power led circuit design

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 akaSat 0:37How are you going to solder the LEDs? What are you going to mount them on? tlfong015153#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. OYPS185@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. OYPS185Sat 0:37@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. OYPS185@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… user289103386Sat 4:03Regarding 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  8 hours later… tlfong015153Sat 12:27@user28910 Yes, I fully agree. It is not just marginal, but out of margin.

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@OYPS Some more references you might find useful:(1) What’s the voltage used to power LEDs inside an LED light bulb?
electronics.stackexchange.com/…(2) Control a 3W RGB LED with an Arduino
electronics.stackexchange.com/…(3) Constant current source for resistance measurement
electronics.stackexchange.com/…(4) Constant Current Source Learning Notes V0.1 – tlfong01 2021feb02
penzu.com/public/bda5986a  6 hours later… OYPS185Sat 18:56@tlfong01 thanks for the links. I did not read all of them yet, but they were already pretty useful. After getting all the answers to my question i was able to filter the existing MOSFETs much more precise. Especially in regards of Rdson. Is an Rdson of 250Ohm@2.5V suitable for my application. I was not able find one at 2.7 or 3.3V.For example this one from Infineon: mouser.de/datasheet/2/196/…OYPSSat 19:15250mOhm@2.5V of course.  20 hours later… tlfong015153yst 15:27@OYPS Ah, 2.5V is just a “benchmark” for you to “extrapolate“. I think your choice is already very good. It would be perfect if you can find something similar, but with a DPAK package which has a SMD heatsink. 🙂

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  3 hours later… tlfong015153yst 18:10@OYPS So far we are discussing on low side current switching. You might like to explore high side switching, as shown below.

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Yet there is one more switching configuration, or constant current switching/limiting/source called “True Floating CCS”, which sits between Vcc and ground, in other words, and does not connect to Vcc or ground, as referred above:(4) LM334Z CCS Learning Notes V0.1 – tlfong01 2021feb02
penzu.com/public/bda5986atlfong015153yst 18:29@OYPS This is P-channel power MOSFET, also very low level Vgs(th) which you might consider high side switching.

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  9 hours later… OYPS1853:39@tlfong01 Thanks for your reply. What is the advantage/disadvantage of high or low side switching ?On which use case would I choose on over the other?Are the 250mOhm a problem? Some answers on the original question suggested, that Rdson could/should be lower than 200mohm. As far as I understand it, I should just try to* minimize this  8 hours later… tlfong01515311:38This might help:High-side versus low-side switching – Lednique
lednique.com/gpio-tricks/…And for ninjas only: GPIO high-side driver fail – Lednique
lednique.com/…tlfong01515311:59Well, comparing to the power NPN BJT such as Darlington TIP120/122 with high Vce(sat), power MOSFET’s ridiculously low Rds(on) is too good to be true. You might like to read the following to get a rough idea of what I am talking about:
Determine if a heatsink is required (Darlington TIP120)
electronics.stackexchange.com/…tlfong01515312:12So for low currents, say, less than 1A, you don’t need to bother Rds(on). What you need to care is the current limiting series resistor of the power LED in your inefficient, power wasting circuit. And that is why I always strongly recommend the power LED newbies to at least use the much more efficient “Two NPN BJT current mirroring, negative feedback CCS (See Wiki on ‘Current Source’)”. 

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