MOSFET transient dissipation
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From: John Fields on 4 Dec 2009 09:23 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 JF
From: pimpom on 4 Dec 2009 10:18 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 $0.37 US each). > To get a better idea, suppose you are pulling a max of 4A > steady > state(ignore any short term transients from turn on since they > will > not have any long term effect) _That_ is the question. *Can* the transient be ignored? Instinct says it can, but instinctive assumptions are always correct. I want to make a bullet-proof design as far as is practicable. > and your Rds_on is 25mOhms. If a 50% > duty cycle then that is 0.2W. 0.2*65 = 13C above ambient. This > is not > much. Of course you need to derate a little and design for > worse > case.. Already covered. Regarding the turn-on surge, I've drawn a load line on the MOSFET characteristics curve with an assumed cold filament resistance of 0.25 ohms. It intersects the 10V Vgs curve at about 45A Id and 0.7V Vds. That's more than 30W dissipation for a brief moment (ignoring gate drive rise time). The fall to the steady-state dissipation of less than 0.4W will be non-linear. What I'm concerned about is the thermal inertia during that brief period.
From: Jan Panteltje on 4 Dec 2009 10:38 On a sunny day (Fri, 4 Dec 2009 20:48:36 +0530) it happened "pimpom" <pimpom(a)invalid.com> wrote in <hfb98d$st6$1(a)news.albasani.net>: > >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. I would mount those MOSFETs on the metal case of the box it is in. This is what I do, same MOSFET, I use a mica washer. It may live without anything to conduct the heat away, but having some heatsink makes you worry less. http://panteltje.com/panteltje/cb/ (bottom page, for diagram) http://panteltje.com/panteltje/pic/swr_pic/index.html (that switch is no in this box). The battery is fused with 2 x 30A fuse.
From: Raveninghorde on 4 Dec 2009 10:47 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. > If you look at the safe operating area graph you will see the FET can withstand about 50A with 1V drop across it. It has a thermal resistance junction to ambient of 62C/Watt and a junction temp limit of 175C so should cope with 2.4W continuous without a heatsink. With an RDS of 0.0175 ohms 2.4 Watts equates to 11.7A continuous. So start worrying somewhere between 11.7A and 50A.
From: Wimpie on 4 Dec 2009 11:02
On 4 dic, 12:25, "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. Hello, Steady state dissipation is about 0.4W, as Rthjc = 62C/W, this results in 25K temperature rise. So the steady state doesn't require a heat sink. Assuming a cold resistance of 12 times below hot resistance, the mosfet has to supply about 50A. This would result in about 63W (0.025 Ohm), but for a very short time as lamp temperature rises rapidly and dissipation is proportional with I^2. Let us assume that 63W is dissipated during 0.1s, that is 6.3J. Look to figure 8 of the datasheet and extrapolates the graph to 100ms. Then you will see that you are close or above the SOA limit (for Dutch speaking readers, Safe Operating Area). When the inrush current reduces to 60% of peak value well within 0.1s, you are within the safe operating area (at 60%, the dissipation drops to 36% of max. dissipation). Do you need a heat sink for the inrush current? Assuming 0.8 gram of copper in the tap results in an average temperature rise of 20K in case of 6.3J added heat. For the transient energy, a heat sink is not required. So my first conclusion is: heatsink is not required as long as inrush current to reach 60% is well below 0.1s (based on 50A peak current). When inrush current > 0.1s, use another mosfet. I think you should measure the inrush current versus time, or try to get reliable data. AFAIK, halogen head lights have more inrush current. Best regards, Wim PA3DJS www.tetech.nl without abc, the address is valid. |