Confused - electrolytic vented? (and LC filters) - Page 1
Forum section: electronics / projects

author: Things
author_rank: Regular Contributor
timestamp: September 18, 2014, 05:34:17 am
content: So I have a MCU controlling a few blower fans using PWM, and I installed a low pass filter to try and quieten them down a bit. This seemed to work fine for a while, but I went to fire it up today and after around a minute at full power, I heard the fans start slowing down and heard a hissing noise. Immediately I knew it was the capThis is the circuit here:The circle is the blower fans (drawing about 5A at max power), the cap is rated at either 16 or 25V (haven't pulled it apart to re-check yet), and it's running off 12V.PWM frequency is only 490hz.Anyone have any idea why it could have failed? Perhaps the inrush on each PWM pulse was too much for it? Was only a cheap cap so maybe I just got a bad one - but it hasn't been running for more than maybe 6 hours total.

author: German_EE
author_rank: Super Contributor
timestamp: September 18, 2014, 07:20:54 am
content: Two guesses:1) Electrolytic capacitors might not like being fed a 490 Hz signal at power levels.2) If the fan is just a motor without an internal controller there may be large voltage spikes across the terminals. Try fitting a reverse diode just as you would with a relay.

author: Things
author_rank: Regular Contributor
timestamp: September 18, 2014, 07:33:13 am
content: They are DC brushless blowers, so they do have internal controllers. I have the same thing driving a normal brushed DC fan as well, but that cap seems to be fine.Thought 490hz would have been pretty low and safe for an electrolytic .. Guess I'll try swap the cap out and see what happens, could have just been a bad one too.

author: wraper
author_rank: Unknown
timestamp: September 18, 2014, 08:08:20 am
content: Quote from: Things on September 18, 2014, 07:33:13 amActually maximum ripple current is lower at lower frequencies. What you are doing here is intentionally killing capacitor in this schematic. I bet power supply is not happy at all about it too. You basically make almost a short circuit when mosfet opens. PWM cannot be done in a such a way. You should either do not use a capacitor at all if the fan can handle raw PWM or make a controlled DC/DC converter (needs coil).

author: tggzzz
author_rank: Super Contributor
timestamp: September 18, 2014, 08:20:05 am
content: Here's anaiveanalysis that mayor may notoffer some pointers...Assume the capacitor has zero ESR, and the motor is "sufficiently" inductive for the effects hypothesised below to have some validity.Start with the mosfet non-conducting, then turn it on. Initially there will be zero current through the inductor (motor), and the current through the capacitor will be limited by the 250m? resistor to 12/0.25 = 48A.So, does the capacitor have sufficient ESR to keep the inrush current within its stated max current rating?After that, the motor's inductance will ring with the capacitance, producing voltages that might exceed the capacitor's rating (in either direction).To reduce those effects I'd insert a resistor in series with the capacitor and a resistor+reverse biassed diode in parallel with the motor. (The resistor reduces the duration of a current spike through the diode at the expence of increased reverse voltage spike).I'm also curious why you chose a 4700uF capacitor - it seems rather high and may well have too much internal inductance to effectively suppress RF. 4700uF across the power supply seems more reasonable.

author: Things
author_rank: Regular Contributor
timestamp: September 18, 2014, 08:33:12 am
content: Well, I have a feeling the inductive kickback from the motors won't be great, as they have their own internal controllers that should be copping most of it. Just the current spikes on the PWM pulses I feel could have been too much for it - it's a cheap cap, nothing fancy. Maybe if I use a bunch of smaller caps in parallel I could share the stress across more of them?Without the filter the fans are just far too noisy, as in it'd probably drown out most laptop speakers on full volume even, and DC/DC converters aren't really an option cost-wise. I'll have to stick to something like this, just somehow reduce the stress on the capacitor(s).Would using more, smaller caps in parallel help you think?

author: trevwhite
author_rank: Frequent Contributor
timestamp: September 18, 2014, 10:19:21 am
content: Maybe just put a small resistor in series with the capacitor and maybe a resistor across the fan? See what it looks like on a scope?

author: tggzzz
author_rank: Super Contributor
timestamp: September 18, 2014, 10:21:43 am
content: Quote from: Things on September 18, 2014, 08:33:12 amAn ounce of measurement is worth a pound of speculation!QuoteWhat do these "controllers" do? Is your PWM compromising their ability to control?QuoteCapacitor tolerance => unequal sharing.QuoteDrown out acoustically or electrically? If electrically, what is the conduction path between the high currents and sensitive electronics?QuotePossibly, but I think you need to do some research into noise suppression. In particular, look at ferrites, common mode inductors, appropriate wiring to keep high currents away from sensitive electronics. Googling will find many many application notes and guides; I don't have a favourite.

author: Things
author_rank: Regular Contributor
timestamp: September 18, 2014, 11:14:38 am
content: Quote from: tggzzz on September 18, 2014, 10:21:43 amJust plain ol brushless motor controllers like you'd find in a computer fan - unfortunately I don't think they have a PWM input on them directly (No wire at least, but I didn't see any unconnected pads either).I've run them PWM before and the controllers seemed OK with it - but using the low pass filters they're basically silent once they've actually started spinning.QuoteAcoustically. They're mounted in a large wooden box so the entire thing just acts as a speaker - and at 490hz, it's more than audible!I have tried above-audible PWM frequencies, but neither of the fans seemed to work at all that high.I've stuck a 35V 2200uF cap in it just for now - even at half the capacitance they're still nice and quiet, so you were correct in regards to such a high value being unnecessary. Hopefully maybe that'll help prolong the life a bit too. They're mounted above a large fan so they have ample cooling, just found it odd this one decided to fail so early.

author: bobcat
author_rank: Regular Contributor
timestamp: September 18, 2014, 12:00:05 pm
content: You don't necessarily have to change the PWM frequency to above audible frequencies to quiet a motor. Try varying the PWM until you find the sweet spot that reduces the noise.

author: mikerj
author_rank: Super Contributor
timestamp: September 18, 2014, 12:25:47 pm
content: Quote from: Things on September 18, 2014, 11:14:38 amRaise the frequency, replace the resistor with a suitable inductor and add a flyback diode from the MOSFET Drain to the supply rail. This will effectively give you a buck converter.

author: wraper
author_rank: Unknown
timestamp: September 18, 2014, 01:41:19 pm
content: Quote from: Things on September 18, 2014, 08:33:12 amNonsense. Total BOM will cost maybe slightly above your current part cost. You cannot just add capacitors in parallel. You basically shorting your PSU. Increasing resistor value also is a no way as your PWM will become a linear regulator (Edit: in terms of heat dissipation) with all power dissipated on a resistor. Well, you can add more capacitors, but then your PSU will die with blown capacitors anyway.Edit:Your circuit is able to work somehow only because you are sagging your PSU voltage (and killing it slowly at the same time) when mosfet is opening. If your PSU would be able to supply infinite current at given voltage, your circuit would blow up almost instantly. You cannot PWM from DC to DC (on capacitor and fan) without a coil. Well you actually can if using a high value resistor, if the load current consumption is very low. In such a case it would be a low pass RC filter. But heat dissipation on the resistor will be the same as if using linear regulator. Therefore you only can do PWM without an inductor only if: 1. load is completely resistive, like a bulb or some kind of heater. 2. load is inductive by itself like a brushed motor, stepper motor.

author: DanielS
author_rank: Frequent Contributor
timestamp: September 18, 2014, 01:49:22 pm
content: My guess on the capacitor would be excessive AC current ripple.If you buy random "low-ESR" caps, you often end up with something rated at 40mOhms and ~1A RMS. If you shop for low-ESR caps like you really mean it, you can find caps rated at 8-15mOhms and 3-5A RMS. Even those 5A-rated caps might have a hard time with hard ripple across them in a high-power RC filter.As wraper said, you want to use a DLC filter - the same thing as a buck converter - so the inductor and capacitor can split the current feed between them. At 500Hz though, you would need a huge inductor so you might want to consider options for raising that to 10kHz or more.

author: Hamster
author_rank: Regular Contributor
timestamp: September 18, 2014, 01:55:46 pm
content: What is the goal here? Control fan speed? or Reduce the Current draw when you turn it on? You should not feed a Brushless controller with a PWM Signal unless its designed to have one, i would start off at a lower voltage then ramp up to 12v to reduce inrush current.

author: Things
author_rank: Regular Contributor
timestamp: September 18, 2014, 11:19:14 pm
content: The goal is to quietly control fan speed.Thing is though, I thought provided I chose a good capacitor value, the capacitor shouldn't fully discharge between PWM off times - otherwise it'd be pretty useless as a filter. Therefore the load on the FET's/PSU is only a fraction of what it'd be if it had to completely re-charge the cap each time. Looking at the voltage across the cap with my scope, the ripple is only around 1V ?I suppose I could use an inductor LC filter if it's only one addition, but I have no idea how to calculate it

author: SirNick
author_rank: Frequent Contributor
timestamp: September 18, 2014, 11:54:24 pm
content: Looks like you have a circuit sim right there...

author: wraper
author_rank: Unknown
timestamp: September 19, 2014, 12:11:15 am
content: Quote from: Things on September 18, 2014, 11:19:14 pmProblem is not about discharging at all. It is all about charging current which is virtually unlimited. And how big is a ripple voltage on the PSU?

author: wraper
author_rank: Unknown
timestamp: September 19, 2014, 12:50:31 am
content: Simplest you can do. Component values are from a thin air but something thereabouts should work at 20-60 kHz. As inductor must be pretty beefy for 5A, it might be cheaper to get a core and wind it yourself. Also as PWM frequency is pretty high, you must be sure that MOSFET gate is charged fast enough if driven by MCU output directly. It may be not enough and additional driving circuit must be added in a such case. Diode must be fast and preferably Schottky to reduce heat dissipation in it.

author: TMM
author_rank: Frequent Contributor
timestamp: September 19, 2014, 05:40:47 am
content: The electro popped because it's having to do too much work. Just because it's not fully discharging doesn't mean it's not charging/discharging substantial currents.Increase the PWM frequency so you can use a smaller plastic film capacitor and inductor.Circuit as in wraper's post, 50KHz pwm, 100uH inductor, 10uF plastic film cap.This may be of use in regards to winding your own inductor:http://www.diyaudio.com/forums/class-d/89018-big-t-6.html#post1393703

author: wraper
author_rank: Unknown
timestamp: September 19, 2014, 10:53:16 am
content: Quote from: TMM on September 19, 2014, 05:40:47 am10uF film capacitor for 5A current? Maybe it will survive, but certainly won't do the job well. And will be more expensive than electrolytic one. 2200uF is not because of the capacitance as much smaller is needed but because of the ripple current it can hold, LOW ESR one is preferable.

author: TMM
author_rank: Frequent Contributor
timestamp: September 19, 2014, 01:00:29 pm
content: The ripple current through the cap is much less than 5A due to the inductor. Assuming a 2ohm load, i'm getting 0.17Arms through the capacitor at 50% duty cycle, 50KHz switching.Film cap will last for pretty much forever in that application. A 10uF electro will not, even if 0.17Arms ripple current is within it's specification. A 1000uF+ cap will last a lot longer, but then transient response of the circuit is terrible (probably not a huge issue for a couple of fans..)You could get away with as low as 2.2uF with acceptable voltage ripple on the output.

author: wraper
author_rank: Unknown
timestamp: September 19, 2014, 03:05:41 pm
content: Quote from: TMM on September 19, 2014, 01:00:29 pmAnd what output voltage ripple did you calculate with 10uF cap and with 2.2uF one?

author: Things
author_rank: Regular Contributor
timestamp: September 20, 2014, 07:19:57 am
content: Hmm, assuming I'm not able to change the PWM frequency due to other reasons, would an inductor from say, a computer power supply be suitable? Like the large ones they have on the output themselves?

author: TMM
author_rank: Frequent Contributor
timestamp: September 20, 2014, 08:07:15 am
content: Quote from: wraper on September 19, 2014, 03:05:41 pm150mVp-p for 10uF600mVp-p for 2.2uFGood enough for some fans and better than what the OP was getting with their original circuit.Quote from: Things on September 20, 2014, 07:19:57 amThe toroidal ones with maybe 20 turns of heavy gauge wiring? not even close for 490Hz.At 490Hz the inductor needs to be increased to something around 10mH and the capacitor 1000uF.What might be of use is if you find a powersupply that has passive power factor correction and take it's PFC inductor. You know it's the right inductor when it looks like a transformer about 5x5x5cm but only has two wires going to it. I have one out of an old PC powersupply and it is about 50mH but the DC resistance of the windings are around 1.3ohm which is going to cause a fair bit of loss when your load is around 2ohm. If you found one of those and unwound it to around 10mH the loss caused by the DCR might be small enough to be acceptable.

author: Things
author_rank: Regular Contributor
timestamp: September 20, 2014, 09:32:27 am
content: Hmm, damn, was just looking at the exact inductor you mentioned (has about 22 turns). There is another larger one there with a couple of windings it looks like.OK - The PWM frequency options I have are: 62.5Khz, 31.250Khz, 7.812Khz, 3.910Khz, then a few at even lower frequencies.When I tried running the fan on the 2 highest frequencies directly, they didn't do anything at all, so I was a bit concerned about using them. However if the filter is going to be smaller/easier/more efficient at higher frequencies (and give me a smoother output), then that's great.I've tried looking online for low pass LC filter calculators, but there seems to be many different configurations, I can't really find any with just the single inductor, cap and diode. Does anyone have any links perhaps?I'm just a bit confused as to what I'm actually aiming for here - in terms of ripple attenuation values etc.I've basically never used inductors in my circuits before, so I was a bit hesistant about the LC filter, but seems like what I'm going to have to go with. Just not sure what component values I'd need to go with assuming those higher frequencies? Are inductors rated purely by Irms? So in my case, I'd aim for something around 6A? Would 62Khz and 32Khz work better with ferrite or powdered iron inductors etc?Cheers

author: wraper
author_rank: Unknown
timestamp: September 20, 2014, 09:45:13 am
content: Quote from: TMM on September 20, 2014, 08:07:15 amHow did you calculate that? If using Ti toolhttp://www.ti.com/tool/buck-convcalcI get 2x higher ripple with such capacitance. I think there is very good reason why film capacitors are not used for buck converters, size and price. Why to use big very expensive capacitors if you will get worse performance. I would at least use ceramic or tantalum capacitors if completely do not want to use electrolytic ones.

author: wraper
author_rank: Unknown
timestamp: September 20, 2014, 09:53:46 am
content: Quote from: Things on September 20, 2014, 07:19:57 amThat inductor has iron powder core with very low permeability. Well, you can use it but need to wind much more turns compared to the ferrite one.

author: Things
author_rank: Regular Contributor
timestamp: September 20, 2014, 11:04:08 am
content: The main thing stopping me from trying it is I don't have any diodes really beyond some basic rectifiers, nothing even close to fast enough to up the PWM frequency.But then if I'm gonna buy some diodes, I may as well just buy the proper inductors and caps while I'm at it. I just can't seem to find any info on what values I should be looking atBasically, I could use either 62Khz or 32Khz PWM, 12V, would need to handle ~6A constant, and as for ripple, probably even 1V would be more than quiet enough, but if it's as simple as changing the capacitor or inductor value slightly for better ripple, then that's greatI just don't know where to start. 10uH? 100uH? 1000uH? 1H?? 10uF? 100uF? 1000uF?

author: wraper
author_rank: Unknown
timestamp: September 20, 2014, 11:23:52 am
content: Use 62KHz, can use values given with it. Industance can be reduced to about 47 uH and even lower if you are not going to use it with a load lower than 1A. As of capacitor, 1000uF 25V low esr would do well, or use 2x of 1000uf for general purpose to be safe. If you have computer psu, then you can get schottky diode from it. You can use inductor like this:http://lv.farnell.com/bourns/2309-v-rc/inductor-toroid-v-47uh-15-10-7a/dp/1929759

author: Things
author_rank: Regular Contributor
timestamp: September 20, 2014, 11:31:54 am
content: Looks pretty similar to this one I got out of the computer power supply, do you think it'd work too?

author: wraper
author_rank: Unknown
timestamp: September 20, 2014, 11:38:21 am
content: Quote from: Things on September 20, 2014, 11:31:54 amThis one is too small. Only if rewinding biggest inductor.

author: Things
author_rank: Regular Contributor
timestamp: September 20, 2014, 11:39:06 am
content: OK, will see what I can do, thanks for your help

author: wraper
author_rank: Unknown
timestamp: September 20, 2014, 11:42:20 am
content: Actually it might work, but then you would need to know how to measure if it is doing well.

author: Things
author_rank: Regular Contributor
timestamp: September 20, 2014, 11:45:11 am
content: I do have a crappy pocket scope, so I can probably at least do comparisons using various inductor + cap combinations. I'll have to mess around with it in the morning.

author: SeanB
author_rank: Super Contributor
timestamp: September 20, 2014, 12:13:20 pm
content: It would work, but may run warm. Try it, worst case it heats the core up till it changes colour.

author: Things
author_rank: Regular Contributor
timestamp: September 20, 2014, 12:32:58 pm
content: Just tried it at 3.9Khz with that inductor and a 1000uF cap - didn't work all that well. Will try a higher freq tomorrow. I also noticed the FET heated up a lot faster too

author: DanielS
author_rank: Frequent Contributor
timestamp: September 20, 2014, 02:10:57 pm
content: Quote from: Things on September 20, 2014, 12:32:58 pmThere are three main reasons for it to heat up more than it did before bumping the frequency up:1- switching too slowly (dissipates more power during rise/fall since there are more transitions)2- not switching correctly (ex.: too high impedance gate drive causes the gate voltage to oscillate or do other stuff due to CDGand Miller effect)3- slow diode causes voltage across the inductor to rise and trigger avalanche operation (the FET starts conducting even with VGSnear 0V)

author: mikerj
author_rank: Super Contributor
timestamp: September 20, 2014, 06:11:16 pm
content: Quote from: Things on September 20, 2014, 12:32:58 pmJust to check, have you definitely fitted the required flyback diode? If not then you can expect bad things to happen to the FET.

author: Things
author_rank: Regular Contributor
timestamp: September 21, 2014, 06:56:47 am
content: Forum is back finally!So I've been experimenting and confusing myself with this a lot .. and I have a working solution now (almost)Just using the random inductor I posted a pic of earlier, a 1000uF cap, and a FR104 diode out of the same PSU, running at 32Khz, everything is dead silent, which is great .. best of all it works, and nothing is heating up excessively.Measuring directly across the legs of the inductor, I seem to get about 70Vpp, however measuring across the FET between drain and source, I get about 20Vpp, which seems acceptable. The FET isn't heating up either, so it must be OK with it.However, now I seem to have an EMI issue with the microcontrollerThis may have been an issue even with the RC filter - but I assume it's because I upped the PWM frequency to 32Khz, so it's just spewing out RF everywhere. The inductor probably isn't helping either.What would be some ways I could try cut down on this EMI a bit? Ferrite bead on the wires running between the FET and filter perhaps?I'm glad the LC filter works, and works well

author: TMM
author_rank: Frequent Contributor
timestamp: September 21, 2014, 08:31:27 am
content: Quote from: wraper on September 20, 2014, 09:45:13 amLTspice, ideal voltage source to generate the PWM and ideal inductor/capacitor. I see that TI's spreadsheet accounts for parasitics, that should explain the worse ripple.I suggested a plastic film because I figure the average hobbiest would most likely already have through hole electros and polyester caps on hand to prototype, or at least easily available to them. If it is being optimised for mass production, absolutely consider using MLCCs and tants instead.

author: mikerj
author_rank: Super Contributor
timestamp: September 21, 2014, 10:07:56 am
content: Quote from: Things on September 21, 2014, 06:56:47 amThe peak-peak inductor voltage should not be greater than the supply voltage for a buck configuration. Could you post a schematic to show how your components are connected, as it sounds like something is not wired correctly?

author: Things
author_rank: Regular Contributor
timestamp: September 21, 2014, 10:14:16 am
content: I also tried it like this, but didn't seem to change much (if anything)Waveform across source-drain on the FET:It could also possibly be the scope, but I'm not sure.

author: mikerj
author_rank: Super Contributor
timestamp: September 21, 2014, 01:25:53 pm
content: That looks ok, but the high voltages suggest your diode may not be doing anything (maybe open circuit?).I'd personally go for the second configuration as it means you won't have high frequency switching voltages on one of your fan wires which could cause EMI problems.

author: Things
author_rank: Regular Contributor
timestamp: September 21, 2014, 09:38:58 pm
content: Is the FR104 diode even suitable? I just randomly ripped it out of the computer power supply from near the inductors. They're fast recovery and rated at 400V 1A, but I'm not sure exactly what specs I need. The diode definitely makes a difference though - stops the power supply sounding angry and the FET runs a lot cooler

author: Things
author_rank: Regular Contributor
timestamp: September 22, 2014, 11:27:46 am
content: So I've ripped 2 similar inductors from computer power supplies, and wired them up with 2 of the FR104 diodes in parallel as they seemed to be getting quite hot at higher powers.One of the inductors seems to be getting pretty warm though, even at "low" power. Not entirely sure how much current it's handling, but the wire should be more than sufficient for 6A.Is there anything besides excessive current that can cause an inductor to heat? Or do I simply need a beefier inductor?Edit: Just measured using my DMM (so it's probably only getting an average), but it's heating even at only 1A .. seems odd.

author: mikerj
author_rank: Super Contributor
timestamp: September 22, 2014, 01:28:20 pm
content: Quote from: Things on September 21, 2014, 09:38:58 pmIf you are trying to supply up to 5 amps then a 1 Amp diode isn't really going to be up to the jobCheck all the TO220 packages in your PSU, you might find some of them are high current fast rectifiers even if they have three legs.

author: Things
author_rank: Regular Contributor
timestamp: September 22, 2014, 02:15:08 pm
content: I found a nice beefy dual diode, though it's maximum repetitive reverse voltage is only 40V, not sure that'd be high enough considering the ~80Vpp I measured across the inductor ... Though it is good for up to 250A surge

author: wraper
author_rank: Unknown
timestamp: September 22, 2014, 03:28:39 pm
content: Quote from: Things on September 22, 2014, 02:15:08 pmShould work. That voltage you are seeing with your oscilloscope likely is not true at all. Usually you will see very high and short voltage spikes which actually are not there. Anyway voltage on the mosfet drain actually cannot go below zero as there is another diode in the mosfet itself. As of fighting the noise, add another capacitor between +12V and GND as close as possible to the schematic.

author: wraper
author_rank: Unknown
timestamp: September 22, 2014, 03:49:17 pm
content: Quote from: Things on September 21, 2014, 10:14:16 amThis is not good at all although will work. Reason is that your fan power rails will be floating with PWM frequency. Therefore you are making radio transmitter.

author: DanielS
author_rank: Frequent Contributor
timestamp: September 22, 2014, 04:47:11 pm
content: Quote from: Things on September 21, 2014, 09:38:58 pm1.3V forward loss and only 1A rating, not really. For a low-voltage switching supply, you want to use a Schottky diode which has no reverse recovery time and a much lower 0.3-0.6V forward drop. You also want it to be rated for at least half your load current since it will be seeing full load current for part of the time where the FET is off.This will make your circuit even more efficient and clean up some of the ringing and spiking you might be seeing with fast diodes.

author: Things
author_rank: Regular Contributor
timestamp: September 22, 2014, 08:54:54 pm
content: OK, will give the other diode a tryI'm going to order some beefier inductors from e14, is that 47uH the best choice for 32Khz? I decided on that vs 62Khz as my gate drive is only 5V.

author: DanielS
author_rank: Frequent Contributor
timestamp: September 22, 2014, 10:29:45 pm
content: Quote from: Things on September 22, 2014, 08:54:54 pmThere is no "best choice" since that depends on a hundred other parameters like how much you can afford to pay, how much space you can afford for components, transient response requirements, regulation, availability, desired efficiency, current, etc.You cannot pick any random inductor based on inductance value alone either since you may run into unexpected issues with core losses, core saturation and a handful of other parameters.

author: Things
author_rank: Regular Contributor
timestamp: September 22, 2014, 10:45:10 pm
content: This is not something that'll be going into production, so in terms of availability and price I don't really care. Just a hobby project.Basically all I want, from 12V 32Khz PWM, is as close to DC as I can get (though realistically even 500mV ripple is probably acceptable), at a max current of around 6A - but the output voltage will be variable. I have found numerous LC filter calculators online but they all seem to calculate for a specific voltage.

author: kingofkya
author_rank: Regular Contributor
timestamp: September 22, 2014, 11:01:49 pm
content: Reading the first post, your not trying to control a pc/server fan right? Because you can just feed a square wave into the 4th wire on most of them.section 2.4 has some info.http://www.formfactors.org/developer%5Cspecs%5Crev1_2_public.pdf

author: Things
author_rank: Regular Contributor
timestamp: September 22, 2014, 11:05:39 pm
content: Quote from: kingofkya on September 22, 2014, 11:01:49 pmUnfortunately no, I'm essentially dealing with a car radiator fan and a bunch of DC brushless centrifugal blowers (with no PWM input).

author: wraper
author_rank: Unknown
timestamp: September 23, 2014, 07:58:50 am
content: Quote from: Things on September 22, 2014, 08:54:54 pmfor 32 kHz better use 100 or 68 uH. As of gate voltage, frequency is not dependent on it at all. However pwm source must have enough current output capacity to provide fast rise/fall time.

author: Things
author_rank: Regular Contributor
timestamp: September 23, 2014, 12:24:46 pm
content: Have decided to move all the motor drive stuff to it's own board and enclose it an an aluminium box .. hopefully that'll keep the microcontroller from resetting all the time due to EMI.I also went out and bought a couple of toroids, the specs say they're good for up to 50Khz so should be OK. Bought the heaviest magnet wire Jaycar had (1.25mm) which should handle the current fine, and wound about 15 turns on. Not exactly sure what inductance this gives me but I'll soon find out of it works or not I guess

author: wraper
author_rank: Unknown
timestamp: September 23, 2014, 08:26:18 pm
content: That looks like a iron powder core. IMO you would need like a 2x more turns for that. off course you should measure actual inductance rather than guessing. If you don't have anything to measure inductance and no signal generator, you can use your MCU + 2 mosfets to make a square wave.

author: Things
author_rank: Regular Contributor
timestamp: September 23, 2014, 11:52:47 pm
content: I've just had a chance to test it this morning, and it seems to work wellNothing gets even warm and the fans are dead silent in terms of PWM noiseAlso just moving all the power stuff off to it's own board seems to have stopped the resetting issue.

author: Things
author_rank: Regular Contributor
timestamp: September 24, 2014, 06:32:42 am
content: I've completely finished rebuilding the controller now, and everything works perfectThe inductors and diode get a little warm after a few minutes, but I just put a small 40mm fan in the box and that fixed it right up.No more squealing fans or power supplies, and no microcontroller EMI resets, woohoo!Thanks everyone for the help