MOSFET transient dissipation
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From: pimpom on 4 Dec 2009 06:25 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.
From: pimpom on 4 Dec 2009 06:29 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). I used the Alt key code for the ohm symbol here and it looked ok while composing, but it's turned into a question mark after posting. So please read m? as milliohm.
From: Brane2 on 4 Dec 2009 07:03 On 4 dec., 12:29, "pimpom" <pim...(a)invalid.com> 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). I'd go for the FETs that are used on motherboards in VRM - for confersion of 12V to 1.something at 100 amps for CPU core. Those things are cheap and plentiful and should have low Rdson...
From: Raveninghorde on 4 Dec 2009 07:53 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. > Get the data sheet for the FET and there will be a graph for safe operating area.
From: Jon Slaughter on 4 Dec 2009 08:19
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. 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) 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. 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) 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.. |