MOSFET transient dissipation
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From: John Fields on 6 Dec 2009 10:41 On Sun, 6 Dec 2009 14:05:09 +0530, "pimpom" <pimpom(a)address.invalid> wrote: >John Fields wrote: >> On Sat, 05 Dec 2009 12:56:55 -0500, ehsjr >> <ehsjr(a)nospamverizon.net> >> wrote: >> >> >>> Hi John, >>> >>> I must say that I have the same problem as pimpom - click >>> on the link but get nothing. What I have to do is use >>> usenet replayer: (watch the line wrap) >>> http://www.usenet-replayer.com/cgi/content/archive?scan=alt.binaries.schematics.electronic >>> >>> The effort to see your posts is always worthwhile, but having >>> to use usenet replayer is a royal PITA. I don't think there >>> is anything you can do at your end of things to change that. >>> :-( >>> >>> One thing I'll request: can you post a test jpeg on abse? The >>> pdf's >>> you post don't show up for me using news.eternal-september.org >>> but jpegs do. I just want to make sure that the difference is >>> jpeg vs pdf and not something else. Maybe Rich (or others who >>> use the same thing) has better luck. Wouldn't be the first >>> time I had a fouled up setting. :-( >> >> --- >> Done. :-) >> >> JF > >John, I sent you the email the same day you asked me to. Did you >get it? --- No. Wanna try again? Or maybe post it here, but disguised? JF
From: Jim Thompson on 6 Dec 2009 10:47 On Sun, 06 Dec 2009 03:26:19 -0800, "JosephKK"<quiettechblue(a)yahoo.com> wrote: >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. Of course they do, but they limit the surge. ...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 | In wine there is wisdom, In beer there is freedom, In water there is bacteria - Benjamin Franklin
From: pimpom on 6 Dec 2009 13:45
John Fields wrote: > On Sun, 6 Dec 2009 14:05:09 +0530, "pimpom" > <pimpom(a)address.invalid> > wrote: > >> >> John, I sent you the email the same day you asked me to. Did >> you >> get it? > > --- > No. > > Wanna try again? Or maybe post it here, but disguised? > > JF OK. Here's an address I haven't used in years. I was surprised to see that it hasn't expired. pimpom(at)37delete-all-letters-and-leave-just-the-figures(dot)com - now let a bot harvest /that/ address. :-) |