MOSFET transient dissipation
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From: JosephKK on 6 Dec 2009 06:22 On Fri, 04 Dec 2009 14:22:42 -0600, John Fields <jfields(a)austininstruments.com> 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 only looked at two curves most similar to the intended application and saw what i expected. No more than 10x steady state current with an exponential transient that decays to 95% (3 TC) within 10 ms.
From: JosephKK on 6 Dec 2009 06:26 On Fri, 04 Dec 2009 12:31:06 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com/Snicker> wrote: >On Fri, 04 Dec 2009 12:25:21 -0700, Jim Thompson ><To-Email-Use-The-Envelope-Icon(a)My-Web-Site.com/Snicker> wrote: > >>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 > >Or a current limit at maybe 2X operating current... avoids the typical >10X+ surge. > > ...Jim Thompson Both of those increase dissipation in the transistor.
From: JosephKK on 6 Dec 2009 06:29 On Sat, 5 Dec 2009 01:25:30 +0530, "pimpom" <pimpom(a)invalid.com> wrote: >Jim Thompson wrote: >> On Fri, 4 Dec 2009 23:00:39 +0530, "pimpom" >> <pimpom(a)invalid.com> >> wrote: >> >>> 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 ?? >> >Well, a linear ramp would raise the MOSFET's turn-on >dissipation - just the thing I want to limit. Pulsed drive with a >rising duty cycle will stretch out the lamp's turn-on time which >is also unacceptable. Since the lights are used for timing the >racers, they have to be lit up as nearly instantaneously as >possible. > >What I meant by pre-heating was either by pulsing the MOSFETs at >perhaps 25% duty in standby or with a resistor in parallel with >the MOSFET. But either scheme would waste battery power. > At some added complexity you could preheat the next lamp, but this would be counterproductive on the conversion to LED.
From: JosephKK on 6 Dec 2009 06:40 On Sat, 05 Dec 2009 13:14:23 GMT, Jan Panteltje <pNaonStpealmtje(a)yahoo.com> wrote: >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. > They can be commutated, but it is complex, parts intensive, and strange. I have worked with systems that did it, but never designed such myself.
From: Wimpie on 6 Dec 2009 06:57
On 6 dic, 12:22, "JosephKK"<quiettechb...(a)yahoo.com> wrote: > On Fri, 04 Dec 2009 14:22:42 -0600, John Fields > > > > <jfie...(a)austininstruments.com> wrote: > >On Sat, 5 Dec 2009 00:11:10 +0530, "pimpom" <pim...(a)invalid.com> wrote: > > >>John Fields wrote: > >>> On Fri, 04 Dec 2009 08:23:59 -0600, John Fields > >>> <jfie...(a)austininstruments.com> wrote: > > >>>> On Fri, 4 Dec 2009 16:55:24 +0530, "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. > > >>>> --- > >>>>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 only looked at two curves most similar to the intended application > and saw what i expected. No more than 10x steady state current with > an exponential transient that decays to 95% (3 TC) within 10 ms. Hello Joseph, Maybe you can exchange the details with Pimpom. With this numbers, he can use the mosfet without any heat sink and other additional circuits (as I used 100ms for decay to 60% of peak value). With 10ms to 5%, he we be far within the SOA boundary. Best regards, Wim PA3DJS www.tetech.nl without abc, PM will reach me. |