IRC log for #qi-hardware on 20160315

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10:12.37DocScrutinizer05>>I measured the gate of the MOSFET with a scope and it gives me a very flat 24V.<<  !!!!!
10:18.47DocScrutinizer05provide circuit diagram with testpoints you used for the measurement and I can tell you what exactly is wrong. So far I just can tell that either your probing or your circuit (or both) is flawed
10:21.26DocScrutinizer05for the FET we also need the particular type you used (either part number or at very least whether it's P or N, depletion or enrichment type)
10:24.52DocScrutinizer05shevek: ^^^
10:25.40DocScrutinizer05also please provide details about the PSUs you used
10:48.59DocScrutinizer05searching Mouser I hardly found FETs with Vgs >25V (and that's ABS MAX). Vgs_th (recommended) maximum I found is 12V, usual are up to maybe 5V
10:50.11DocScrutinizer05most are 0.6 to 2V though
10:52.59DocScrutinizer05I hope you didn't connect drain to +24V and load from source to GND
10:55.58DocScrutinizer05otherwise odds are your FET autoadjusts to Vsource = Vgate - Vgs_th (up to 3V maybe), so when you tie gate to source level (24V) you have a autoadjusting voltage of Vds_th between drain and source. multiply that with the current of 4A and you're in the 2figure Watts dissipated by FET
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11:12.02DocScrutinizer05s/Vds/Vgs/
11:18.08DocScrutinizer05first datasheet I found. a tad to the high A and V side, but anyway. Also Vgs_max and Vgs_op are unusually high (15V, 18V)  http://www.mouser.com/ds/2/90/3m0065090d-838565.pdf
11:34.28DocScrutinizer05another arbitrary chosen one: http://paste.opensuse.org/17948781
11:34.41DocScrutinizer05oops
11:34.58DocScrutinizer05http://wstaw.org/m/2016/03/15/plasma-desktoprW2219.png
11:37.32DocScrutinizer05you see there's no need to go higher than 5-10V Vgs with the latter one, and when you have load in Source and tie Gate to Drain, you'll have a ~3V across the Drain-Source pins, no matter which current you draw
11:42.42DocScrutinizer05you also see that in a load-in-Source circuit, you need to drive the Gate with 4-10V *higher* voltage than Vdrain
11:43.45DocScrutinizer05yes, the last is Vdrain, not Vsource
11:45.03DocScrutinizer05there's honestly hardly any good reason to use load-in-Source circuit for on/off applications
11:48.40DocScrutinizer05the exact opposite applies for analog regulators etc, like Voltage regulators and audio amps, where you regularly use load-in-Source circuit - and you regularly expect the FET to get hot
11:54.24DocScrutinizer05another aspect: both of the quoted FET types would escape magic blue smoke with Vgs=24V, it's far beyond ABS MAX for both of them. No matter where the load
11:55.54DocScrutinizer05and even when you got your load in Source, in the moment you apply Vhigh (=24V) to Gate, your Source ist still 0V since the FET hasn't opened yet, so you'll blow the FET any way
11:57.18DocScrutinizer05to mitigate that effect, you need a series R in gate line, to get a sufficiently slow rising slope on gate's capacitance
12:58.14wpwrakyes, Vgs(max) tends to be quite limited. surprised me, too. in anelok, i have some FETs with the gate on VBUS. then i tried to vary them, just to notice that the new ones i was looking at had Vgs(max) = 5.0 V. not good :)
13:00.28wpwrakshevek: but in any case you should post a circuit diagram. hard to tell what's going on in your circuit. if the FET is intended to be either fully on or fully off, it shouldn't burn much power (if properly dimensioned). if it ends up somewhere in the resistive zone of operation, it may very well become an indian communication device (smoke signals)
13:00.55wpwrakalso, how is the gate modulated ? just on/off at a low frequency ? PWM ? ...
13:06.21DocScrutinizer05yeah, another possible failure mode I considered (but rejected as too little plausible) is (PWM) modulation with very high frequency, several MHz
13:07.11DocScrutinizer05anyway I wonder how an arduino(?) is driving a 24V gate voltage
13:07.45DocScrutinizer05without schematics that's all fortune telling
13:12.29rohwell.. either not properly -> smoke signals, or with atleast one more transistor as driver i guess ;)
13:12.58rohwhat i dont quite get: why not just use classic low side switching and a logic level fet?
13:13.06rohits only a heating coil, isnt it?
13:13.12DocScrutinizer05yeah
13:13.43DocScrutinizer05source to GND, drain to coil, gate to arduino, done
13:14.18DocScrutinizer05basically
13:14.50DocScrutinizer05maybe use a normal transistor to control the gate
13:15.17DocScrutinizer05depending on VDD of arduino, and Vgs_th of FET
13:17.16roh4A is 100W.. i guess one could even still switch that bipolar, with a tip121 or such
13:17.40rohi would always suggest using a base or gate resistor still...
13:17.43DocScrutinizer05sure, but a FET has better properties
13:17.54DocScrutinizer05Rds_on is lower
13:19.48DocScrutinizer05yes, base R is mandatory, particularly in common-emitter circuit. Gate R is never a bad idea too
13:19.57rohor use a schematic like this: https://github.com/Traumflug/Generation_7_Electronics/blob/Gen7Board-ARM-2.0/release%20documents/Gen7Board-ARM%202.0%20Schematic.pdf
13:21.19DocScrutinizer05you mean the part around Q1/Q2?
13:23.09DocScrutinizer05and Q4 obviously
13:24.34DocScrutinizer05Q1, R5, R11, D1. Looks decent
13:31.48rohjup
13:32.04rohwas just the first example schem which came up
13:32.10DocScrutinizer05sure
13:32.24DocScrutinizer05they are legion
13:32.57DocScrutinizer05always same
13:33.05DocScrutinizer05FET 101
13:33.28rohstill.. i'd use better fet.. some irf stuff which i know from the ebike controllers.. those can do 72-96V peak at ~20A a piece
13:34.00rohhaving something speced for 4 or 5 A and 30V and use it at 24V 4A is not enough headroom
13:34.19DocScrutinizer05btw... roh you should look for better targets for your explosive experiments ;-)
13:34.42roh?
13:34.47DocScrutinizer05massive traffic jam is not nice
13:35.17rohbaah.. just regular crazies here in berlin.
13:35.39DocScrutinizer05hehe https://www.youtube.com/watch?v=D6mTn0qDKrw
13:37.00*** join/#qi-hardware solrize (~solrize@unaffiliated/solrize)
13:38.36DocScrutinizer05yeah, news tell about the killed guy being involved into drug dealing and fake money
13:38.59DocScrutinizer05pretty unusual execution method
13:39.19DocScrutinizer05so yes, cracy criminals
13:40.38DocScrutinizer05hard to believe some gangster boss spends thousands on C4 when a bullet for 50ct would do
13:42.02DocScrutinizer05I rather suspect the killed guy tried a new business: explosives or weapons
14:14.21wpwrakmaybe the boss wanted to show off
14:51.48*** join/#qi-hardware xiangfu_ (~xiangfu@114.242.250.102)
15:12.32shevekYes, I was surprised by the 24V as well.  Hang on, I'll look for part numbers.
15:15.16shevekhttp://www.digikey.com/product-detail/en/STD17NF03LT4/497-3155-1-ND/654484 is the one I'm driving directly from an Arduino.  I have no resistors at all, just digital pin to gate, source to ground and drain through heater to 24V.
15:17.46shevekThe other is http://www.digikey.com/product-detail/en/STD17NF03LT4/497-3155-1-ND/654484.  It's on a chinese board with a controller to handle things.  But now I see the datasheet it isn't supposed to handle Vgs > 20V.
15:18.58shevekSo you're saying I need a resistor on the gate?  I don't understand what would be its purpose.  I thought there's never supposed to be any current going through the gate?
15:21.14shevekI was originally using PWM on them, and that worked without the parts getting overly hot.  Later I switched to full on/off because I thought this could be causing the heat.
15:41.02shevek(They started getting hot without me making any changes AFAIK.  So I wanted to solve that problem, but I have no idea why it wasn't happening originally.)
15:41.51shevekIs it possible that the lack of a resistor on the gate could break the FET?  Is there a current through the gate that I wasn't told about?
15:43.00shevekOr maybe the limited inductance of the heater coil (it's wrapped around a steel pipe, so there is some) could cause the FET to break?  Would a diode like in the schematic that was referenced fix it?
15:43.27*** join/#qi-hardware pcercuei (~paul@137.71.226.54)
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15:56.01*** join/#qi-hardware sandeepkr__ (~sandeep@111.235.64.4)
15:56.08shevekThe schematic is here: https://github.com/mtu-most/filament_extruder
15:59.53shevekAnd I made a pdf for you, which I put at http://wijnen.dtdns.net/~shevek/extruder.pdf
16:32.33DocScrutinizer05shevek: the first FET in source-to-GND circuit is absolutely OK. Recommended Vgs-on is 5V. You must make sure that arduino delivers an active 5V (or *at least* 3V) to gate for ON, and pulls gate to GND solidly for OFF
16:33.24shevekYes, I'm using 5V power on the Arduino, it's at 4.8V on the digital output.
16:33.39DocScrutinizer05is the output a push-pull?
16:33.59shevekWhen it boots, the first thing I do is set the pin to output, and for off I set it low.  The gate is never floating.
16:34.18DocScrutinizer05resp a internal pullup resistor configured to be enabled?
16:34.43shevekThe internal pullup can only be enabled when it is set to input, and that never happens.
16:34.53DocScrutinizer05ouch
16:35.02shevekIs that a problem?
16:35.12DocScrutinizer05so how is 4.8V going to happen on gate?
16:35.25shevekThe pin is set to output and I set it high.
16:35.26DocScrutinizer05is the output a pushpull?
16:35.52shevekI think so.  It's an atmega328p, just like on an Arduino Uno.
16:36.06DocScrutinizer05for many GPIO on SoC you can configure if pushpull or open-collector
16:36.34DocScrutinizer05I'd add a 10k resistor from gate to +5V
16:36.48shevekAh right.  Yes, the atmega is pushpull in output, and either floating or pullup in input.  I set it to output, so it is always pushpull.
16:37.26DocScrutinizer05when it's set to input, the gate is floating and FET in resistive mode
16:37.45DocScrutinizer05add a pullup or pulldown R
16:37.47shevekIf the output is floating for some reason (because the chip is in reset), I'd prefer the channel to be closed, so pulling it to ground would seem better?
16:37.57DocScrutinizer05yes
16:38.21DocScrutinizer05this is also the explanation why it escaped magic smoke
16:38.50DocScrutinizer05keeping CPU in reset for a few seconds might already suffice
16:38.52shevekBut it shouldn't ever have been floating.  When 24V is applied, it is booting for a few milliseconds, after that the output is asserted.
16:38.58DocScrutinizer05^^^
16:39.06shevekIt's nowhere near seconds.
16:39.24DocScrutinizer05add a pulldown and you're safe
16:39.34DocScrutinizer05100k
16:40.07DocScrutinizer05or 22k, depending on what the internal pullup rresistor of SoC
16:40.24DocScrutinizer05it should be internal R * 0.1
16:41.09DocScrutinizer05I dunno if arduino has weak or strong pullups
16:41.59shevekI'm not sure, but that pullup is never active.
16:43.17DocScrutinizer05ATMEGA328 has no PowerOn reset input?
16:43.59DocScrutinizer05aaa pin 29
16:44.37DocScrutinizer05so that'
16:44.45DocScrutinizer05s on P12, pin2
16:45.00DocScrutinizer05pull that to GND and see your FET go BBQ
16:45.19shevekRight; I don't do that. ;-)
16:45.24shevekBut I am aware of that risk.
16:45.38DocScrutinizer05add a pulldown R to gate and you're safe
16:46.05shevekYes, I'm planning on that for that reason.
16:46.05DocScrutinizer05best EE practice
16:46.23shevekBut I don't understand why it blew up already; I don't think I did anything that was wrong.
16:46.28DocScrutinizer05never build circuits with self destruct function ;-)
16:47.25DocScrutinizer05your crystal might be faulty so the CPU fails for a while (or forever) to generate clock
16:47.57DocScrutinizer05seen that with AT89c51 chip
16:49.05DocScrutinizer05the circuit needed a finger snip to start up, literally
16:49.52DocScrutinizer05knock the PCB, so crystal starts mechanical vibration and suddenly the CPU booted
16:49.58DocScrutinizer05reproducable
16:50.21wpwrakregarding gpio input = Z: careful with clamp diodes. first, don't burn them. second, they can mess up the voltage. e.g. it's perfectly safe to have a NPN transistor (or an n-FET) to drive a p-FET gate at, say, 24 V, with an MCU at 3.3 V or such.
16:50.56DocScrutinizer05hmm?
16:51.52wpwrakdo the same without transistor in the middle, you either burn the clamp diode or worse, or you get at least Vg clamped to the MCU's VCC. so the gate may be at ~3.6 V instead of the desired 24 V, when the FET is "closed"
16:52.16DocScrutinizer05huh?
16:52.23DocScrutinizer05you seen the circuit?
16:52.31DocScrutinizer05http://wijnen.dtdns.net/~shevek/extruder.pdf
16:52.32wpwrakthis was a general remark
16:52.53DocScrutinizer05I don't see how a 24V get on gate there
16:53.04shevekNo, that's the other board.
16:53.25DocScrutinizer05forget the other board
16:53.33shevekThat's a chinese board with a controller on it; it takes a logic input and for some reason puts the high voltage on the gate.
16:53.38shevekYes, let's forget about it.
16:54.16wpwrakthe one with 24 V / FET looks quite right. low-side switching keeps this simple
16:54.37DocScrutinizer05yes
16:54.56DocScrutinizer05just lacks a pulldown for gate when GPIO = Z
16:55.09shevekThe load is 48 windings of nichrome around a steel pipe of d=2cm.  You think that could be a problem with inductance?
16:55.27DocScrutinizer05umm yes
16:55.42DocScrutinizer05add a snubber diode
16:56.21shevekOk.  If that's what went wrong, I can have some confidence that with a diode it won't happen again.
16:56.30DocScrutinizer0548 windings around a steel pipe sounds like a pretty decent inductivity
16:56.35shevekI'll also add a pulldown, of course.
16:56.45shevekBut I'm pretty sure that wasn't the problem.
16:56.56DocScrutinizer05:nod:
16:57.40shevekI didn't quite understand what you were saying about a resistor on the gate.  I understand it's not needed here, but I don't see how it would ever do anything.  Isn't the gate supposed to be zero current at all times?
16:58.08DocScrutinizer05yes, except for charging the gate capacitor
16:58.23shevekAh, ok.  With a capacitor it makes sense indeed.
16:58.31shevekThen it's just a low pass filter.
16:58.44DocScrutinizer05a FET has parasitic gate capacitance
16:58.55DocScrutinizer05some few pF
16:58.58shevekOk.
16:59.16DocScrutinizer05and yes, the R forms a low pass
16:59.47shevekI think I should be able to make this work now.  Thanks!
16:59.57DocScrutinizer05:-) welcome
17:02.52DocScrutinizer05shevek: see https://raw.githubusercontent.com/Traumflug/Generation_7_Electronics/Gen7Board-ARM-2.0/release documents/Gen7Board-ARM 2.0 Schematic.pdf Q1, R5, R11, D1.
17:03.20DocScrutinizer05https://github.com/Traumflug/Generation_7_Electronics/blob/Gen7Board-ARM-2.0/release documents/Gen7Board-ARM 2.0 Schematic.pdf
17:05.28DocScrutinizer05http://wstaw.org/m/2016/03/15/plasma-desktopyw2219.png
17:07.22shevekYes, I saw that.  Should D1 be large, to dissipate the energy, or doesn't that matter much?
17:07.31DocScrutinizer05actually I seem to recall FETs don't like too steep voltage slope on gate, thus the 10R series resistor
17:08.00shevekI can put that in as well, if it's useful.
17:08.06DocScrutinizer05it is
17:08.09shevekOk.
17:08.14DocScrutinizer05common best practice
17:08.23DocScrutinizer05at least for power mosfets
17:08.36shevekThat's what this is.
17:08.42DocScrutinizer05sure :-)
17:09.15shevekBut D1, can I just use a small SMD diode, or must it be large?
17:09.37DocScrutinizer05no, any semi-decent diode will do
17:10.24DocScrutinizer05needs to cope with 4A for a few milliseconds
17:11.36shevekHm, the ones I have here say they go up to 3A; is that a problem then?
17:11.43DocScrutinizer05no
17:11.48DocScrutinizer05absolutely fine
17:11.56shevekOk, good.
17:12.03shevek1A is good enough as well?
17:12.09DocScrutinizer05as long as the have 40V
17:12.28DocScrutinizer05yes, 1A is also fine, when it's continuous rating
17:12.47shevekI have a set which goes from 1A to 3A and from 20V to 90V.
17:13.03DocScrutinizer05you need >30V
17:13.19shevekI'll just use 1A 40V then.
17:13.25DocScrutinizer05sounds fine
17:25.10DocScrutinizer05if you want to do even more to defeat the surge from inductance, you add a 1nF from gate to GND, after the series R
17:25.33DocScrutinizer05or a 4.7nF
17:25.38shevek"after" meaning on the side of the FET?
17:25.42DocScrutinizer05yes
17:26.10shevekWhy would that change anything?
17:26.35DocScrutinizer05it slows down the switching, so the FET takes a tad longer to go from on to off state
17:26.44DocScrutinizer05and vice versa
17:27.08shevekAh right.  That makes sense.
17:27.38DocScrutinizer05so you don't stop from 50mph err 4A to stop in 50cm but in 10m
17:31.53DocScrutinizer05you even should use a 10nF and make the series resistor a 100 or 500R
17:32.03wpwrak(gate resistor) if directly driven by the MCU, you already get some 100 R from the FET at the port
17:32.42DocScrutinizer05right
17:32.48shevekSo I can omit the external gate resistor, or is it a good idea to keep it anyway?
17:33.03DocScrutinizer05I'd keep it, make it 1k even
17:33.14shevekOk.
17:33.23DocScrutinizer05or make it 220R and use a 100nF
17:33.49wpwrakif the FET burns up, is there a failure mode where gate could get shorted to high voltage ? if yes, a gate resistor would protect the MCU
17:33.57DocScrutinizer05http://www.wolframalpha.com/input/?i=100nF+*+220+Ohm
17:34.40DocScrutinizer05still 22us
17:35.18DocScrutinizer05I'm not used to such long timing constants we should use here :-)
17:35.51DocScrutinizer05make that 220kR
17:35.54DocScrutinizer05;-)
17:36.12DocScrutinizer05and 100nF
17:36.19DocScrutinizer05wow, that sounds odd
17:36.39shevekIf the FET burns up, I think I'll throw away the board and make a new one, so protecting the controller isn't really a priority in that case.  Not unless FETs burning up is supposed to be a regular thing, but in that case I want to make them easily swappable. ;-)
17:37.17DocScrutinizer05FET should work infinitely
17:38.01DocScrutinizer05wpwrak: could you please check the timing constant for 220kR -- 100nF ?
17:38.16DocScrutinizer05is it really 22ms?
17:39.08DocScrutinizer05I'm not used to that stuff in this magnitude anymore
17:41.11DocScrutinizer05hmm, well, a 22s for 100uF * 220k sounds right again, so yes, it's prolly 100nF and 220kR you should use
17:46.58DocScrutinizer05shevek: you can use this softstart method _only_ when you don't use ('high' frequency) PWM to regulate the heater
17:47.22shevekAh yes, that's a good point.  I do actually want that.
17:47.54DocScrutinizer05when you use PWM with a frequency >1Hz, you can't use that softstart and you need a diode that has 4A continuous
17:48.23shevekOk.  I'll do that then.
17:49.27DocScrutinizer05or you at least must use a softstart timing that's shorter than 1% of your PWM frequency. So when you use 10Hz PWM, softstart should have a timing for 1ms
17:49.58DocScrutinizer05IOW 100kR and 10nF
17:50.00shevekRight.
17:50.36DocScrutinizer05I suggest slow PWM for a inductive heater
17:51.02DocScrutinizer05fast PWM has no benefits but brings lots of hassle with such stuff
17:51.12shevekThe induction is not its main feature; it's a resistive heater, that happens to have some induction.
17:51.31DocScrutinizer05yeah, I know
17:51.45DocScrutinizer05sorry for the fuzzy wording
17:52.01shevekIt's actually only 12 windings, I was mistaken before; it's 4 of those in parallel.
17:52.28DocScrutinizer05still on a magnetic steel rod
17:52.32shevekYes.
17:52.54DocScrutinizer0512 windings air would be negligible. Steel core changes that completely
17:53.45DocScrutinizer05also take care for proper isolation of the NiCr wire on the steel ;-)
17:53.59shevekYes, of course. :-)
17:54.09shevekIt has a glass fiber coating.
17:54.36DocScrutinizer05good
17:56.05DocScrutinizer05if you want short circuit protection, add a 0.25R/4W resistor between FET source and GND
17:57.31DocScrutinizer05or even as small as 0.1R
17:58.03DocScrutinizer05which then only needs 2W
18:00.35DocScrutinizer05will greatly improve the reliability of your design, avoiding FET to break even on shorts
18:03.10shevekThat might be useful, yes.
18:06.18wpwrakDocScrutinizer05: 22 ms sounds reasonable. this calculator gets tr = 50 ms: http://sim.okawa-denshi.jp/en/CRtool.php
18:07.11DocScrutinizer05for 90%, yes. 22ms is for iirc 63%
18:07.27DocScrutinizer05or 66?
18:07.56wpwraki wonder if adding massive filtering to the gate won't make the FET operate a lot as resistor, thus heating up. kinda not what you want with PWM.
18:08.22DocScrutinizer05as I said: max 1% of PWM freq
18:08.58DocScrutinizer05I strongly discourage to use high freq PWM with heaters of that type
18:09.38wpwrakput a cap across the heater ?
18:10.04DocScrutinizer05the system is very inert, and you get all sorts of trouble, from switching loss like the one we discuss here, to noise generated by the heater itself, wehn you go beyond 50 or 100Hz
18:15.54shevekDocScrutinizer05: I think you mean 63%, 1-1/e.
18:21.08DocScrutinizer05yep, 63
18:23.39DocScrutinizer05and yes, of course the FET gets a little warm with this
18:25.38DocScrutinizer05rough estimation: 100W * 1% * 2 (switch state changes) * 0.5 (very rough estimation for 'efficacy' between ramping FET and load) = 1W
18:26.45DocScrutinizer05probbaly the 0.5 are way too high
18:27.13DocScrutinizer05worst case is half of 24V * half of 4A
18:28.10DocScrutinizer05at 4A voltage is near zero, at 24V current is zero
18:29.47DocScrutinizer05so it's not 0.5 but rather the integral over one half sine * 0.5
18:31.31shevekIntegral, or average?
18:31.42DocScrutinizer05plus the FET going from off to on not during capacitor charging up from 0 to 5V but rather between 2.2V and 2.6V or sth like that
18:32.19wpwrakhmm, not for steady state. what's roughly the DC resistance of the heater ?
18:32.38DocScrutinizer0524v@4A
18:32.41*** join/#qi-hardware GeorgeHahn (~GeorgeHah@c-69-141-92-254.hsd1.nj.comcast.net)
18:32.41shevek6Ω
18:32.47wpwrak60 Ohm ?
18:32.52shevek6
18:32.53wpwrakah . 6 Ohm :)
18:32.57rohon heaters one ususally doesnt do 'fast' pwm... cylces with the length of up to a few seconds are normal
18:33.11DocScrutinizer05what I said
18:33.13wpwrakthe Ω looks like a zero here :)
18:33.13shevekThat makes sense; they don't respond fast anyway.
18:33.29rohanything faster does only heat the fet more, and not help stabilize the temperature much, since there is lots of thermal/mechanical integration happening
18:33.46DocScrutinizer05inertia I called it
18:34.31rohi think our pwm runs on something like a single digit frequency in hz ;)
18:35.11shevek1Hz seems reasonable, I think.
18:35.27DocScrutinizer05microwave ovens have a PWM at ~0.1Hz or less
18:35.30DocScrutinizer05;-)
18:35.52shevekThey have a very slow startup.
18:36.12DocScrutinizer05yeah, sort of
18:36.55DocScrutinizer05the magnetron anode heating takes some time
18:37.22DocScrutinizer05err kathode
18:37.36wpwrakworst-case FET load would be around 2 A, where it would have to burn 24 W. so if the PWM uses the corresponding range, the smoke may return
18:38.05DocScrutinizer05huh?
18:38.38DocScrutinizer05[2016-03-15 Tue 19:25:37] <DocScrutinizer05> rough estimation: 100W * 1% * 2 (switch state changes) * 0.5 (very rough estimation for 'efficacy' between ramping FET and load) = 1W
18:39.49DocScrutinizer05[2016-03-15 Tue 18:49:27] <DocScrutinizer05> or you at least must use a softstart timing that's shorter than 1% of your PWM frequency. So when you use 10Hz PWM, softstart should have a timing for 1ms
18:40.20wpwrakthat's if you quickly ramp through the resistive zone. if you switch fast enough (just a few Hz, it seems, with the large filter), you can hold the gate voltage such that the FET becomes a ~6 Ohm resistor
18:40.30wpwrakso that's what you want to avoid
18:40.30DocScrutinizer05^^^^^
18:41.28DocScrutinizer05I said 10Hz, _not_ 10kHz
18:41.51wpwrakyour low-pass filter is around 7 Hz
18:42.05DocScrutinizer05*sigh*
18:42.15DocScrutinizer05[2016-03-15 Tue 18:49:58] <DocScrutinizer05> IOW 100kR and 10nF
18:43.04wpwrakah, you made it smaller. good. now it's 160 Hz
18:45.09DocScrutinizer05http://www.wolframalpha.com/input/?i=10nF+*+100+kOhm = 1ms
18:46.03DocScrutinizer051000ms/1ms = 0.1%
18:46.30DocScrutinizer05100ms (aka 10Hz)/1ms = 1%
18:47.56DocScrutinizer05[2016-03-15 Tue 19:25:37] <DocScrutinizer05> rough estimation: 100W * 1% * 2 (switch state changes) * 0.5 (very rough estimation for 'efficacy' between ramping FET and load) = 1W. probbaly the 0.5 are way too high. worst case is half of 24V * half of 4A
18:48.24DocScrutinizer05[2016-03-15 Tue 19:31:42] <DocScrutinizer05> plus the FET going from off to on not during capacitor charging up from 0 to 5V but rather between 2.2V and 2.6V or sth like that
18:49.22DocScrutinizer05that's significantly less than the 63% for one T
18:49.59DocScrutinizer05or even the 90% your calculator seems to use
18:50.58DocScrutinizer05so no smoke signals, the FET will just get a little warm
18:51.17wpwrak(my calculator) tr is now 2.3 ms
18:52.16DocScrutinizer05yes, for 90%
18:52.45DocScrutinizer05not for 2.2V to 2.8V, out of 0 to 5V
18:53.47wpwrak(smoke) what i mean that there is/was a fairly low frequency limit before Vg becomes "DC" and keeps the FET operating in resistive mode.
18:53.55DocScrutinizer05yet you avoid transients of 4A resp 24V @ <1us
18:54.41wpwrakwith fc = 160 Hz and a switching frequency around 1 Hz, that would be fine, though
18:55.00DocScrutinizer05we're assuming frequency << timing constant of the low pass filter
18:55.36wpwrakyes. i was worried about the frequency you had before.
18:55.57DocScrutinizer05you should read the complete backscroll
18:56.28wpwrakit's sometimes hard to spot things in those walls of text ;-)
18:57.06DocScrutinizer05the point is that you must not 'spot things' since everxy part of info is equaly relevant
18:57.40wpwraknot if it supersedes previous information :)
18:58.04DocScrutinizer05in that case it's even more relevant
18:58.27wpwrakexactly. hence it's important to spot those bits. qed :)
18:58.48shevekAll info is equally relevent.  Some info is more equally relevant than others. :-p
18:59.03DocScrutinizer05no, since my and roh's suggestion is still to use sub-1Hz PWM frequency
18:59.30DocScrutinizer05thus the original suggestion still is valid
19:00.57DocScrutinizer05the 10Hz was an example why we need to take care, since even at 10Hz the filter timing gets relevant, then I gave an example how to calculate IF the PWM was at 10Hz
19:01.01wpwrakyes, but i was warning about a dangerously near "danger zone". it's not a "suggestion" but a fairly hard speed limit
19:01.24DocScrutinizer05ohmy, we all got that meanwhile
19:01.57wpwraksee, it's simple ;-)
19:02.00DocScrutinizer05Ffilter<<Fpwm != Ffilter >> Fpwm
19:03.30DocScrutinizer05you seen my initial:
19:03.33DocScrutinizer05[2016-03-15 Tue 18:47:53] <DocScrutinizer05> when you use PWM with a frequency >1Hz, you can't use that softstart and you need a diode that has 4A continuous
19:03.35DocScrutinizer05[2016-03-15 Tue 18:48:23] <shevek> Ok.  I'll do that then.
19:03.36DocScrutinizer05[2016-03-15 Tue 18:49:27] <DocScrutinizer05> or you at least must use a softstart timing that's shorter than 1% of your PWM frequency. So when you use 10Hz PWM, softstart should have a timing for 1ms
19:05.27DocScrutinizer05it's really not helping to replace a percieved wall of text with a 10 times larger wall of discussion text just to pick up the lost details in that first wall
19:12.08DocScrutinizer05let's rather evaluate which snubber diodes would work for a heavily inductive load at 24V@4A, for freq of <1Hz and for 50Hz
19:13.13DocScrutinizer05obviously a 4A diode with a Wattage of 4*0.7W should always suffice
19:13.42DocScrutinizer05but I guess we can go significantly 'cheaper'
19:14.38DocScrutinizer05particularly since our inductivity has a ESR of 6 Ohms
19:18.15DocScrutinizer05((would always suffice)) not true, for frequencies of several kHz and way above, you need a schottky (low paraistic capacitance)
21:00.15*** join/#qi-hardware sandeepkr__ (~sandeep@111.235.64.4)

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