MOSFET transient dissipation
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From: pimpom on 5 Dec 2009 07:31 Jan Panteltje wrote: > On a sunny day (Sat, 5 Dec 2009 11:42:24 +0530) it happened > "pimpom" > <pimpom(a)invalid.com> wrote in <hfctk9$drt$1(a)news.albasani.net>: > >> Thanks. At the moment, I don't have any current limiting >> circuit >> integrated into the design. The only protection mechanism is >> several fuses. > > Here is a differnt solution: > > > > SRC parallel with MOSFET. > To switch bulb on, trigger the SRC, a pulse a few ms wide will > do. > That SRC will handle the surge caused by the light bulb's cold > resistance no problem. > To switch the bulb off, for a short moment activate the MOSFET. > The MOSFET will pull the SRC voltage close to zero, the SRC > will stop > conducting, > the MOSFET only handles the current caused by the higher 'hot' > resistance of the bulb, > and that only for a few ms seconds. > All you need now is an 'on' pulse to the SRC, and an 'off' > pulse to > the MOSFET. > Much simpler. > Of course, to limit voltage drop and heat in the SRC, you can > also > switch and keep the MOSFET on after the first few ms. Timing > for that > could be easily done with a PIC. > hehe I assume you meant "SCR"? Technically, it's a nice idea. Can't find any flaw in it. Perhaps a TIC126. However, I don't think I'll have time to make the substantial changes it entails - at least for this upcoming race. I'm in the process of putting the finishing touches to the pcb design. I'll certainly keep your suggestion in mind for later. This brings up another question: Why are SCRs rated to withstand their maximum surge currents for a full mains half cycle while MOSFETs are so rated for microseconds?
From: Jan Panteltje on 5 Dec 2009 08:14 On a sunny day (Sat, 5 Dec 2009 18:01:56 +0530) it happened "pimpom" <pimpom(a)invalid.com> wrote in <hfdjrt$f95$1(a)news.albasani.net>: >> SRC parallel with MOSFET. >> To switch bulb on, trigger the SRC, a pulse a few ms wide will >> do. >> That SRC will handle the surge caused by the light bulb's cold >> resistance no problem. >> To switch the bulb off, for a short moment activate the MOSFET. >> The MOSFET will pull the SRC voltage close to zero, the SRC >> will stop >> conducting, >> the MOSFET only handles the current caused by the higher 'hot' >> resistance of the bulb, >> and that only for a few ms seconds. >> All you need now is an 'on' pulse to the SRC, and an 'off' >> pulse to >> the MOSFET. >> Much simpler. >> Of course, to limit voltage drop and heat in the SRC, you can >> also >> switch and keep the MOSFET on after the first few ms. Timing >> for that >> could be easily done with a PIC. >> hehe > >I assume you meant "SCR"? Yes, sorry, typo. >Technically, it's a nice idea. Can't >find any flaw in it. Perhaps a TIC126. However, I don't think >I'll have time to make the substantial changes it entails - at >least for this upcoming race. I'm in the process of putting the >finishing touches to the pcb design. I'll certainly keep your >suggestion in mind for later. > >This brings up another question: Why are SCRs rated to withstand >their maximum surge currents for a full mains half cycle while >MOSFETs are so rated for microseconds? Perhaps because MOSFETS are normally used in switchers, and SCRs in mains 50 Hz or 60Hz power circuits? Once on, in such a circuit, the SCR cannot be switched off, so 'no choice' for 20 or 16 mS.
From: Jim Thompson on 5 Dec 2009 10:31 On Sat, 5 Dec 2009 11:42:24 +0530, "pimpom" <pimpom(a)invalid.com> wrote: >legg wrote: >> On Fri, 4 Dec 2009 16:55:24 +0530, "pimpom" >> <pimpom(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 at http://www.analog-innovations.com | 1962 | In wine there is wisdom, In beer there is freedom, In water there is bacteria - Benjamin Franklin
From: ehsjr on 5 Dec 2009 12:56 John Fields wrote: > On Sat, 5 Dec 2009 00:11:10 +0530, "pimpom" <pimpom(a)invalid.com> wrote: > > >>John Fields wrote: >> >>>On Fri, 04 Dec 2009 08:23:59 -0600, John Fields >>><jfields(a)austininstruments.com> wrote: >>> >>> >>>>On Fri, 4 Dec 2009 16:55:24 +0530, "pimpom" >>>><pimpom(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. >>>> >>>>--- >>>>news:c76ih5lqg5193ombg21q6gqc9skjlhjnhs(a)4ax.com >>> >>>--- >>>Pimpom, did you miss my earlier post? Click on the link above. >>> >>>JF >> >>Nothing happens when I click on it. That kind of link is new to >>me. What's supposed to happen? I'm using OE6 and IE8. > > > --- > It's a link to USENET and it's supposed to take you to > alt.binaries.schematics.electronic where I posted some PDF's of > incandescent lamp surge current waveforms for you. > > If it doesn't work, then just go over there manually and check the post > with the same subject as this one. > > If that still doesn't work then you probably don't have access to the > alt.* groups through your ISP. > > If that's the case, email me your email address and I'll get the stuff > to you that way. > > JF 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. :-( Thanks, Ed
From: John Fields on 5 Dec 2009 14:09
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 |