Buck converter design - MOSFET power rating calculation? - Page 1
Forum section: electronics / beginners

author: nsayer
author_rank: Regular Contributor
timestamp: July 13, 2014, 06:30:48 am
content: I've designed a Raspberry Pi power supply. I call it Pi Power.Preliminary tests show it performs very well. I've put 2.5 ohm, 5 ohm and 10 ohm load resistors on it, and the output voltage has stayed inside of 1% with around 25 mV P-P of ripple. The schematic is athttp://www.geppettoelectronics.com/2014/07/pi-power-schematic.html(there's one error - the inductor is actually 15 uH, not 33).The only issue I'm seeing is that under high loads, the MOSFET is gettingquitehot. I mean, "Ow!" hot. In fact, I blew one.Powering an actual Raspberry Pi, it's fine. But I intended this design to work up to 2A in order to be able to support extra peripheral loads folks might want to use.The MOSFET in question is this one:http://www.digikey.com/product-detail/en/SSM3J307T(TE85L,F)/SSM3J307T(TE85LF)CT-ND/2257794It is rated at 5 amps. I wouldn't think I'd be exceeding that. It has a maximum power dissipation of 700 mW. There's a formula in the datasheet that when I plug in the numbers would seem to say that the power dissipation in the MOSFET is only around 100 mW, but I'm not sure I did that calculation correctly.I could swap it out for one of these:http://www.digikey.com/product-detail/en/AO3415A/785-1193-1-ND/2353911They're only two cents more in single-unit quantities, and they're the same form factor. But I'd rather not just *guess*.

author: rs20
author_rank: Super Contributor
timestamp: July 13, 2014, 06:43:39 am
content: There's three things to check when speccing a MOSFET:-- Current ( & voltage): fine-- Power: you did this too, fine-- Temperature: no evidence that you did thisHave you got a 1 square inch heatsink patch of copper attached directly to the appropriate pin of the device? If not, your thermal resistance is 350 degrees C per watt, which works out to 43 degrees C above ambient when multiplied by the 124mW load you're putting on it. That's seems borderline, especially since that's ignoring switching losses entirely.Check that the switching frequency isn't ridiculously high. Calculate the rise and fall times you would expect with that driver and transistor combination, and probe the waveforms around the gate and drain of the FET to check that it's switching fast and cleanly at that speed. While you're at it, have a go at calculating the switching losses you wouldexpectif your circuit was acting as expected (LTSpice can tell you power dissipation) and see if it's more than 124mW.That alternative transistor you suggested has a higher on resistance, but a lower gate capacitance. Which one is better will depend on whether your current set up's power loss is dominated by switching losses or on-state resistance.What test input voltage are you using?

author: nsayer
author_rank: Regular Contributor
timestamp: July 13, 2014, 06:59:37 am
content: Quote from: rs20 on July 13, 2014, 06:43:39 amNot even close. There's barely a square inch of board total to this thing.QuoteYou're right that I didn't do anything in particular in the way of thermal planning. Where are you finding that stuff in the datasheet? I see an absolute maximum junction temperature of 150 deg C, but I don't see anything in there about thermal impedance other than a graph that talks about transient impedance.I may be able to add a tiny bit of a heat spreader to the board. Does the other part look any better to you from a thermal perspective? I at least see some specifications about thermal resistance in its datasheet.QuoteI will. I don't have any particular evidence that suggests to me that it's not, but I will anyway.

author: nsayer
author_rank: Regular Contributor
timestamp: July 13, 2014, 07:09:04 am
content: Oh, the test input voltage is 12 volts.

author: rs20
author_rank: Super Contributor
timestamp: July 13, 2014, 07:16:45 am
content: Quote from: nsayer on July 13, 2014, 06:59:37 amYou're right, the only thing there is that graph. That's unusual, normally FET datasheets have it as a proper figure. But in any case, the graph is good enough in a pinch -- you can see how the curves flatten off on the right around 1000 seconds (which, for a little component like that, is pretty much forever -- that's why the curve is almost flat at that point). Just extrapolate out from there -- take the 1000s figure, and add a bit.Quote from: nsayer on July 13, 2014, 06:59:37 amAs I said, whether the other part will be better depends on whether your switching losses or on-state losses are greater. The two parts are rather similar, though, the difference won't be that great. It's worth eliminating the possibility that your circuit is switching at 1 MHz or something silly like that first, and calculating the properties of the ideal part.Quote from: nsayer on July 13, 2014, 06:59:37 amIf a MOSFET is switching slowly or too often (i.e., spending too great a fraction of time neither fully on nor off), it will get hotter. So the fact that it's hot is good reason to check.Quote from: nsayer on July 13, 2014, 07:09:04 amThat means that the input current should be 2A with less than 50% duty cycle, right? That means it should be reaching only 20 degrees or so above ambient if switching is neglected. Looks like switching is the dominant source of losses.

author: nsayer
author_rank: Regular Contributor
timestamp: July 13, 2014, 07:28:08 am
content: FWIW, with a 1.5 A load, it was drawing 750 mA @ 12 VDC. That's around 83% efficiency, FWIW. Of course, the MOSFET was getting quite warm, but that's why we're having this conversation. Looking at the output ripple frequency, it appears to me to be switching at somewhere around 150 kHz. I'm being vague because I'm going by memory - it wasn't something I actually examined closely at the time.

author: nsayer
author_rank: Regular Contributor
timestamp: July 13, 2014, 07:31:39 am
content: Without a tremendous board redesign, I can make a 34 square mil heat spreader, with 8 vias in it.It doesn't seem sensible to me that that would have a meaningful impact on this situation.I could start looking at way, way over-spec'ing the MOSFET - like using a D-PAK package unit. It'd be twice as expensive, though.

author: rs20
author_rank: Super Contributor
timestamp: July 13, 2014, 07:53:07 am
content: Quote from: nsayer on July 13, 2014, 07:31:39 am34 square mils? Do you mean 0.034 square inches, i.e. 34000 square mils? (There's 1000 mm in a meter, there's 1000000 square mm in a square metre, etc). In any case, yes, not a huge change.Quote from: nsayer on July 13, 2014, 07:31:39 amIt's not really overspeccing if it's running at 20% of theoretical capacity. The figures in the datasheets are really bleeding-edge marketing stats for non-switching, very-well heatsunk devices in ideal conditions. It's perfectly normal to use a device at well less than its maximum current or power in real applications -- you'll get better efficiency as well as a cooler device (since these go hand-in-hand, of course). I'd pay the extra 50c if I was your customer.All this aside, though, you haven't yet figured out why your device is getting so hot. It may well be the case that getting a higher resistance mosfet with a lower gate capacitance (to reduce switching losses) will give better results -- generate less heat in the first place, rather than figuring out how to dissipate the heat.

author: rob77
author_rank: Super Contributor
timestamp: July 13, 2014, 08:44:16 am
content: i would definitely change the mosfet for a bigger one (in terms of package) sot-23 is way too small to dissipate any reasonable amount of heat.it will be far cheaper to redesign the board and use a bigger mosfet, than stay with sot-23 and then fixing the boards all the time with blown mosfetsalso pay attention to Rdson and related Ugs voltage when choosing the mosfet. - for example : the one you used is a "logic level" with 1,5V gate voltage , but the declared Rdson of 31mOhm is valid only while gate voltage Ugs is 4,5V.

author: nsayer
author_rank: Regular Contributor
timestamp: July 13, 2014, 08:55:03 am
content: The LM3485 "should" be able to swing the gate pretty hard. I haven't yet looked at the gate of mine on the scope (it's way, way past my bedtime), but the datasheet suggests that it should be a substantial fraction of Vin. At 12 volts input, it probably is upwards of 5 volts relative to the drain - my guess.This one looks like a likely candidate:https://www.digikey.com/product-detail/en/SI3443CDV-T1-GE3/SI3443CDV-T1-GE3CT-ND/3305367I had to make my own TSOP-6 library, but the capacitance appears to be half the other one, there's 4 pins of drain for thermal transfer, and I can add some chimneys under it that might help a little. Even without a square inch of copper, things look better to my relatively naive eyes, and the price is almost the same.

author: max_torque
author_rank: Super Contributor
timestamp: July 13, 2014, 11:38:40 am
content: Sounds to me that you need to start at the beginning and do some proper thermal analysis of your converter! Especially if you're going to sell it to the general public! It looks like you are also suggesting it can be used in an automotive environment, in which case, you need to take some very carefull estimates of operating ambient temp (remember, the inside of a black car, parked in a parking lot in the sun in Arizona can easily reach 50degC!). Also, your 16v TVS across the inputs is WAY low for automotive load dump........

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 13, 2014, 03:14:48 pm
content: I think Vgs(th) min on the transistor is too marginal.There is literally no description or specification on what the PGATE pin looks like. Is it a bipolar drive output? CMOS? What voltage is it guaranteed to swing to, and at what current? Evidently, the "on" state voltage is controlled in the 5-8.3V range, so it's not simply switching from VCC to GND.Besides that, I don't like the "hysteretic control". For starters, you aren't working with something very much better than a 34063 to begin with! I don't see enough, nor clear enough, discussion of design and use. I would not choose this part.I've used this before,http://www.ti.com.cn/cn/lit/ds/symlink/tps54233.pdfit seems to do the job. Missing features: VSENSE input bias current. Turns out it's a BiCMOS process, and they claim it's a CMOS input, so the feedback resistors can be almost any value, not just the rather low values given in the examples (10k feedback for a "near zero leakage" pin?!). You might like a little more current capacity, but there's more than a few in the series so I don't think that'll be a big deal anyway.Tim

author: nsayer
author_rank: Regular Contributor
timestamp: July 13, 2014, 03:27:23 pm
content: One of the big surprises in all of this is just how hot a 10w rated resistor (as a load) gets with 5 watts running through it.I've examined the gate and drain waveforms. They look *fairly* square. There's a little bit of rounding, but also from what I can tell the switching frequency is on the order of 300 kHz or so, so there's going to be.Going from 500 mA to 1 amp of load, nothing really changed except the duty cycle of the gate signal - it obviously went down from about 50% to closer to 33%. But it still ranged from a high of +12 (Vin) to around 8 volts or so.One of my prototypes has been running an R-Pi for a day straight now, and under stress none of the other components get more than finger-detectably warm.QuoteCan you elaborate on that? The purpose of that part is to try and protect the input from excessive surge voltages. It has a peak pulse current rating of 22A, and a standoff voltage of 17V - comfortably above Vsd max for the MOSFET (which at Vin of 17 volts would be 12 since the output voltage is 5) and well below the maximum input voltage of the LM3485.http://www.digikey.com/product-detail/en/SMBJP6KE20A-TP/SMBJP6KE20A-TPMSCT-ND/2345728

author: nsayer
author_rank: Regular Contributor
timestamp: July 13, 2014, 03:31:48 pm
content: Quote from: T3sl4co1l on July 13, 2014, 03:14:48 pmI don't believe there's any reason to redesign the whole thing at this point. Everything is working just fine except that the MOSFET is getting too hot under high (but within intended design spec) load. Lesson learned - I'm going to upgrade the MOSFET and hopefully it will meet the top-end current spec.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 13, 2014, 03:55:29 pm
content: Quote from: nsayer on July 13, 2014, 03:27:23 pmEven under normal operation, that TVS might end up toasted -- automotive is generally considered 8-16VDC, with transients ranging from teeny spikes up to the infamous load dump.Load dump is where the battery comes disconnected, or the alternator goes nutzo, or whatever, and over the course of a few hundred ms (well longer than <100us semiconductor failure periods), voltage and current ramps up and down, reaching a peak of maybe 80V and as many amps. The test is usually given as a specified voltage source, with 0.5 to 2 ohms ESR to define the current capacity. The pulse is long, so the available energy is huge (an 80V 1 ohm test delivers a peak 1600W into 1 ohm, and remains above half power for probably 100ms = 80J total energy -- try dumping that into a MOSFET!).Most automotive spec parts are rated at 60-100V, so they simply ride it out. You could also use an MOV, or rather massive TVS, to absorb some energy, but because these have significant internal resistance, you can't simply pick a 20V device and assume it clamps everything past 20.0V -- in fact, the peak is more like double the rated voltage.If this is intended for automotive environments, I suggest at least a fuse at the input -- so when (not if) the converter fails, it doesn't start a fire. Beyond that, a series polarity-protection diode would be nice, as well as some LC filtering (keeps automotive noise off your circuit, keeps your circuit's noise away from anything else!), and a beefy TVS (say, SMCJ18A or something like that). And going from there, at least 30V tolerant components (the controller and switch).For "gold standard" reliability, you'd want either an input protection circuit (e.g., a 20V LDO to ride out the load dump), or automotive grade circuits (who cares about 80V transients if you can operate smoothly throughout the voltage range?), and beefier protection (MOVs instead of TVSs -- much higher energy rating, more likely to survive). You may also want an isolated output, to prevent ground loops in case the attached load is grounded to other things (say, running audio from the rPi to the stereo system?).Tim

author: nsayer
author_rank: Regular Contributor
timestamp: July 13, 2014, 04:15:25 pm
content: Well, in lieu of all of that, I think I'll just stop talking about using it a car. Either that, or folks using it in a car can add a fuse and MOV on the input cable. As it is, I don't want to make the board any larger than it already is.Even leaving that aside, the TVS diode serves a useful purpose in other contexts, as well as being useful as a reverse-polarity protection.

author: nsayer
author_rank: Regular Contributor
timestamp: July 13, 2014, 07:20:38 pm
content: I went into the office to use the EE lab today and put it through its paces with a much better scope than I have at home.I am going to try to use the adjustable load on Monday, but with my fixed resistors, I got some good tests in anyway.Regulation is quite accurate everywhere from 6 to 14.75 volts. The bench supply couldn't quite make it up to 15. Below 5.75 or so, the output begins to drop out, and lower than 7 the ripple starts to increase a bit, up to a maximum of around 45 mV. But above 9, it's around 25.The switching frequency varies quite a bit depending on input and load, but I must have been wrong earlier - the lowest I saw here was aroun 66 kHz and the high was around 150.The gate signal is quite clean, as is the drain.I was able to try it with 2.5 ohms load, and ran it for a few minutes at 12 volts. It held on and was around 88% efficient. But, again, the MOSFET got - at least in my opinion, too hot.The new board design has a footprint for a TSOP6 MOSFET instead, with a smaller heat spreader than probably would be recommended, but it's on both sides and there are 3 chimneys under the part. That, and looking at the numbers of around a worst-case steady-state thermal resistance of 100 deg/W and a dissipation of 125 mW... I think it'll do better.I'll know in two weeks. :/Meanwhile, this design has no problem at all running all day long at 1A. That's good enough at least for MY R-Pi.

author: nsayer
author_rank: Regular Contributor
timestamp: July 17, 2014, 05:36:31 pm
content: I decided late last night to try and kludge the new MOSFET onto the old board. It turns out that the gate and source pins on the TSOP-6 lined up fairly well with the same pins on the SOT-23-3 footprint. I just added a bit of 22 gauge wire to connect two of the drain pins up to the cathode of the catch diode and it worked.That allowed me to spot check the performance of the new circuit and it was largely comparable to the old one. With 2A of load the MOSFET got warm, but I put a thermometer on it and it stopped at around 55 degrees C - and that's without the heat spreaders and connecting all of the drain pins. I'm fairly confident now.Since I have time, though, I do have some other questions.Most of the component values for this came from an online TI design tool. And most of them make quite a bit of sense - except for one.There are two resistors for the current-limiting feature of the LM3485. One of them replaces the measurement of Rds(on) of the MOSFET. Going with a resistor for this purpose should be less temperature sensitive. It was the TI design tool's idea, but I concur. It's a 25 milliohm 1/2 watt resistor.The other resistor calibrates the current limiter. The formula from the datasheet are:Vadj = Vin - (Radj * 3.0uA)Visense = Vin - (Rdson * Iind_pk)In this case, Rdson is actually the 25 milliohm resistor. Radj is 39K, per the TI design tool. Iind_pk is given later:Iind_pk = (Iout + Iripple/2)*1.1Iripple is the ripple current, and for >2A, is Iout * .3When I put all that together, I get closer to a 22 k-ohm resistor rather than 39k. At 39k, it seems to me like the current limiter is simply not going to do anything at all.Have I missed something? I'd have to guess TI's online design tool is probably smarter than me, so I'd have to say so, but I can't think what it might be.