MOSFET transient dissipation
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From: pimpom on 4 Dec 2009 16:25 Jan Panteltje wrote: > On a sunny day (Fri, 04 Dec 2009 12:24:27 -0800) it happened > Rich > Grise <richgrise(a)example.net> wrote in > <pan.2009.12.04.20.24.25.234138(a)example.net>: > >> On Fri, 04 Dec 2009 16:55:24 +0530, 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. >>> >>> 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. >> >> Select a power resistor value such that the bulb filaments are >> just >> starting to glow. Shunt the FETs with them (one each). This >> not only >> dramatically reduces the inrush current, increasing the life >> of the >> bulb, but you get a much "snappier" turn-on (i.e., the bulb >> reaches >> incandescence quicker.) >> >> If you can't find the right power resistors, then lurk ebay or >> whatever for some nichrome resistance wire, and make your own. >> :-) >> >> Cheers! >> Rich > > mmm why has nobody mentioned PWM yet? > From a PIC of course. Actually, I did mention it in my reply to Jim Thompson's post. Only I didn't use the term PWM then. But as with paralleling the MOSFET with a resistor, it will be a continuous drain on the battery in standby mode. This is very significant because, out of 14 groups of bulbs, a maximum of only three groups will light up at any one time. And those times will be much shorter than the standby periods.
From: D from BC on 4 Dec 2009 17:14 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. I measured the cold resistance of a 55W halogen (auto).. 0.2 ohm at 22C. Racing can happen in colder weather so there could be less R. The initial current I'm guessing will be about 60 amps. The initial power of the mosfet is about 63watts. The warm resistance is 2.6. The final power of the mosfet is 372mW. Assuming 1 second to reach max brightness. Making the false assumption of linear dynamic load, the energy put into the mosfet is 1/2*(1sec)*63Watts = 31joules (Not sure if that's right.) So... what sort of mosfet heat rise is to be expected from 31joules... I dunno.. But 31 joules of energy might raise the temperature of a Canadian penny 31.4C degrees (If I did the math right..) > >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. > Huh?? The load is wayyy more dynamic than the mosfet... iows.. For this app, assume your mosfet instantly turns into a resistor. All your heat is from the dynamic load current^2* Rdson. The material in the mosfet will rise in temperature depending on the internal energy and thermal conduction to surroundings.
From: legg on 5 Dec 2009 00:41 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. > One thing you might watch out for, if there is any over-current protection involved, are circuit stresses occuring if limiters act during the initial surge. This is easy to overlook. RL
From: pimpom on 5 Dec 2009 01:12 legg 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. >> > > One thing you might watch out for, if there is any over-current > protection involved, are circuit stresses occuring if limiters > act > during the initial surge. This is easy to overlook. > > RL Thanks. At the moment, I don't have any current limiting circuit integrated into the design. The only protection mechanism is several fuses.
From: Jan Panteltje on 5 Dec 2009 06:57
On a sunny day (Sat, 5 Dec 2009 11:42:24 +0530) it happened "pimpom" <pimpom(a)invalid.com> wrote in <hfctk9$drt$1(a)news.albasani.net>: >Thanks. At the moment, I don't have any current limiting circuit >integrated into the design. The only protection mechanism is >several fuses. Here is a differnt solution: 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 |