How to select a MOSFET? | MOSFET parameters | MOSFET selection

we use mosfets in every circuit but have you ever thought what makes mosfet so common and what are its parameters which help us to select a suitable mosfet for our application well don't worry about it we are going to check that right now [Music] we use power mosfets in most of our circuits like smps and motor drivers etc these power mosfets have high voltage and current capabilities and their switching frequency is very high we'll see which parameters are important to select a mosfet and for that we'll refer to a data sheet of the mosfet this is the buk-7y3r5 series mosfet from nextperia first we need to start with drain to source breakdown voltage this rating is important in selecting mosfets if the application voltage exceeds this voltage value then it might result in the destruction of a mosfet we should choose mosfets with a vds sufficiently higher than the voltage at which we'll be using it this rating denotes that it can block this voltage without damaging itself when it is in off state but it comes with conditions and dependencies let's see this data sheet the breakdown voltage of this mosfet is around 42.7 volts only when the junction temperature is 25 degrees celsius gate to source voltage should be zero volts now what is gate to source voltage and junction temperature the gate to source voltage is required to turn on the mosfet this signal allows the drain current to start flowing through the mosfet when we apply this gate voltage signal it should be at least more than the threshold value only then the mosfet will turn on for this mosfet we need at least 3 volts to turn it on this vgs depends upon the drain current which has to flow through mosfet and the vds if we see this graph which is known as transfer characteristics of a mosfet it shows the theoretical relation between drain current and vgs let's say a load is 50 amperes for that we need around 4.5 volts of vgs to turn on the mosfet provided the vds is 12 volts again it is the theoretical relation between vgs and drain current we can give gate voltage in the range of minus 10 volts to 20 volts to this mosfet next is the continuous drain current it is the maximum continuous current which a mosfet can carry when the mosfet is turned on this mosfet can carry 120 amperes when we keep 10 volts vgs but at the junction temperature of 25 degrees celsius and 93 amperes if the junction temperature is up to 100 degrees celsius this drain current capability varies with the change in junction temperature and vgs if you see the drain current versus junction temperature graph when the junction temperature of the mosfet increases the current handling capacity of the mosfet decreases exponentially and eventually it goes to 0 at 175 degree celsius even if we give this sufficient gate voltage next parameter is rds on it is the resistance between drain and source this parameter plays a very important role in the mosfet selection it is the key parameter in calculating conduction losses and eventually rise and junction temperature of the mosfet this rdson increases if the vds of a mosfet increases that means if you take a different mosfet with high vds let's say 150 volts then its rds on will definitely be higher than the mosfet which has vds of 40 volts if we apply less vgs still this rds on increases for the same mosfet the mosfet is a positive temperature coefficient device so if the junction temperature of a mosfet increases the rds on also increases this is the formula to calculate the conduction loss of the mosfet now you see why rdson is so important this is the rds on for this mosfet with the condition of we just should be 10 volts drain current should be only 25 amperes and the junction temperature of the mosfet is 25 degree celsius now if we see the drain current versus rds on graph then we can see the rds on and drain current are very dependent on vgs if the vgs is less and drain current carrying capacity is also low but if the vgs is high up to 10 volts then this guy can handle up to 200 ampere current and still theoretically its rds on will be less than 5 milli ohms next coming up is drain leakage current this is also a drain to source current when vgs is 0 volts that means when the mosfet is turned off even if we don't give any gate voltage signal to the mosfet still this fellow allows some small amount of current to flow through it we can call it a leakage current this leakage current should be as small as possible for this mosfet it typically ranges from few nine amperes to 500 microamperes with different vds and junction temperature conditions so this leakage current depends on the drain to source voltage and junction temperature when the mosfet is off any junction temperature is rising in that case the leakage current of the mosfet increases so its conduction loss increases which eventually dissipates more power next is the peak drain current it is the maximum drain current which can flow through the mosfet for only 10 microseconds or less for the mosfet which you are referring to it is 526 amperes if the junction temperature is 25 degree celsius only this current capacity changes with the change in pulse type and vds if the pulse duration is more this peak drain current decreases this graph is called as safe operating area of the mosfet it shows a mosfet can work without exploding like a granite we can talk more about this later now we need to know the thermal characteristics of a mosfet this is the thermal resistance of the mosfet from junction to case or we can call it as junction to mounting base it denotes the capability of the mosfet to conduct the excessive heat out of it if you want to calculate the temperature rise in a mosfet then this formula is very helpful where this is the thermal resistance between junction to ambient the pd is the power dissipation across mosfet this is the ambient temperature where our circuit or system is working well the r theta j and r theta jc both are different quantities but we cannot use r theta jc in our calculation this r theta jc is given in this data sheet but many mosfet data sheets have r theta j a value the power dissipation of the mosfet is addition of different power losses which are conduction power loss switching power loss gate charge loss and dead time loss etc we'll see about this losses in detail in coming videos when we are using a mosfet in the third quadrant operation or let's say there is an inductive load then this body dial of the mosfet comes really handy its forward voltage drop is actually the source to drain voltage of the mosfet well these are all important parameters of mosfet we need to consider while checking the data sheet next time we'll see how a mosfet turns on or turns off with its waveforms till then stay tuned i've added all the reference on this topic in the description i hope you got something from this if you haven't you can watch the video again still if you don't you can ask your doubts in the comment box below hit the like button if you like this video subscribe to my channel and finally thanks for watching you

Channel: Foolish Engineer

Views: 48,860

Keywords: foolish engineer, electronics, voltage, current, electronics basics, power electronics, arduino, transistor, battery charger, BJT, NPN BJT, Transistor as a switch, Low side switch, NPN BJT as a low side switch, PNP BJT, Bipolar Junction Transistors, BJT basics, BJT working, PNP BJT workig, overvoltage protection, BJT as a protection, MOSFET, MOSFET working, MOSFET switch, MOSFET transistor, BJT vs MOSFET, Difference between MOSFET & BJT

Id: mZKTafaF3xc

Length: 9min 0sec (540 seconds)

Published: Thu Jan 13 2022