PC high-current PWM fan controller issues - Page 1
Forum section: electronics / projects

author: ratdude747
author_rank: Regular Contributor
timestamp: August 28, 2017, 11:22:12 pm
content: I recently aquired a somewhat old server with loud non-PWM fans hooked to a straight up 12V power distribution board ("molex" plug and 6 3 pin fan ports, one side 12V, the other ground). Since the motherboard in it (and the one that I'm swapping in) supports PWM, I decided to make my own replacement distribution board with two PWM channels; while controllers can be bought, each of the 4 fans is rated at 1.5A... so a cheaply made 2A max unit of ebay wasn't going to cut it.The PCBs came in today (ordered from allPCB, they turned out nice), but unfortunately it's not throttling connected fans.Attached are the schematic for the board, gerber files for the PCB, and the intel PWM spec I designed it to meet.The topology is based on a schematic I found here:https://electronics.stackexchange.com/questions/275561/use-motherboard-pwm-to-control-non-pwm-fans-with-mosfet...and I verified the circuit with some rough back of the envelope calculations.Here's what I've done for diganosticsNo PWM signal: Fans run at full blastgrounded PWM signal: Fan shuts offconnected to computer with known working PWM fan connector (tested before and after with a PWM fan): No throttling, fans at full blastTach signal: present on outputgrounding the ground connector on the PWM in ports: No changeSwapping D1 for two more 2.7K resistor's in parallel: no changetesting a PWM fan with a jumper wire on the PWM port: grounding slows fan, connecting to 12V does nothing, confirming that PWM fan signals are NPN, not PNP in natureIdeas on what to check next. Sadly, unlike my college days, I don't have good scope access; I do have a couple of good mutimeters though.

author: floobydust
author_rank: Super Contributor
timestamp: August 29, 2017, 03:47:34 am
content: I don't see how that author's circuit is going to work with a tach.If you are modulating the fan's low-side then the tach signal is scrambled by the PWM because you are (pulsing) lifting the fan's ground.Imagine the MOSFET is off, so your fan GND and TACH are sitting at 12V.You might get backfeed from the fan's tach output seeking lower potential, into the motherboard's input gate. It looks like needs to be a 3.3V logic signal, so your tach output signals make sure resistors drop to 3.3V max. Even then, the tach freq. is mistakenly seen as too high by the motherboard, IMHO.

author: mihnen
author_rank: Newbie
timestamp: August 29, 2017, 04:38:11 am
content: According to the fan connector pin-out you are switching the ground side of the fan with your mosfet, you should have a flyback diode across that output from +12V to V_Outx.The previous poster is correct about the tach not being valid because of the low side switching although it should still work without the tach signal being clean depending on the motherboard firmware.What is your PWM frequency? the pwm output of a motherboard is meant for 4-wire fans and so should be around 20kHz which may be to fast when your trying to PWM the actual supply voltage for the fan. If you say it runs at full blast with no PWM signal that is what is expected since you have a pullup on the gate. When you ground the signal it shuts off is also good since fet is off so no current should flow through the fan.You can find the details for PC fan interfaces here:http://www.formfactors.org/developer/specs/4_wire_pwm_spec.pdfIf your just looking for something to get your fans working I have used this in the past. It takes a pwm signal (4-wire) from your motherboard and allows you to use it to control up 6 dc fans.http://www.phanteks.com/PH-PWHUB.htmlAlso if your interested I made a fan controller in the past for my PC. Its special feature was that it had both high side DC control with a switcher controlled by a dac as well as standard PWM control. It ran off a micro-controller but you could look at the fan control parts for some inspiration. See attached

author: Zero999
author_rank: Super Contributor
timestamp: August 29, 2017, 08:55:51 am
content: As others have said you're messing up the tachometer signal by PWM'ing the fans' 0V terminal.Ordinary fans don't work well with PWM. You're better off filtering the motherboard's PWM to make a DC voltage and amplifying that to drive the fans.This is all a bodge. Replace the fans with proper 4 wire fans which have the PWM signal.

author: ratdude747
author_rank: Regular Contributor
timestamp: August 29, 2017, 12:41:40 pm
content: ^The fans are in a special enclosure/hotswap, so a direct swap would have been a hack to say the least.As for the off-the-shelf controller... It's only good for 2.5A. No bueno.Would putting a large filter cap on the fan outputs help at all? I might try it when I get off work.

author: ratdude747
author_rank: Regular Contributor
timestamp: August 29, 2017, 03:03:15 pm
content: I did think about it some more... Technically I could rewire the hot swap plugs (which happen to have 4 positions) for PWM and recrimp some appropriately rated PWM fans. Would be a PITA and such fans are not cheap but I do have some extra PWM fan extenders left over from this project that I could crimp in.Still, I would much prefer to make a system to control non-PWM fans via PWM (working tach signals are nice but not required). What would I need to do? Use a transistor to drive a P-channel mosfet (or perhaps a darlington) amplifier with filtering?

author: ratdude747
author_rank: Regular Contributor
timestamp: August 29, 2017, 03:23:25 pm
content: Would this be a better circuit to use?Note that I'm using the transistor as my input snubber (0.7V is well under 5.25V) . I assume that's a valid move, or do I need to add a series resistor and a zener to ground in case the transistor fails? Edit- Yeah, I probably should, to ensure switching signal "completely" turns off the transistor.

author: Zero999
author_rank: Super Contributor
timestamp: August 29, 2017, 03:34:28 pm
content: Quote from: ratdude747 on August 29, 2017, 12:41:40 pmA filter capacitor would be a bad idea, because every time the transistor turns on, it will have to charge the capacitor, resulting in a high current surge.Quote from: ratdude747 on August 29, 2017, 03:03:15 pmI was thinking a PWM circuit with a filter and emitter follower to drive the motor. The PWM voltage is 12V, which can be obtained from the 5V using a MOSFET driver. The output stage will always drop around a volt, so even with 100% PWM it will never run at full speed.

author: ratdude747
author_rank: Regular Contributor
timestamp: August 29, 2017, 03:36:35 pm
content: Here's the two stage idea I posted above with a better input design. Should accomplish what I want while keeping a consistant ground?Side note- I'm OK with not getting full speed. I'm not running 15K drives or anything (these are case fans; the CPU fans I swapped for PWM already)

author: Zero999
author_rank: Super Contributor
timestamp: August 29, 2017, 03:40:29 pm
content: Quote from: ratdude747 on August 29, 2017, 03:36:35 pmAs I said above, the capacitor across the motor will result in large current surges, which will put strain on the transistor.

author: ratdude747
author_rank: Regular Contributor
timestamp: August 29, 2017, 03:45:04 pm
content: Quote from: Hero999 on August 29, 2017, 03:34:28 pmIsn't that the principle of how the secondary of a SMPS works? The capacitor smooths out the switching ripple. I have an intentionally overspec'd MOSFET so inrush current isn't a huge concern.Quote from: Hero999 on August 29, 2017, 03:34:28 pmYou do know that the PWM signal is open collector; it must be pulled up to no more than 5.25V in the fan (or in this case, the amplfier board). Not to say that wouldn't work, just know that the PWM signal's voltage is set in the circuit via pull-up.

author: Zero999
author_rank: Super Contributor
timestamp: August 29, 2017, 09:58:56 pm
content: Quote from: ratdude747 on August 29, 2017, 03:45:04 pmTotally different, since in a switched mode power supply there's a big fat inductor to momentarily act as a current source, thus limiting the current. In the circuit you've depicted, there's nothing to limit the current, other than the capacitor's ESR and the MOSFET's on resistance.QuoteYes, that's why I said you need something to level shift the 5V PWM to 12V for this to work. You could use a MOSFET driver such as the MCP1402 or TC4427 or (cheaper but it will have a higher output resistance) a level shifter IC such as the CD40109 or CD4504.http://ww1.microchip.com/downloads/en/DeviceDoc/20002052D.pdfhttp://ww1.microchip.com/downloads/en/DeviceDoc/21422c.pdfhttp://www.ti.com/lit/ds/symlink/cd40109b-q1.pdfhttp://www.ti.com/lit/ds/symlink/cd4504b.pdfQuote from: Hero999 on August 29, 2017, 03:34:28 pm

author: ratdude747
author_rank: Regular Contributor
timestamp: August 29, 2017, 10:38:17 pm
content: ^I did realize that. Actually, why not design it as a SMPS:Using this calculator:https://learn.adafruit.com/diy-boost-calc/the-calculatorVin Max= 12V, Vin Min =10VVout Max=12V, Vout Max = 6V (50% is the lowest any motherboard will ever command)Which yielded me this:Maybe this is the way to go? (Yes, I know Q3's pin numbers are wrong)

author: NiHaoMike
author_rank: Super Contributor
timestamp: August 30, 2017, 01:24:25 am
content: Move the diode to the other side of the inductor, then you have a good starting point.

author: ratdude747
author_rank: Regular Contributor
timestamp: August 30, 2017, 02:48:55 am
content: ^good catch. Doh!See attached schematic. Since we have two transistor stages, I'm going back to an old setup that switched the PWM signals and not the raw output for channel 2. Direct driving the mosfet was too much to double up but even with a measly gain of 50 I'll be more than OK to double them (old design also had passthrough jacks for the input, but I'm not putting those back in).I'm working on PCB artwork now but won't order until I've had a chance to breadboard this... lesson learned: not having stuff from college readily available is a poor excuse not to test things!Edit- Pulled schematic, I forgot to swap the connections on the ouput connectors.Edit #2: Fixed Schematic.

author: ratdude747
author_rank: Regular Contributor
timestamp: August 30, 2017, 06:24:31 am
content: Final updates before I go to bed. See attached files.

author: Zero999
author_rank: Super Contributor
timestamp: August 30, 2017, 10:36:03 am
content: Quote from: ratdude747 on August 30, 2017, 02:48:55 amI strongly recommend using a proper MOSFET driver, rather than Q3. The MOSFET's gate will have to discharge through R3, which will take awhile. If you can't stretch to a proper MOSFET driver IC, such as the MCP1402 or TC442, then you can make do with the CD40109 or CD4504, with several of logic gates connected in parallel, to increase the output drive. Alternatively you can make your own proper driver with several transistors.

author: ratdude747
author_rank: Regular Contributor
timestamp: August 30, 2017, 02:23:38 pm
content: The TC4426 looks like a winner... if I'm reading the datasheet and counting my inversions right. This chip require any bypass caps on the power rail?

author: b_force
author_rank: Super Contributor
timestamp: August 30, 2017, 04:11:49 pm
content: You can also use a simple totem pole buffer with just two transistors.You can also just use a buck converter and use that to control the fan.OR connect the positive side of the fan to the power supple and the negative part at the output of the buck converter.Btw, what is the reason why normal fans are not fan (lol pun) of PWM signals?

author: ratdude747
author_rank: Regular Contributor
timestamp: August 30, 2017, 04:28:51 pm
content: ^for a push-pull totem pole to work I'd need to add 4 transistors (2 per channel) as I still need Q3/Q4 to convert from 4.7V logic to 12V logic, which would then feed the totem pole. . Technically 4v and above will work with the Mosfet, but since the max is 25V, I think 12V will do a better job of ensuring saturation.Could I? Yes. But at $1.40 for the microchip driver (which is designed for what I'm doing) and a lot of circuitry removed (4 resistors and two transistors removed) it's an appealing choice. The costs are compareable; while the totem pole gives more board space/routing options (getting traces around power busses is a slight pain on this board), the chip is hopefully less fussy. Flip a coin?As for a buck converter, I already have the PWM signal so half of a Switch mode converter is already there. Although without feedback, one concern I have is stability and output variation due to lack of load. Maybe I do have to put dedicated supply in and use a cap to convert the PWM to DC for a set voltage? Once I have parts (and breadboards, all of my existing ones are tied up in an old college project I don't want to scrap), we shall see.Edit- lack of feedback might not be an issue after skimming this:http://www.electronics-tutorials.ws/power/switch-mode-power-supply.html...Basically what I made IS a buck converter.

author: Zero999
author_rank: Super Contributor
timestamp: August 30, 2017, 06:44:51 pm
content: Quote from: b_force on August 30, 2017, 04:11:49 pmI don't see how it can be done with just two transistors. I tried but couldn't get the current and voltage gain to drive the MOSFET at any decent speed.Quote from: ratdude747 on August 30, 2017, 02:23:38 pmYes, it needs bypass capacitors, as does the circuit I posted with transistors.

author: b_force
author_rank: Super Contributor
timestamp: August 30, 2017, 09:36:09 pm
content: Quote from: Hero999 on August 30, 2017, 06:44:51 pmThis totem pole circuit is done in a lot of switching regulators and Class-D power amplifiers.Basically standard circuit for everything that uses high current MOSFETs to be honest.You can simply use a NPN and PNP, but there are also smaller options available from Zetex (Dioded Inc)Like the ZXGD3005 and similar. Which are a lot smaller and cheaper than the TC4426.

author: Zero999
author_rank: Super Contributor
timestamp: August 30, 2017, 10:03:40 pm
content: Quote from: b_force on August 30, 2017, 09:36:09 pmA totem pole circuit has four transistors.https://electronics.stackexchange.com/questions/16864/totem-pole-output-driverQuoteNo, that won't do, as it has no voltage gain, only current gain. The voltage needs to be level shifted i.e. boosted, from 5V logic levels, to 12V to control the P-MOSFET.

author: b_force
author_rank: Super Contributor
timestamp: August 30, 2017, 10:24:48 pm
content: Mm, I guess there is some miscommunication what a totem pole is.To my knowledge it is just a NPN/PNP pair.As you can see here, page 17http://www.ti.com/lit/ml/slua618/slua618.pdfLevel shifting with just a ground is pretty easy actually.(it's a bit more a pain when working with a symmetrical power supply)I guess what you're looking for here as well, is a bootstrap circuit.

author: ratdude747
author_rank: Regular Contributor
timestamp: August 31, 2017, 03:23:16 am
content: Here's another round of schematics and artwork.Changes from last time:Swapped 1st stage from BJT's and resistors to a TC4426 based driverAdded main filter cap C3 and bypass cap C4---The TC4426 is cheap enough. I don't want to use SMD if I can at all avoid it... This isn't a mass production project (I'm making 3-5 units, for me and a buddy or two). IMHO not worth it (not to mention not as easy to breadboard).

author: Zero999
author_rank: Super Contributor
timestamp: August 31, 2017, 08:06:17 am
content: Quote from: b_force on August 30, 2017, 10:24:48 pmThat's what I thought you meant but wanted you to post a schematic to confirm. The trouble is, that circuit has no voltage gain. The output simply follows the input minus 0.6V on positive cycles and plus 0.6V in negative cycles. Another transistor is required to provide voltage gain, to get 12V pulses from 5V pulses, hence why the circuit I posted previously has three transistors.QuoteThe trouble with a bootstrap, is it will never allow full duty cycle. An off period is required to recharge the capacitor. Other than that it's a good idea and enables lower on resistance N-channel to switch the high side.Quote from: ratdude747 on August 31, 2017, 03:23:16 amThat looks good so far. Yes, the TC4426 is inverting and is just what you want.The 1N4001 is only rated for 1A at mains frequencies. D4 and D2 should be replaced with some faster, higher current, diodes, such as the 1N5817.I haven't checked the inductor calculations. I suggest doing it again, either manually or using a different calculator page, for a sanity check and round up the result to the nearest available value, never down. The current rating should be higher, than what the fans will draw, to avoid core saturation.

author: b_force
author_rank: Super Contributor
timestamp: August 31, 2017, 10:33:52 am
content: With a buck converter (or half bridge), you always need a bootstrap?I don't know what the further idea is, switching freq, needed deadtimes etc. But there are enough halfbridge drivers that can do the job and deliver enough current.There are also little high side drivers etc.I think I would just go for half bridge configuration though.But isn't all of this just a bit much for just a simple fan?

author: Zero999
author_rank: Super Contributor
timestamp: August 31, 2017, 02:00:35 pm
content: Quote from: b_force on August 31, 2017, 10:33:52 amA bootstrap is only needed, if the high side device is N-type, as the gate voltage needs to exceed the output voltage, by enough to turn on the MOSFET. In this case the high side device is P-type so it's not needed.

author: b_force
author_rank: Super Contributor
timestamp: August 31, 2017, 02:05:13 pm
content: Quote from: Hero999 on August 31, 2017, 02:00:35 pmThat is correct.The thing is that using two N-types is far more common nowadays.Doesn't mean it's impossible of course. Just wouldn't be my path to make it work I think.

author: ratdude747
author_rank: Regular Contributor
timestamp: August 31, 2017, 08:33:51 pm
content: Quote from: b_force on August 31, 2017, 02:05:13 pmOff-topic: I can speak on this a bit.In school I was taught to use NPN/N-channel to sink, and PNP/P-channel to source. Hence my design. I also read (on wikipedia) that NPN/N channel is often cheaper as it's more widely used (and thus made in bigger bulk), so I could understand why one would find a way to source with one if they had to.In my day job, I'm an engineer at an automotive component plant (body shell division) who works with a lot of PLCs, welders, robots, and automation. NPN/PNP comes up a lot as some devices have sinking inputs/outputs (referred to as NPN devices), and others have sourcing inputs/outputs (referred to as PNP devices). This is both for controller I/O and sensors. The thing is, this is a regional issue too. We're a Japanese owned company and most Japanese equipment is NPN (Mitsubishi PLCs, Kawasaki/Motoman robots, and many Keyence/Omron sensors), so our company standard is NPN. However, for safety devices (required by OSHA/CSA), the standard is PNP (as is most American equipment), which in the case of one my projects where I'm updating the safety systems on some old robot cells, is a bit of a pain (micro relays for conversion all over the place, ugh).

author: b_force
author_rank: Super Contributor
timestamp: August 31, 2017, 08:59:01 pm
content: There is a lot to say to use two (close to) identical devices.Besides the differences in behavior of a p-channel and n-channel, finding complementary MOSFETs (to get specs as close as possible) is difficult.I work with high power Class-D amplifier (500-1000W) on a regular basis and these are all n-channels MOSFETs.With these currents, something simple like a difference in even Rds(on) can ruin your day.Not speaking about difference in capacitance and all kinds of other parameters, which will result in different switching ringing noise and so on.Having more similar parts on your BOM is always cheaper in the end.

author: ratdude747
author_rank: Regular Contributor
timestamp: September 02, 2017, 04:26:06 am
content: Inductors:https://www.digikey.com/product-detail/en/abracon-llc/AIUR-06-4R7K/AIUR-06-4R7K-ND/2343590I designed it around 5A per channel, although 6A is the absolute max I'd expect each channel to run. These are rated at 6.2A. Sadly, the calculator I used was forboostconverters, so they're probably no good. I'm not getting consistant results from buck calculators; it seems they're not designed for the currents I'm running. I might have to do some old fashioned math myself on this one...

author: ratdude747
author_rank: Regular Contributor
timestamp: September 02, 2017, 04:43:54 am
content: Worst case scenario: high load, ~2V drop across mosfet (as per datasheet and ohm's law) low fan speed requested (50% duty cycle). Used this calculator:http://www.daycounter.com/Calculators/Switching-Converter-Calculator.phtml230uf and 160uH is what I'm getting. I have way more capacitence and nowhere near enough inductance. Whoops. That said, at 50% I bet it's only running 3A, not 6.Perhaps this:https://www.digikey.com/product-detail/en/signal-transformer/HCTI-220-5.8/595-1732-ND/7362983

author: Zero999
author_rank: Super Contributor
timestamp: September 02, 2017, 09:38:23 am
content: I agree with your conclusions: you need more inductance and the inductors need to be rated for a much higher peak current, than the output current. The current drawn, when the load is the maximum inductor current, in this case.A voltage drop across the MOSFET sounds too high. It would give a power dissipation of 12W at 6A, so you'll need a fairly large heat sink. Find a MOSFET with a lower on resistance.I think you need to develop a better understanding of buck converters, even if you don't fully understand the calculations, otherwise you're just blindly trusting some calculator program. Refer to the links below:http://www.learnabout-electronics.org/PSU/psu31.phphttp://www.radio-electronics.com/info/power-management/switching-mode-power-supply/step-down-buck-regulator-converter-basics.phphttps://en.wikipedia.org/wiki/Buck_converterhttp://www.ti.com/lit/an/slva477b/slva477b.pdfhttp://www.ti.com/lit/an/snva038b/snva038b.pdf

author: ratdude747
author_rank: Regular Contributor
timestamp: September 03, 2017, 12:13:22 am
content: Other information:I will be running a heat sink. See attached picture. But its' only rated for 7W at ambient... and that's for both devices.Supposedly my Mosfet was rated for 10A (or more) at ambient temperatures... but I didn't realize what sorts of voltage drop I'd be talking.Datasheet for my mosfet:https://www.fairchildsemi.com/datasheets/FQ/FQP7P06.pdfAccording to that, with 10V gate voltage, I'd get 0.35 Ohms at 6A... so 2.1V drop, and therefore 12.6W. Darn. (and to think I used to do this math all the time in college...)That said, for not a low more there are better options. This one is overkill:https://www.digikey.com/product-detail/en/fairchild-on-semiconductor/FQP27P06/FQP27P06-ND/965349But from what I'm seeing it's the cheapest unit with a low enough resistance not to burn a ton of power. 1.44W to be exact. Comparing it's 1400pf max capacitence to TC4226 datasheet, I'd get a rise time of 30ns and a fall time of 35ns. Given that a 25khz has a period of 40000ns, that should be fine, right?I am indeed rusty on buck coonverters... sadly it wasn't covered much in college (other than what they are and why they're better (and sometimes worse) than linear supplies. I did skim this though:http://www.daycounter.com/LabBook/BuckConverter/Buck-Converter-Equations.phtmlPlugging in new numbers for the new mosfet, 48uf and 180uH. Is going bigger on either one OK, or is that going to cause an issue? AFAIK 25khz square waves don't have a harmonic that would match the resonant frequency... which is a good thing for stablity? (although being PWM I'd have to look at my old fourier tables to see what I could get at common PWM fan duty cycles).

author: ratdude747
author_rank: Regular Contributor
timestamp: September 03, 2017, 12:44:04 am
content: Found another good reference:http://www.ti.com/lit/an/slva477b/slva477b.pdfAccording to that calculation, since worst case is 50% PWM (top part of the equation is a parabola, vertex is indeed at Vout = 0.5*Vin), I'm getting 66.7uH at 30% current ripple. at 20% (what I originally used for the calculator above) I'm getting 100uH.Also, from that, I'm getting a minimum capitence of 200uf (20% of 6V for output swing (ripple?)... although I'd want to go higher, so 1000uf per rail should be more than adequate. To make room for the inductors, I'll swap to an 8mm 1000uf 16V (Nichicon HE, my favorite cap for PSU work; I do a lot of power supply recapping on the side).These numbers seem more reasonable?

author: ratdude747
author_rank: Regular Contributor
timestamp: September 03, 2017, 06:20:45 pm
content: ^ based on the above, I'm thinking this inductor would work:https://www.digikey.com/product-detail/en/signal-transformer/HCTI-100-7.0/595-1728-ND/7362979rated for 7A, which should be good, right?edit- I'm thinking these for the diodes:https://www.digikey.com/product-detail/en/smc-diode-solutions/SB3200TA/1655-1523-1-ND/6022968and capcitors, some Panasonic FR's should fit the bill:https://www.digikey.com/product-detail/en/panasonic-electronic-components/EEU-FR1C102LB/P15332CT-ND/307221220% current ripple at worst case 6A is 1.2A. These have a high-frequency rating of 1.56A, and while I'm not running at the spec'd 100Khz, the 120hz rating of 1.17A tells me that at 25kHz 1.2A shouldn't be an issue. All of this is worst case (3 2.0A fans per channel, not a likely configuration).Working on a revised PCB design now...

author: ratdude747
author_rank: Regular Contributor
timestamp: September 04, 2017, 05:37:30 am
content: Fresh schematics and PCB.I had to add half an inch to the board size to accommodate the torroids (which after a review of the datasheet, led me to choose 120uH instead of 100uH). Also, I decided to remove some of the solder mask from the back side in current-dense areas to give me an edge. Still, maybe I should shoot for 2oz copper on the next order?

author: Zero999
author_rank: Super Contributor
timestamp: September 04, 2017, 10:08:31 pm
content: That's much more sensible: more beefy components and a larger inductor. Please click on the links I posted and at least scan through them. I know you haven't because you went on to post one of them.One thing I think you might have overlooked is, the capacitance seen by the MOSFET driver, is higher than the static gate capacitance, so the switching time is slower, than you've estimated. This is due to the Miller effect. Look up MOSFET gate charge.

author: ratdude747
author_rank: Regular Contributor
timestamp: September 04, 2017, 11:44:28 pm
content: Quote from: Hero999 on September 04, 2017, 10:08:31 pmYeah, looking back I saw that and probably should have edited my post.---Looked at:http://www.ti.com/lit/an/snva038b/snva038b.pdf. I got 68uH using some of their assumptions (r = 0.3, etc.), which is in the ball park of what I already had.All of you other links (minus the one genius here found independantly) didn't have equations for L in them, so obviously I can't post about such. 100uH (120 nominal) seems to be a safe number.----As for the miller effect, from what I read, that's the reverse transfer capacitence, which for my MOSFET, at 1MHz has a max of 155pf. When combined with the Drain-source static resistance, I'm getting 109ns for T, or 218ns of of charge time (IIRC 2T is generally what's used for "full" charge?). But I'm sure I did something bone headed there. Still, I don't think this will be a huge issue at 25kHz.MOSFET Datasheet:https://www.fairchildsemi.com/datasheets/FQ/FQP27P06.pdf

author: ratdude747
author_rank: Regular Contributor
timestamp: September 05, 2017, 05:36:54 am
content: One more PCB refresh, moved diodes D2 and D4 (and by extension, C1) due to a heatsink clearance miscalculation.

author: Zero999
author_rank: Super Contributor
timestamp: September 05, 2017, 10:11:25 am
content: Quote from: ratdude747 on September 04, 2017, 11:44:28 pmThe characteristic, you're after is the gate charge, which takes the Miller effect into account.https://www.microsemi.com/document-portal/doc_view/14692-mosfet-tutorialI haven't done the calculations, but I think it will be fine.Quote from: ratdude747 on September 05, 2017, 05:36:54 amShould be fine but I haven't checked everything. The fat traces and planes have a low inductance and resistance and are good practise for high currents.

author: ratdude747
author_rank: Regular Contributor
timestamp: September 05, 2017, 11:49:23 am
content: ^PCB wise, I did checks using an actual heatsink and a PCB printout. The same way I've checked new custom footprints for proper pitch and the like.The Heatsink "fingers" go edge-to-edge, but the IC, resistors, capacitor c4, and diodes D1 and D3 will fit underneath. I don't like putting things under a heat sink but the PCB size was getting a tad excessive. The issue with D2 and D4 was they were in the flat/bend area where things won't fit (hence the empty channels flanking Q1 and Q2); I'm sure D2 will also fit underneath (as it is in the current PCB layout)

author: ratdude747
author_rank: Regular Contributor
timestamp: September 05, 2017, 12:35:24 pm
content: Quote from: Hero999 on September 05, 2017, 10:11:25 amI'll look into it later. Both driver IC and mosfet are designed for the SMPS application so I'm confident it "should" work... then again, I have come across exceptions.Quote from: Hero999 on September 05, 2017, 10:11:25 amThat was the goal... plenty of track width for high current traces (and where I couldn't make the back side wide enough, I exposed solder mask to do the old solder flow trick), decent track width for in-between stuff (driver to mosfet connections), and 0.5mm tracks for signal wires (PWM and Tach signals). No need to go to 0.25mm traces on this board, so I won't.

author: ratdude747
author_rank: Regular Contributor
timestamp: September 06, 2017, 12:20:51 pm
content: Barring any other issues, I'll probably order another round of 5 boards after work today.

author: texaspyro
author_rank: Super Contributor
timestamp: September 07, 2017, 04:28:53 am
content: PWM the fans at 1 Hz! Use a minimum pulse width of say 100 ms. When staring up you can give a couple of seconds of 100% duty cycle to make sure they start spinning. This has worked on every two wire fan I have tried. Of course the tach output wont work very well with many fans, but it does with a lot of them.My PWM fan controller PID code n Lady Heather can control the temperature of a GPSDO to millidegree levels... micro-degree long term (actual absolute accuracy dependent upon the temp sensor)

author: ratdude747
author_rank: Regular Contributor
timestamp: September 07, 2017, 04:41:15 pm
content: Quote from: texaspyro on September 07, 2017, 04:28:53 amThat's not the point of this project.The intent was to use the PWM control already present on many PC/Server motherboards to control high-current non-PWM fans. In my case, a rackable systems NAS server (which I'll use for other things too) that had 4 high current fans on proprietary hot swap connectors going to a dummy boards that took a standard "molex" plug's 12V and put it on 6 standard fan ports. The Tach was wired up (but not used by the dummy board) but the fans are not PWM. I have a buddy with a similar situation (old dual-motherboard monster case, far newer motherboards to drop in), hence the idea.To do what you're suggesting would require the addition of a microcontroller with few exisiting components removed.As for fan spinup, Motherboards already handle this. Since it's a pulled up signal (on the fan/PWM amp side), no signal=full speed. Likewise, PC motheboards from my experience run the fans at full blast during startup as normal procedure either intentionally (to spin the fans up) or because the low level controls haven't started yet.Using the onboard PWM is desired as it both is an already controlled (sometimes with tachometer feedback) signal source that can also be controlled from software on some platforms. In addition, for non-PWM systems there are already 3rd party PWM controllers for computer fans out there, so one could combine this with that (or do both and use one channel for each) making this a partially universal solution.I see no need to go through a bunch of trouble to convert/interpret a PWM signal to a lower frequency when the hardware to directly use and drive with the higher frequency exists and isn't cost prohibitive.Why reinvent the wheel?

author: ratdude747
author_rank: Regular Contributor
timestamp: September 08, 2017, 12:22:32 pm
content: Ordered more PCBs... 10 of them. The cost difference is marginal and even after adding 10 ofanother of my projectsjust for fun DHL sipping still costs more... Yay cheap overseas PCB makers.

author: ratdude747
author_rank: Regular Contributor
timestamp: September 14, 2017, 11:04:27 pm
content: New parts and PCBs came in, still not working.Some intial issues:Somehow the bypass caps didn't make the order, so I'm running withoutInductors were too big (lead size)... so I had to trim them and solder on thinner leads (from diode trimmings)Past that, same as before, Tach signal works great, but no speed modulation. Grounding the input kills the fan.I wonder if the lack of direct feedback is causing drift due to low load... if so, then this concept is DOA and I've wasted my time and money. I might as well run extension cables to the existing fans and learn to deal with the 6000RPM whine (or once I get the house it's for bought, put it in a closet or something).Unless there's a chance the lack of a bypass cap is causing the driver to not pick up the PWM signal, I'm out of ideas.

author: Zero999
author_rank: Super Contributor
timestamp: September 15, 2017, 08:50:19 am
content: Quote from: ratdude747 on September 14, 2017, 11:04:27 pmBummer.Is the PWM actually changing the output voltage? Have you tested it with a PWM signal source from a signal generator or 555 timer circuit?

author: ratdude747
author_rank: Regular Contributor
timestamp: August 29, 2018, 04:32:44 am
content: Reviving this as I've seen some inspiration that may allow for a retry of this design:My suspicion (which was never verified, as I moved right around the time I posted last) was that the issue related to the LC output configuration not having anywhere to drop ripple voltage. Which makes sense, as a "normal" linear supply filter (no inline resistor before the Cap) like I was using creates an output equivalent to the Vpeak of the input rectified waveform, not the RMS or midpoint. Freshman level stuff, and I flunked it. What is needed is a decoupling capacitor (to convert the PWM signal to an analog DC value) and then an RC stage (aka the linked capacitance multiplier, ideally using the same model MOSFET as the first stage) to filter out the ripple, no massive inductor needed. This will probably throw a wrench in the works for the PCB layout (double the # of mosfets with heatsinks, which I may try to run without), but that's besides the point since the old design was junk and not working.I've been bogged down with work and homeowner chores, and may not get to this any time soon... But with any luck I'll have some time over the next few weeks to breadboard a circuit and give it a test spin.Am I possibly on the right track?

author: Ian.M
author_rank: Super Contributor
timestamp: August 29, 2018, 05:58:26 am
content: The easy option would be to use aLM2596or similar buck converter module. They are readily and cheaply available on EBAY. To add PWM output voltage control, simply disconnect the bottom of the voltage set divider chain from ground and inject a control voltage derived from the low pass filtered PWM signal + an OPAMP. The reference voltage is about 1.2V, Inject that voltage and the output will go to zero, Inject 0V and it will go to the voltage set by the divider ratio. Inbetween you get linear control from Vref up to the max voltage set. You'll need a small 5V regulator feeding a dual rail-to-rail I/O OPAMP, one section as a comparator to invert and buffer the PWM and limit it before filtering so the result isn't dependent on the motherboard logic levels, and to get it the right way round (100%=0V) for the LM2596 and the other to buffer the filtered analog output voltage to provide a low impedance drive to the bottom of the LM2596 voltage set divider chain.

author: xani
author_rank: Frequent Contributor
timestamp: August 29, 2018, 06:14:27 am
content: Did you actually scope the input, output and drive voltage, and the waveform coming from mobo ? Or at least simulate it in LTSpice.From what I understand motherboards will output 0-100% duty cycle PWM. That is NOT what a buck converter wants as a drive signal, or rather the relation between duty cycle and output voltage/current is not linear. IIRC mobo will produce 20-100% duty cycle, which means that even on slowest setting duty cycle might be high enough to drive your fan at high speedI've built a similar project out of junkbox parts as I wanted to use PC fan in my ARM cluster, and just used micro to drive it (that used tacho and ADC for feedback). But it maxes speed at around ~30% duty cycle, mostly because I oversized inductor (or rather didn't had any other on hand).My guess is your circuit "works" but mobo does not go to the very low duty cycle required to actually make your fans spin slower.Doing it the "analog" way is very much possible, but considering that you could just get literally any micro that has an ADC and a timer to do it, it probably wouldn't be cheaper, or easier.

author: oPossum
author_rank: Super Contributor
timestamp: August 30, 2018, 03:43:38 am
content: I built and characterized some 4 wire to 3 wire converters. They are described is this thread:https://www.eevblog.com/forum/projects/pc-motherboard-pwm-4-pins-to-3-pins-digital-converter/msg24112/#msg24112

author: xani
author_rank: Frequent Contributor
timestamp: August 30, 2018, 05:56:09 am
content: And have you tried same method (feeding PWM on from 0 to 100%) of characterizing on your project ?Honestly it looks like it should work just fine, just that with no any real negative feedback the output voltage will depend both on actual PWM signal frequency, and current consumption of the fan (which will then vary on spinup)