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Understanding MOSFET characteristics

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I am trying to select a N-Channel MOSFET to switch a 12V power rail. The use case here is to switch on/off a couple of LEDS which are wired in series. The LEDs (datasheet) can draw a maximum of 700mA. I want to control the MOSFET via a MCU (3,3V logic level). I am now struggling to choose a suitable MOSFET, mostly because I do not understand some of their characteristics.

What I know so far is:

  • MOSFET needs SMD package for my application
  • Vgs(th) should be smaller than my 3,3V logic level
  • FET type should be N-Channel

What I dont know:

  • What does Continuous Drain Current (Id) mean? What is important here?
  • What does Drain to Source Resistance mean? What is important here?
  • How important is the Maximum Power Dissipation?

For more information on the circuit you can look at my original question here. Thanks a lot in advance!


EDIT:

As suggested in the comments, I added an example circuit. 


EDIT:

Following the input from the comments, is this MOSFET suitable for the use case?mosfetcomponent-selectionShareEditFollowFlagedited 8 hours agoasked 15 hours agoOYPS4166 bronze badges New contributor

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5 Answers

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What does Continuous Drain Current (Id) mean? What is important here?

It means continuous current (as apposed to pulses of current) between the Drain and Source. Generally it is specified in the ‘maximum’ or ‘absolute maximum’ ratings section, which means it’s the absolute maximum current the FET can pass continuously (at the specified case temperature) without getting so hot that it burns out. You need to keep the actual Drain current well below this for reliability and efficiency.

What does Drain to Source Resistance mean? What is important here?

It’s the ratio of voltage drop across the Drain-Source junction to current passing through it, when the FET is fully turned on. A Drain current should be specified for this because at some higher current the resistance will increase dramatically as the FET ‘saturates’. Temperature should also be specified because the ‘ON’ resistance also increases as the FET gets hotter. Several Gate voltages may also be specified because at lower Gate voltage the FET will not turn on as hard (ie. it will have higher ‘ON’ resistance) and it will reach saturation at lower current.

RDSon is the most important characteristic you need to calculate voltage drop, power loss, and temperature rise.

How important is the Maximum Power Dissipation?

Not very important, because it is not a realistic number. What is actually important is how hot the FET gets (ie. its junction temperature), which is dependent on heat sink performance and ambient temperature as well as power dissipation. In practice these numbers are poorly defined, so temperature cannot be accurately predicted and you cannot run the FET at its ‘maximum’ power dissipation rating without severe risk of overheating it – which makes the figure almost useless.

For reliability and safety the FET should be operated well below its maximum junction temperature, and for efficiency you want it to dissipate negligible power compared to the load. Therefore in practice the actual power dissipation generally needs to be much lower than the ‘optimistic’ datasheet spec.

What I know so far is:… Vgs(th) should be smaller than my 3,3V logic level

Not just smaller, much smaller. ‘Threshold’ voltage is the Gate voltage at which the FET just begins to turn on. It is typically specified at a Drain current of only 0.25 mA, and varies over a wide range between individual units. If the threshold voltage is 3.0 V the FET will not turn on fully with 3.3 V. The minimum voltage you should use is the one specified in the RDSon spec.ShareEditFollowFlaganswered 5 hours agoBruce Abbott42.6k11 gold badge3434 silver badges6262 bronze badgesadd a comment4

These parameters are related to each other and are important because they determine how hot the MOSFET gets during operation. Excessive junction temperature will destroy the device quickly. It is important to note that these parameters are specified under a certain set of conditions, such as junction temperature, gate voltage, drain current. Datasheets have graphs describing how the parameters change as these conditions change.

A MOSFET is not an ideal switch; it has a small amount of resistance when it is on. This is the drain-source resistance parameter. Note that this is normally spec’d at 25 deg C junction temperature; its value can double at max temperatures.

The drain current is the maximum continuous current the device can conduct. Note that this value is usually spec’d at 25 deg C case temperature, which is normally hard to maintain. It should be regarded more as a figure of merit rather than a realistic maximum current for the device.

The drain current flowing through the drain-source resistance causes a power loss in the device, which is manifested by heating at the junction. This heat has to dissipate into the environment; heat flow is limited by the thermal resistance of the package, which is spec’d on the data sheet. The maximum power dissipation is the maximum amount of power that the package can dissipate without exceeding the maximum junction temperature. Again, this value for the condition that the case is maintained at a certain temperature. As in the case of the drain current, this figure is only a starting point.

So, how do you know if the chosen MOSFET is suitable, from a heating point of view? Most MOSFETS have a max junction temperature of 175 deg C, but you really don’t want to be operating that high. 100 to 125 deg max is more reasonable. So, take your device, look up its drain-source resistance at 100 degrees. You know what the current is, so calculate the power dissipation: power = amps squared times resistance. Multiply this power by the thermal resistance, add this temperature rise to the ambient temperature, and see how close the junction temperature is to 100 degrees. If it’s under 100, then you’re good. If it’s way under, you could maybe choose a smaller MOSFET. But if it’s over by more than a little, you should choose a device having lower resistance, or in a package having a lower thermal resistance.ShareEditFollowFlaganswered 6 hours agouser289103,1411212 silver badges1313 bronze badgesadd a comment3

The two things I would look at first are drain to source resistance which is abbreviated “RDS(on)” and the VGS at which RDS(on) is specified.

It is a mistake to focus too much on VGS(th) because this is the point where the transistor starts to turn on. You want to make sure that when you turn it on, it is “on like Donkey Kong” at 3.3V. It cannot be just kind of sort of starting to turn on.

Practically speaking you are going to be looking for a transistor where RDS(on) is very low and is specified at 3V or 2.7V.

There may be transistors out there that do not specify RDS(on) at 2.7V but would work for you. But it is more complicated to try to figure that out so I would start off looking for something that is fully specified at the voltage you want to use.

In your case you probably want the RDS(on) to be less than 0.2 ohms (you want the maximum less than this) to keep heat and voltage drop reasonable. That will give you 0.14 V of drop, and around 100 mW of power dissipation in the transistor. You can probably find a transistor with RDS(on) much lower than 0.2 Ohms, and in that case the dissipation and voltage drop will be negligible, which could be a good thing for peace of mind.ShareEditFollowFlaganswered 5 hours agomkeith18.1k11 gold badge1818 silver badges4848 bronze badgesadd a comment2

A FET acts more or less as a current-limiting device between drain and source, with the magnitude of the current being set by the gate-source voltage. Graphs of current vs voltage are typically published in datasheets so it’s fairly easy to predict how a particular device will behave in-circuit. In your application you’ll want to find a device that can easily pass 700mA with a gate voltage of 3V or somewhat less. As a FET nears saturation (I.e. switched fully on) it behaves increasingly like a resistor. The exact behaviour varies somewhat with temperature and can vary from one device to another, so it’s wise to allow a good safety margin, but most unwise to rely on the device as a current-limiter – for example the datasheet may show that the FET will allow 700mA to flow when 3.3V is applied to the gate, but this should certainly not be relied upon- use a discrete resistor instead. You can, however, use the FET’s resistance value to estimate the power dissipation for the device. Without significant heat sinking you could estimate 300mW for a SOT-23, 1W for a SOT-223, 3W for a DPAK. The headline power figures in the datasheets often call for significant heat sinking.ShareEditFollowFlaganswered 5 hours agoFrog1,97422 silver badges77 bronze badgesadd a comment2

What does Continuous Drain Current (Id) mean? What is important here?

A: This is continuous DC drain current the transistor can handle. Normally you want this about 1.5x your target current.

What does Drain to Source Resistance mean? What is important here? A: Allows you to calculate the IR drop in the transistor itself. Normally this is specified for a certain Vgs.

How important is the Maximum Power Dissipation? A: Maximum power going through the transistor that the device can handle. Again you want this to be about 2x the power going into your load.ShareEditFollowFlaganswered 7 hours agoBrian G6133 bronze badges New contributor

  • 1Thanks a lot for your reply. This clarifies a lot of questions i had, especially your recommendations! – OYPS 7 hours ago
  • 2Those factors seem arbitrary and are far from sufficient for selecting a suitable MOSFET. Maybe it would also bei helpful to explain why the proposed MOSFET is not suitable here. – Lars Hankeln 6 hours ago
  • 1By long tradition, continuous drain current in power MOSFET’s is kind of a fantasy specification. – mkeith 5 hours ago

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Q: Understanding MOSFET characteristics

OYPSI am trying to select a N-Channel MOSFET to switch a 12V power rail. The use case here is to switch on/off a couple of LEDS which are wired in series. The LEDs (datasheet) can draw a maximum of 700mA. I want to control the MOSFET via a MCU (3,3V logic level). I am now struggling to choose a suita…mosfetcomponent-selectiontlfong01For prototyping, I would suggest to try TO220 through hole 3V3 logical level Vgs(th) IRL540N N-channel power MOSFET: infineon.com/cms/en/product/power/mosfet/…. It might be risky to use SMD device. Bimpelrekkie72.1kWhat does xyz mean? What do you think it means? What do you think would happen of xyz was very small/large? Also it is strongly recommended to add a schematic so that we can see HOW you will be using the MOSFET. Your “What does xyz mean?” tells me you’re not that experienced so in order to give sound advice I need to see how you’re going to use the MOSFET. For that, include a schematic. Some xyz are more/less important depending on how you use the MOSFET. OYPS@tlfong01 thanks for your reply. I will try that one for prototyping.@Bimpelrekkie thanks again for your reply. I will add a schematic shortly if it helps to clarify the question Andy akaAn N channel MOSFET is an inappropriate choice for controlling a 12 volt power rail unless you are prepared to add a voltage boost circuit. It’s better to control the 0 volt rail with an N channel MOSFET. Bimpelrekkie72.1kI agree with starting with a commonly used MOSFET like the IRL540. Chances are that it will do the job. As a beginner you’re over-worrying about selecting the right component while experienced designers (like me) know that many commonly used devices can do the job (assuming you’re not doing anything extraordinary). Also beginners often over-worry about the components but then use the wrong circuit!What Andy mentions is a extremely common trap for beginners that 9 out 10 fall into (and then come here and ask why it doesn’t work). And that’s why we need to see a schematic. Are you making a source follower or not? Andy aka0:37How are you going to solder the LEDs? What are you going to mount them on? tlfong015145#OYPS. I usually strongly recommend power LED lamp newbies, before starting to do any circuit design, to read the following: (1) One Watt LED – Components 101 2018mar17: components101.com/diodes/1-watt-led. After reading this short, newbie friendly tutorial, you might like to let me know if there was at least one important thing that you didn’t know that you didn’t know. Happy reading. Cheers.#OYPS, you might also like to watch how other guys are playing with power LEDs. (2) Power LED’s – Simplest Light With Constant-current Circuit – dan, Monkeylectric, instructables.com/… Happy reading. Cheers. OYPS@tlfong01 thanks for your useful articles.@Bimpelrekkie I have added an example schematic to the question. I hope this helps to clarify things.@Andyaka they will be reflow soldered on to a pcb. Andy akaAre you hoping to drive the LEDs at around 100 mA @OYPS OYPS@Andyaka no, i am pretty sure they will draw more than that. Maximum of 700mA. Why? Andy aka328kWell, 700 mA through an 18 ohm resistor drops 12.6 volts and that’s more voltage than the supply. Whereas 100 mA through an 18 ohm resistor drops 1.8 volts leaving 10.2 across 3x LEDs or 3.4 volts each. You also need to specify which LED specifically you are using.And, 700 mA is the absolute maximum rating where they may get destroyed if you go a little bit more. You should aim for no more than 500 mA. OYPS1650:37@Andyaka You are right. The schematic had the wrong Resistor value. It should be 1,8Ohm as calculated via this amplifiedparts.com/tech-articles/led-parallel-series-calcula‌​tor. The LEDs datasheet is listed the question. I can increase the value a bit to limit the current to 500mA. Thanks for pointing this out. BimpelrekkieYou didn’t try to use a source follower which is good. You’re switching the – side with an NMOS also good. Re-calculate that resistor and also how much power it is going to dissipate! It will need to be able to handle that or you will get a burned resistor. Hearth16kYou say that Vth should be smaller than your logic level. Good! That’s right. But also it must be much smaller than your logic level. A common pitfall people fall into is thinking that Vth is the voltage at which the FET can be considered fully on, when that’s not true at all. It’s the voltage at which it just barely begins to conduct, and depending on the FET you may need to be as much as several times Vth to turn it fully on. You want a FET with an Rdson specified at a Vgs of your logic level or less, and specified to be low. OYPS165@Hearth thanks for pointing that out. Is the following MOSFET suitable for this application: lcsc.com/product-detail/…. I selected it on behalf of your information, selecting low Rdson at the 3,3V logic level.I updated my question with a MOSFET suggestion based on your inputs.I updated my question with a MOSFET suggestion based on your inputs. @Bimpelrekkie I updated the resistor and calculated the power dissipation. The resistor should be able to handle 2W. Thanks for pointing that out again!  3 hours later… user289104:03Regarding the DMN10H120SE MOSFET… If you look at figure 2 on the data sheet, “Typical Transfer Characteristics” you will notice that at 3.3V, the device is barely on, if at all. You really should have minimum 4.5V gate-source. Drain-source resistance for this device is spec’d at 6V and 10V, btw  The last message was posted 5 hours ago.

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