Confirm Mosfet Power Dissipation Calculation - Page 1
Forum section: electronics / general

author: LittleRain
author_rank: Contributor
timestamp: May 09, 2022, 11:57:58 pm
content: Can someone please confirm if I have calculated the power dissipation for a mosfet correctly?I will be using an N-channel mosfet on high side for overvoltage/reverse polarity, with a boost converter driving the gate.The part I used to test the formula is the AO3400.This is the formula and values I used. Not sure if everything correct.Code:I = Current = 5AT = TJ(HOT) = 150°CR = RDS(on) = 52 / 1000 (52mΩ)S = TSPEC = 25°CH = RDS(on)HOT = 1 + R * (T - S) or R * 1.7V = VIN = 3.3VO = VOUT = V - (I *R)P = PDResistive = ((I * I) * H) * ( 1 - (O / I)) = 0.166439WWhat I'm really unsure about is if I used the correct values for TJ(Hot), resulting in RDS(on)HOT also being incorrect.Does everything check out?Edit: Updated the equation.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 10, 2022, 12:42:55 am
content: An easy way to spot certain kinds of errors: attach a unit to everything, and follow the units through as you would any other algebraic factor.Any time you have addition,the units must match.If they do not, a conversion factor, or multiplier or ratio, is needed to get them to match.Multiplication, division and exponentiation work as normal: you can have a unit raised to whatever power, including negative (flip sign if it's above/below the division line) or fractional (e.g. 0.5 i.e. sqrt). All that matters is that it's done consistently, and the above condition holds.Also, that whatever unit you get out, is somehow useful.Which solves a whole heck of a lot of physics just as it is: how much weight does 1kg exert? Well, 1kg is mass, it's not force. We need a conversion factor to force. F = ma tells us we need acceleration. If it's  unaccelerated sitting on the Earth, well a = 9.8 m/s^2, and we get kg m/s^2 == N, a unit of force, bingo. And if we have several, well we can't just sum up the masses and forces willy-nilly, but we can take the sum of all one, or all the other, and convert to mass or weight as needed.So here, you've got "R = RDS(on) = 52 / 1000 (52mA)", which I should assume the mA is just a typo for mOhm (mΩ if you like to be proper). But lower down you have,H = RDS(on)HOT = 1 + R * (T - S)and presumably T and S are in °C (well, S is in C, but T isn't in anything at all!), so H is in units of ohm °C, which, isn't very useful, and definitely can't be multiplied by I^2 to get power. (The 1 - (O/I) expression is a nice dimensionless ratio, no worries there. Which, note there's a '1' in the above expression, which needs suitable units as well!)So we can easily spot the problem: you've got temperature without a conversion factor to resistance.And the "1 + R" makes it look like it was supposed to be a normalized change, which should actually be written: R' = R * (1 + normalized_change). But most of the time they give normalized value, not change from nominal, so it's just R' = R * (value on the plot at temp T). This is given in Figure 4.Easier to not fiddle with a stack of equations and just pick a likely Rds(on) from the temp curve. It's nonlinear anyway -- it increases faster at higher temperature -- so a simple (linear) arithmetic expression can't capture the details of it, and in general you'll need to solve for an initial value, then keep testing it against the curve, iteratively, until the result matches (that is, going between P and R, to solve for P and T together). (On the upside, it's a fairly flat curve for this particular part -- lower voltage FETs have less bend in this curve -- and a linear fit wouldn't actually be bad at all. But a fitted value, it is -- a magic number, a conversion factor, or calibration if you will, between T and R. And so it has the combo units of Ω/°C.)You also need to account for Rds(on)max, which is given in the data tables but only for certain points on the temp curve (usually 25C and Tjmax?). Presumably the tempco is the same for nominal or max Rds(on)s, so you can use the same ratio (usually it's given as normalized resistance, not normalized change, so the curve is 1.0 at 25C and is simply the given multiple of Rds(on)nom elsewhere -- no "1+" needed).Max is due to manufacturing variation between parts; you aren't likely to get one quite so bad, the average case really is quite typical -- and maybe it's worth rejecting them if they do perform so badly -- but you'll need to do that test to find out, so if you want a streamlined production test without full current and power and temperature, just design with max in mind.(Another 'dimensional analysis' fact: most functions are dimensionless. Or some special kind of ratio dimension, if you like. e^x works with dimensionless x. You can think of sin x as using units of radians, since its argument is angular. They return dimensionless numbers -- pure ratios. So you can have a sine wave y = sin x, in the abstract -- but a real wave that you measure, might be better expressed as: v(t) = A * sin(2π * (t * F + φ / 360)), with A == (V) (read: "in units of"), 2π == (rad/cycle), F == (cycles/s) and φ == (°), for t == (s) and v == (V).)Tim

author: LittleRain
author_rank: Contributor
timestamp: May 10, 2022, 02:25:21 am
content: Thanks you very much for the informative answer, it is much appreciated.I was in the middle of writing a proper reply, but I keep falling asleep at my desk.So I'm going to have to come back to this and respond tomorrow after a good nights sleep.

author: Faringdon
author_rank: Super Contributor
timestamp: May 10, 2022, 01:10:25 pm
content: Your Vin is only 3.3V and you only have 5A....you could use a 15v/20V SOT23 NFET and get much lower rdson.......rdson is never more than twice the 25degc value.QuoteTo sleep better.....reduce sugar intake (eg cakes, candy) and eat plenty of fibre, eg raw apples and carrots......never eat before bed..unless its high fibre food.As an EE, sat at desk often, you(all of us) will burn out if you dont follow the sleep advice.To cool the SOT23....have it on the biggest pads possible...ie commensurate with your PCB assemblers maximum allowed pad size.....and sneak it up a bit more because they always ask for too small pad sizes......and the Standard ones you can download are too small.....its amazing how much cooler a SOT23 will run if you increase the pad copper area by just 30%.

author: LittleRain
author_rank: Contributor
timestamp: May 11, 2022, 03:42:54 am
content: I have updated the original post with the revised formula.It seems extremely low, so I'm not sure if its correct.Quote from: T3sl4co1l on May 10, 2022, 12:42:55 amYou are correct that is a typo, I originally had it written as mohm, made some changes and accidentally put mA.Ok that makes a lot of sense,I was following this technical documentand the use of square brackets and round brackets was confusing me as to if RDS(ON)SPEC was included in the equation.I suppose if I had known the rules you mentioned I would have realized it would create a useless value (omh/°C).Re-reading the document it is very obvious that it was required, as almost all of the other equations include square brackets. I'm going to have to blame lack of sleep on that one.This is the equation for H from the document.RDS(ON)HOT = RDS(ON)SPEC [1 + 0.005 × (TJ(HOT) - TSPEC)]For some reason I replaced 0.005 with R. I think it may have been that I didn't understand what the 0.005 was, and it being a value fairly close to R I ended up making a stupid assumption that it was an example for 5mΩ.So I have changed the H equation to this.H = R * (1 + 0.005 * (T - S))The result checks out, as it is almost exactly the same as your advice to just multiple R by the value in figure 4 (0.0845 vs 0.0884).Maybe I just got lucky because of the linearity of the part.So now P equals 0.1664... Is that really all the power that is being dissipated, it seems extremely low to me.If I just take the voltage drop (0.26V) and apply ohm's law using 5A I get 1.3W.I originally was going to use TO-252 part, but if it is really this low I think I may actually use a SOT part.If it is still incorrect, could you please show me the correct answer so I can check my result to it?Oh and thanks again, there are a lot of very helpful tips here.I hope to one day have math skills such as yours.Math used to be my best subject in school, and I used to be able to do multiplication and division quickly in my head.Then 7 or so years ago I ended up in a coma, and ever since then my math skills have been lacking.Remembering 2 numbers at once is tough for me now.Weirdly enough my memory is still great, writing programs I can remember where and what every function does and its name, its just if I have to remember more than 1 number at a time.Can't blame that for being bad at algebra though, that's just because I haven't really done any in 15 years.Only really started to again when I started working with FPGAs and battery operated circuits, when I needed to start worrying about power consumption.Quote from: T3sl4co1l on May 10, 2022, 12:42:55 amYes I have been designing with max in mind, I added all the IC's maximum current draw which ends up being around 5amps, as well as using the 2.5V RDS(on) at 150C.The actual circuit will most likely require much less than half, as I made a generous estimate for the FPGA.Quote from: Faringdon on May 10, 2022, 01:10:25 pmJust to confirm you mean 15/20V VDS?I will take a look, but for 50 units for about $1 CAD excluding shipping its hard to beat.I originally wasn't going to use this part, I had planned to use a TO-252, but now that I realize the power dissipation is a lot less than I thought it was I probably will use a SOT-23.Quote from: Faringdon on May 10, 2022, 01:10:25 pmYes I am aware of that thanks.A couple days ago I was looking at a SOT-23 that could dissipate up to 3 watts, but while reading the datasheet I found out it required a full square inch of copper to be able to handle 3 watts, gave me a chuckle.I assume they were trying to abuse the parametric search, who knows.Not sure why anyone wouldn't just upgrade to a larger package to save a ton of space at that point.But I guess if you were already using it else where, and had a lot of extra board space, like on a UI board with buttons and switches that were far apart, it could make sense to keep part count down.It would actually be nice if datasheets included something like that more often, but when it shows up in a parametric search you are under the impression it is for a standard package.Quote from: Faringdon on May 10, 2022, 01:10:25 pmYeah I don't eat much sugar at all, only decaf coffee as well. If I even eat a small piece of candy I'll be up until 5am.I always eat a bowl of raisin bran before bed, the sugar in the raisins probably isn't the best.Thanks though, it is good advice for many.

author: uer166
author_rank: Super Contributor
timestamp: May 11, 2022, 04:16:43 am
content: Maybe a different thought process might help.. Consider that a MOSFET's characteristic when fully "ON", largely match a regular old resistor. Sure, they might change with temperature a little bit, but given a fixed temperature, what would the Pd of a MOSFET be if it matches that of a plain old resistor that had R=Rdson. Hint: you need only Ohm's law to figure this out.

author: LittleRain
author_rank: Contributor
timestamp: May 11, 2022, 04:31:51 am
content: Quote from: uer166 on May 11, 2022, 04:16:43 amOk so for a 100Ω resistor connected to 5V and GND, it would be 0.25 watts.What's really confusing me is for high side, source is 5V, drain is 4.8V, would I use the voltage drop in the calculation?Edit: Never mind, I got it.Looking at all these power dissipation formulas made me think it was much more complicated than that.I had actually used ohms law with the voltage drop in reply #2, but it wasn't even close to what the formula produced, 160mW vs 1.4W, didn't know if either was the correct way.That is something I should have realized much sooner, I am frequently calculating the current in voltage dividers to see how much current is flowing to a regulators feedback pin, don't know why it was so unclear that a mosfet would be exactly the same.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 11, 2022, 08:01:02 am
content: Quote from: LittleRain on May 11, 2022, 03:42:54 amWhat are you using for Vin and Vout?Notice that your reference is discussing synchronous rectification: this is when the transistor is switching on and off, opposite another one -- by averaging the "up" and "down" voltages, a new (lower) output voltage is obtained, without wasting power in the process. (Or put another way, you get more current output than input -- a sort of DC transformer. Or, equally well, it works in reverse, boosting a low voltage and high current, to a higher voltage at lower current.) They aren't very specific about this I think, hence the confusion (and, if you don't know what a synchronous rectifier is, well, that too). Well, for that application, Vin and Vout differ, and the duty cycle is given by the ratio of voltages. That's why they put it here. The switch is only carrying current while it's on, and off the rest of the time, dissipating zero. So P has to be less than I^2 * R, and how much less is given by the duty cycle, given by the voltage ratio (and that one-minus part). So that's what the extra stuff on the right is for.Your OV/RV FETs will simply be at 100% duty, so the whole right side term goes away (=1) and you get P = I^2 R.QuoteAlso, I guess they give as justification, the tempco: 0.5%/C being the upper limit, so they put that in the equation. This must have units of 1/°C, which they kindly omit for... clarity, I'm sure.And as I mentioned, it's worse for higher voltage MOSFETs; this figure isn't true in general -- the Rds(T)/Rds(25C) curve goes up to 2.5-3x for 600V+ parts near Tjmax (150-175°C). And, not that you're working with OV/RV protection at those kinds of voltages... just for flavor.Always check the datasheet (RTFDS, Read The "friendly" Data Sheet)!And yeah, for low voltage parts like this, where the curve is linear, this linear fit works fine.QuoteBingo, you're doing DC (at least, I assume so), so this is the correct result.You might consider getting a lower Rds(on) part. SOT-23 aren't good for much over say 500mW, and even that may be pushing it.Note that supplier/datasheet headline "drain current" ratings are optimistic at best; indeed D(2)PAKs they outright lie to you and assume you have the capability to immerse the part in boiling freon at 25°C -- it'll keep the tab nice and cool sure, but how in the &*$! am I supposed to do that on a PCB? Simple, you don't. SOTs at least they tend to use thermal figures for typical scenarios (minimal footprint, 2-layer, 4-layer, etc.), and DPAKs they usually give a few too (just not the headline figures).So, to put it another way -- you could very well have identical chips in SOT-23 and DPAK packages, and one is rated a measly ampere and the other ten. Because the DPAK can ideally shed off 50 goddamn watts or something like that. But it won't ever in practice, so you need to read below the fold and see what it'sactuallygood for.All that really matters is chip temperature, and all of these typical figures assume, well, typical conditions -- if your ambient isn't 25.0°C, maybe it's still air inside a box, maybe it's got extra cooling (heatsinking, thermal pads, etc.), whatever -- all you can do is estimate the thermal resistance for your application, add the part's to it (RthJA or RthJC as appropriate) and that's your actual temp rise above ambient. And then, whatever current corresponds to the power that can be handled at Tjmax, in that setting, that's what you can run.So yeh, rather roundabout, and in our defense as engineers, heat transfer and convection are notoriously difficult topics. Heat transfer not so much really, in the grand scheme of things, but it's maybe so seemingly trivial that it's easy to overlook -- until you have half a million gadgets getting warranty returns due to a design oversight.Comes to mind, it's like statics class. That is, the study of objects and structures just resting in place. Basically Newtonian physics, but no motion, just force balancing; weight, tension and compression, rotational moment. Such an easy class, but deceptively so; most students did rather poorly in it as I recall.QuoteAh yuck, coma is definitely no good! But you made it through, that's great.On the upside, computing is just applied mathematics -- more on the logical side (program flow, conditional statements, etc.) than numeric, but numbers derive in turn from logic, so it's all the same in the end. Maybe give yourself some exercises, practice symbolic representation of program statements, data flows, expressions, etc. Turn algebraic expressions into program code, and, that can lead into numerous quirks of our machines, like integer overflow, limitations of floating point numbers, etc. (For real numbers, a*(b+c) = a*b + a*c, and (a*b)*c = a*(b*c); but for floating point, these are only true in very particular cases -- understanding how floats are aligned and rounded off, makes a huge difference to the numerical accuracy of a chain of operations.)Might also take a look at little refresher problems, or riddles or what have you. This comes to mind,https://www.youtube.com/c/MindYourDecisionsalso, 3Blue1Brown and other math-education channels have FANTASTIC content, ranging from simple (algebra, geometry?) to ludicrous (say, a surprisingly simple explanation of a topic deep in analytical calculus).I have no idea what kinds of things might capture your interest, or prove helpful, if you're even looking for that sort of thing -- or if you aren't aware of them already! -- but there's lots of good stuff out there to be found, in any case.QuoteKeep in mind, dissipation is a surface thing. You don't magically get more dissipation from a DPAK, if the pours are the same size as on the SOT-23.Well, not more beyond the basic effect of heat spreading: you get a lower Tj for the bigger device, because its tab is thicker and wider -- spreads out heat further before it even flows into the board. Whereas a SOT-23 in the middle of a board can't do any more than heat the tiny bit of board around it, which heats a little more around that, which... Each little ring of board-heating comes with a small temperature drop, and you add up all the drops from all the rings to get the total. Well, that temp drop is a heck of a lot higher for tiny rings with little circumference, so you do have some advantage using a bigger part there.Beyond that though, if you're talking whole watts -- you simply need board area, or heatsinking. A square inch is good for a watt or so, if you don't mind it getting fairly toasty in the process. That's a square inch of board area, component surface, heatsink fins, enclosure, whatever, doesn't matter -- air doesn't know the difference.Tim

author: LittleRain
author_rank: Contributor
timestamp: May 12, 2022, 08:01:08 am
content: Quote from: T3sl4co1l on May 11, 2022, 08:01:02 am3.3 for the VIN, and 3.03 for VOUT.I did notice it was for switching, but then saw this "PDRESISTIVE = [ILOAD² × RDS(ON)HOT] × (VOUT/VIN)," which looked exactly like other equations I had seen for non switching applications.I'll be honest, I basically only read the equations, and used Ctrl + F to look for certain keywords like "TSPEC" to find out which value I need to find.Probably shouldn't try to rush things like this to save myself from the embarrassment of looking like an idiot.I've been trying to finish this power supply for the project, or at least the BOM, before I have to go away for a few weeks, which is in a few days.Should probably have left it until I'm back and have more time, but I need to place an order before any more IC's go out of stock, and have to redesign my other boards for the fourth time.Quote from: T3sl4co1l on May 11, 2022, 08:01:02 amAhh ok that makes a lot more sense.I made another assumption that the right side had something to do with the drain vs source voltage (voltage drop across it).The dividing didn't look right to me for what I believed its purpose to be. Should have trusted my instincts, but this little problem has made my confidence fluctuate like a sine wave.Quote from: T3sl4co1l on May 11, 2022, 08:01:02 amHaha that must be why.I swear I do, but point taken.Quote from: T3sl4co1l on May 11, 2022, 08:01:02 amYes you are correct.Originally I did think it was as simple as that, then I doubted myself and googled "Mosfet Power Dissipation" calculation.After seeing all these complex(more complex) formulas I was really unsure and thought maybe it isn't as simple as I thought.Was too embarrassed to ask such a simple question, so I thought I'd give the formula a go and ask for confirmation instead.Quote from: T3sl4co1l on May 11, 2022, 08:01:02 amAlready on itAfter seeing the voltage drop after 2 or 3 mosfets (third one is required for ORing the USB port after an LDO) I realized it wouldn't meet the voltage requirements of some of my ICs unless I boosted the voltage before hand, which gets messy.The board already has 2 boosts, 3 bucks and 4 LDOs, and I really don't want to add any more.Because a USB port can only supply 500mA, I have to design it with at least two 3V3 rails so the USB port only powers certain components to allow updating/configuring when the main supply is absent.Do not worry a SOT package will not be used, well SOT-23 at least.If I can find a decent SOT-89 or SOT-223 they will be my first pick, if not I will go with a DPAK.I think I see what you are saying, its the DPAKs TAB size which connects to the copper which does the heavy lifting.Quote from: T3sl4co1l on May 11, 2022, 08:01:02 amYes I could see that as being a helpful way for me to look at equations, as it is something I am much more familiar with.Math exercises have definitely been on my to do list for a while, quite a few years now.I think I took 1 little course on khan academy back then, and unfortunately never got back to it.This time around I'll make sure to find some extra time to spend on stuff like this, and keep to it.As the complexity of my projects are ever-growing, I find it is a skill that is required more and more.At first glance that channel looks very familiar, I will definitely check it out more tomorrow.As for 3Blue1Brown I have been subbed for quite a few years now, while I don't watch every video, I can confirm it is great content.Quote from: T3sl4co1l on May 11, 2022, 08:01:02 amOh ok, I realized it was important, but not how important.It is pretty crazy how many things need to be taken into account when working with tight designs.I guess that's why its where the big bucks are.Anyways, thanks again Tim.I really appreciate all the help and advice you have given me, there is a lot of solid stuff you provided.Rain