MOSFET transient dissipation
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From: Wimpie on 5 Dec 2009 14:48 On 5 dic, 16:31, Jim Thompson <To-Email-Use-The-Envelope-I...(a)My-Web- Site.com/Snicker> wrote: > On Sat, 5 Dec 2009 11:42:24 +0530, "pimpom" <pim...(a)invalid.com> > wrote: > > > > >legg wrote: > >> On Fri, 4 Dec 2009 16:55:24 +0530, "pimpom" > >> <pim...(a)invalid.com> > >> wrote: > > >>> For the drag race Christmas tree project I opened for > >>> discussion > >>> several days ago, I intend to use IRFZ44N MOSFETs to switch > >>> car > >>> headlight bulbs. The MOSFET has an ON resistance of 17.5m? > >>> (say > >>> 25m? warm). The lamps are 12V/45W used in small cars and > >>> should > >>> be just enough to get the transistors slightly warm to the > >>> touch > >>> in the on state without a heatsink. > > >>> However, since incandescent lamps draw a high turn-on surge > >>> current, I wondered if it's advisable to bolt the MOSFETs on > >>> to a > >>> larger thermal mass such as a strip of thick aluminium plate. > >>> To > >>> those of you with experience in that kind of application, > >>> would > >>> you say that such a precaution is - > >>> 1. essential > >>> 2. not likely to be needed but good insurance > >>> 3. a waste, not needed at all. > > >>> Switching frequency is expected to be no more than once in > >>> several seconds at most - essentially a single pulse. I've > >>> looked > >>> at the transient thermal Z of the transistors, but I have no > >>> clear idea how long the turn on surge period is except that > >>> it's > >>> a small fraction of a second and is probably an initial peak > >>> with > >>> an inverse exponential decay. I haven't had time to rig up > >>> something to measure it with a scope. Anything other than a > >>> wild > >>> guess will be welcome. Thanks. > > >> One thing you might watch out for, if there is any over-current > >> protection involved, are circuit stresses occuring if limiters > >> act > >> during the initial surge. This is easy to overlook. > > >> RL > > >Thanks. At the moment, I don't have any current limiting circuit > >integrated into the design. The only protection mechanism is > >several fuses. > > One important thing you are overlooking... "surge" is not a > _dissipation_ issue, it's a local hotspot, or wire-bond-blowing event. > You're liable to end up with "cool but dead" MOS devices. > > ...Jim Thompson > -- > | James E.Thompson, CTO | mens | > | Analog Innovations, Inc. | et | > | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | > | Phoenix, Arizona 85048 Skype: Contacts Only | | > | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | > | E-mail Icon athttp://www.analog-innovations.com| 1962 | > > In wine there is wisdom, > In beer there is freedom, > In water there is bacteria > - Benjamin Franklin Hello Jim, I think you are right. In my example I used 50A, some people say 63A. In that case you exceed the SOA. He will ruin the chip in the end. So he don't need a heatsink, but a bigger (larger) mosfet (2 or three in parallel is also possible). Of course he can also use other methodes as suggested by others. Best regards, Wim PA3DJS www.tetech.nl
From: pimpom on 6 Dec 2009 03:35 John Fields wrote: > On Sat, 05 Dec 2009 12:56:55 -0500, ehsjr > <ehsjr(a)nospamverizon.net> > wrote: > > >> Hi John, >> >> I must say that I have the same problem as pimpom - click >> on the link but get nothing. What I have to do is use >> usenet replayer: (watch the line wrap) >> http://www.usenet-replayer.com/cgi/content/archive?scan=alt.binaries.schematics.electronic >> >> The effort to see your posts is always worthwhile, but having >> to use usenet replayer is a royal PITA. I don't think there >> is anything you can do at your end of things to change that. >> :-( >> >> One thing I'll request: can you post a test jpeg on abse? The >> pdf's >> you post don't show up for me using news.eternal-september.org >> but jpegs do. I just want to make sure that the difference is >> jpeg vs pdf and not something else. Maybe Rich (or others who >> use the same thing) has better luck. Wouldn't be the first >> time I had a fouled up setting. :-( > > --- > Done. :-) > > JF John, I sent you the email the same day you asked me to. Did you get it?
From: JosephKK on 6 Dec 2009 05:49 On Fri, 4 Dec 2009 20:48:36 +0530, "pimpom" <pimpom(a)invalid.com> wrote: >Jon Slaughter wrote: >> pimpom wrote: >>> For the drag race Christmas tree project I opened for >>> discussion >>> several days ago, I intend to use IRFZ44N MOSFETs to switch >>> car >>> headlight bulbs. The MOSFET has an ON resistance of 17.5m? >>> (say >>> 25m? warm). The lamps are 12V/45W used in small cars and >>> should >>> be just enough to get the transistors slightly warm to the >>> touch >>> in the on state without a heatsink. >>> >>> However, since incandescent lamps draw a high turn-on surge >>> current, I wondered if it's advisable to bolt the MOSFETs on >>> to a >>> larger thermal mass such as a strip of thick aluminium plate. >>> To >>> those of you with experience in that kind of application, >>> would >>> you say that such a precaution is - >>> 1. essential >>> 2. not likely to be needed but good insurance >>> 3. a waste, not needed at all. >>> >> >> 3. The surge current will only be transitory and not have much >> effect >> once the lamp heats up and should not have any significant >> effect. >> Most devices can handle much more transient effects than steady >> state. >> >Of course. But how much more for how long, that's the question. >The transitory period is brief but finite. > >> Your lamp is drawing about 4A. If, say, the mosfets Rds_on is >> 50mOhms >> then that is just 0.8W dissipation max in the mosfet(assuming >> it is >> always on, which in your case it's not, if it's 50% duty then >> thats >> only about 0.4W) >I'm not concerned about the steady dissipation once the lamp is >fully heated up. The math is simple and, as I said in my initial >post, it should make the transistor only slightly warm to the >touch even without a heatsink. > >> Thats well under under what the mosfet can handle. In fact your >> mosfet is too big. It has a 62C/W for no heat sink. Hence you >> do not >> need to use a heat sink. It will get hot of course but it can >> easily >> withstand double what you are using assuming normal ambient >> temperatures. >I'm using this transistor because 1) I want to be sure it can >handle the turn-on surge, and 2) it's cheap and easily available >here(I got them for the equivalent of $0.37 US each). > >> To get a better idea, suppose you are pulling a max of 4A >> steady >> state(ignore any short term transients from turn on since they >> will >> not have any long term effect) > >_That_ is the question. *Can* the transient be ignored? Instinct >says it can, but instinctive assumptions are always correct. I >want to make a bullet-proof design as far as is practicable. > >> and your Rds_on is 25mOhms. If a 50% >> duty cycle then that is 0.2W. 0.2*65 = 13C above ambient. This >> is not >> much. Of course you need to derate a little and design for >> worse >> case.. > >Already covered. > >Regarding the turn-on surge, I've drawn a load line on the MOSFET >characteristics curve with an assumed cold filament resistance of >0.25 ohms. It intersects the 10V Vgs curve at about 45A Id and >0.7V Vds. That's more than 30W dissipation for a brief moment >(ignoring gate drive rise time). The fall to the steady-state >dissipation of less than 0.4W will be non-linear. What I'm >concerned about is the thermal inertia during that brief period. > So continue the temporal thermal analysis. The curve is exponential like charging a capacitor. Do the rather simple integral for temperature rise. (to simplify assume no heat sink and theta ja)
From: JosephKK on 6 Dec 2009 06:04 On Fri, 4 Dec 2009 12:01:10 -0800, dplatt(a)radagast.org (Dave Platt) wrote: >>Already covered. >> >>Regarding the turn-on surge, I've drawn a load line on the MOSFET >>characteristics curve with an assumed cold filament resistance of >>0.25 ohms. It intersects the 10V Vgs curve at about 45A Id and >>0.7V Vds. That's more than 30W dissipation for a brief moment >>(ignoring gate drive rise time). The fall to the steady-state >>dissipation of less than 0.4W will be non-linear. What I'm >>concerned about is the thermal inertia during that brief period. > >How about slowing down the turn-on pulse, and "warming up" the >filament (and raising its resistance) more gradually? > >Run your gate drive through an RC with a reasonable time constant, and >then feed it to the gate through a gate-stopper resistor. Depending >on the time constant of the RC, and the thermal time constant of the >filament, I imagine that you can probably bring the lamp up to full >brightness in (e.g.) 100 milliseconds or so, without the current >rising to more than double its steady-state value at any point. This >might result in a better-looking power dissipation curve. You may >increase the service life of the bulbs, too. > >If you do decide to bang the MOSFET all the way on as fast as >possible, it wouldn't surprise me if the MOSFET's ability to get rid >of the transient heat pulse is limited by the rate of heat conduction >through the package and tab. The current surge may be over, and the >dissipation settled down to its steady state, before much of the heat >has been conducted out to the far side of the tab. If that turns out >to be the case, adding a heatsink (for additional thermal inertia at >this point) might not buy you much. So you propose to increase the transistor heat transient by an order of magnitude?
From: JosephKK on 6 Dec 2009 06:08
On Sat, 5 Dec 2009 01:47:53 +0530, "pimpom" <pimpom(a)invalid.com> wrote: >Dave Platt wrote: >>> Already covered. >>> >>> Regarding the turn-on surge, I've drawn a load line on the >>> MOSFET >>> characteristics curve with an assumed cold filament resistance >>> of >>> 0.25 ohms. It intersects the 10V Vgs curve at about 45A Id and >>> 0.7V Vds. That's more than 30W dissipation for a brief moment >>> (ignoring gate drive rise time). The fall to the steady-state >>> dissipation of less than 0.4W will be non-linear. What I'm >>> concerned about is the thermal inertia during that brief >>> period. >> >> How about slowing down the turn-on pulse, and "warming up" the >> filament (and raising its resistance) more gradually? >> >> Run your gate drive through an RC with a reasonable time >> constant, and >> then feed it to the gate through a gate-stopper resistor. >> Depending >> on the time constant of the RC, and the thermal time constant >> of the >> filament, I imagine that you can probably bring the lamp up to >> full >> brightness in (e.g.) 100 milliseconds or so, without the >> current >> rising to more than double its steady-state value at any point. >> This >> might result in a better-looking power dissipation curve. You >> may >> increase the service life of the bulbs, too. >> >A slow build-up of light is unacceptable for the reasons I gave >in the reply I just posted to Jim Thompson's post. Please see >that. > >> If you do decide to bang the MOSFET all the way on as fast as >> possible, it wouldn't surprise me if the MOSFET's ability to >> get rid >> of the transient heat pulse is limited by the rate of heat >> conduction >> through the package and tab. The current surge may be over, >> and the >> dissipation settled down to its steady state, before much of >> the heat >> has been conducted out to the far side of the tab. If that >> turns out >> to be the case, adding a heatsink (for additional thermal >> inertia at >> this point) might not buy you much. > >That's what I'm concerned about and is really what this thread is >about. Hmmm > Here is a really crazy idea: Can you build it such that the power transistors are thermally connected to a water mass? Like put the power tabs into a water tank? The thermal mass of even a little bit of water is truly impressive. |