the oscilloscope we can observe a damped oscillation that reaches peaks around 64 volts when the Mosfet switches off and I don't think he will like that for very long a part of the reason for this oscillation are the parasitic capacities between the terminals of the mosfets Which are much bigger than those of a BJT Power that will be small inductance a big current flow and a rise/full time of 280 Nano seconds and you got yourself problems to find a possible solution, I place the 1.15 Ohm resistor between the gates and Arduino to determine the peak gate current that is flowing Which seems to be around 113mA because when turning on the mosfets it is not only about the voltage at the gates but also about the charge and with a constant gate charge, we can increase the rise and fall time by simply decreasing the gate current and for that we can use a simple resistor for me a 470 Ohm did the trick by decreasing the current Peak to 11mA and thus increasing the rise and fall time which then decreased the oscillation to acceptable values this problem of rise/full time becomes even more complex with higher frequencies Which require way higher gate current to switch the mosfet on and off fast enough otherwise the results might look like this another noticeable aspect is the energy loss at the gates since a certain amount of charge has to move into the gates and afterwards to ground those losses to actually exists But with a low frequency like 490Hz of the arduino. They are almost unnoticeable, but on the other hands with a frequency of 1MHz we have switching losses of 80mW so all in all mosfet driver ICs can make your life easier and If you want more information on how to handle Mosfets properly I put a couple of useful links in the video description I hope you like this video if so don't forget to like share and subscribe That would be awesome stay creative, and I will see you next time

Channel: GreatScott!

Views: 923,110

Keywords: mosfet, MOSFET, tutorial, how to, handle, choose, beginner, basic, basics, arduino, arduino uno, n channel, n-channel, p channel, p-channel, driver, bootstrap, ic, IC, bootstraping, guide, electronics, greatscott!, greatscott, IRLZ44N, IRF9540, gate, drain, source, switch, switching application

Id: o4_NeqlJgOs

Length: 6min 22sec (382 seconds)

Published: Sun Jul 24 2016