MOSFET transient dissipation
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From: pimpom on 4 Dec 2009 12:18 Jan Panteltje wrote: > 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. That's what I would normally do without asking myself or the group if it's necessary. But my almost-finished pcb design will be considerably simplified if the MOSFETs didn't have to be provided with extra thermal inertia. Oh well. > 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. This unit is going to drive 14 separate loads (not all at the same time). I intend to provide a profusion of fuses. Thanks for your input.
From: pimpom on 4 Dec 2009 12:30 Wimpie wrote: > 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. > Thanks for the input. I also considered biasing the lamps with a pre-heating current to just below incandescence (a dim red glow wouldn't interfere with the application), but that would add complexity and also consume considerable standby power from the battery.
From: John Fields on 4 Dec 2009 13:21 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
From: pimpom on 4 Dec 2009 13:41 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.
From: Jim Thompson on 4 Dec 2009 14:25
On Fri, 4 Dec 2009 23:00:39 +0530, "pimpom" <pimpom(a)invalid.com> wrote: >Wimpie wrote: >> 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. >> >Thanks for the input. I also considered biasing the lamps with a >pre-heating current to just below incandescence (a dim red glow >wouldn't interfere with the application), but that would add >complexity and also consume considerable standby power from the >battery. > RAMP the MOSFET's ON ?? ...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 | You can never be too prepared for the REPRESSION! |