MOSFET transient dissipation
in [Design]
|
Prev: Voltage controlled high current source problem with low valuesense resistor
Next: SMD identification: please help!
From: John Fields on 4 Dec 2009 15:22 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
From: Rich Grise on 4 Dec 2009 15:24 On Fri, 04 Dec 2009 16:55:24 +0530, 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. > > 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. Select a power resistor value such that the bulb filaments are just starting to glow. Shunt the FETs with them (one each). This not only dramatically reduces the inrush current, increasing the life of the bulb, but you get a much "snappier" turn-on (i.e., the bulb reaches incandescence quicker.) If you can't find the right power resistors, then lurk ebay or whatever for some nichrome resistance wire, and make your own. :-) Cheers! Rich
From: Jan Panteltje on 4 Dec 2009 15:52 On a sunny day (Fri, 04 Dec 2009 12:24:27 -0800) it happened Rich Grise <richgrise(a)example.net> wrote in <pan.2009.12.04.20.24.25.234138(a)example.net>: >On Fri, 04 Dec 2009 16:55:24 +0530, 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. >> >> 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. > >Select a power resistor value such that the bulb filaments are just >starting to glow. Shunt the FETs with them (one each). This not only >dramatically reduces the inrush current, increasing the life of the bulb, >but you get a much "snappier" turn-on (i.e., the bulb reaches >incandescence quicker.) > >If you can't find the right power resistors, then lurk ebay or whatever >for some nichrome resistance wire, and make your own. :-) > >Cheers! >Rich mmm why has nobody mentioned PWM yet? From a PIC of course.
From: pimpom on 4 Dec 2009 16:10 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 I use news.albasani.net most of the time and access alt.* groups regularly. I also use eternal-september which I believe does binaries. The link didn't work with either (I know about abse but have never used it). I even tried GG with a browser. Anyway, I'm sending the email. Thanks.
From: pimpom on 4 Dec 2009 16:16
Rich Grise wrote: > On Fri, 04 Dec 2009 16:55:24 +0530, 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. >> >> 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. > > Select a power resistor value such that the bulb filaments are > just > starting to glow. Shunt the FETs with them (one each). This not > only > dramatically reduces the inrush current, increasing the life of > the > bulb, but you get a much "snappier" turn-on (i.e., the bulb > reaches > incandescence quicker.) > > If you can't find the right power resistors, then lurk ebay or > whatever for some nichrome resistance wire, and make your own. > :-) > That's exactly what I mentioned as a possibility in my reply to Jim Thompson. The drawback is that it will keep draining battery power in standby, which is where it will be more than 90% of the time. |