Dear Mike, Thanks a lot for your reply. I inserted the URL so that it serves as a ready reference for readers to see the diagram of my circuit. As such, the circuit I have constructed is from the Motorola/ON/TI datasheet and the ON Semi design worksheet. There a few things I can pick-up from your reply: * This chip seems to be used quite a lot (like some other people tell me). I had feared that this chip is not so popular for standard products. * This chip should probably be used with a heat sink. There is a thing with the inductor though. I have to use higher values than that are worked out in the calculations. (I use inductors that are wire wound on a powder iron torroidal core which is what quite a few people seem to be using.) As for the testing, I am not testing for peak/transient loads currently. I would first like to ascertain that the heating issue is addressed for steady state load (Vin: 10V, Vout: 5.3V, Iout: 250mA). This power supply design will not plug into a $1000 design but then whatever it plugs into is worth more than that to me! Thanks, Anand On 27-07-2011 01:58, mike wrote:> You asked an open-ended question. Here's an open-ended answer that I > often give fledgeling power supply designers. > > What's the cost of failure? > I've seen people save $20 by copying a random schematic off the web, > spending > a week building it with not-quite-the-same parts, not testing > the result competently, then plugging it into a $1000 device. > Often, it works. One of the significant failure modes is > that the part melts and shorts input to output. How lucky do you feel > today? > Got a spare $1000 device? > > IF you can afford to fail, put on your boots and wade on into the > power supply swamp. > > Power supply design is an art. Yes, you can simulate the schematic > quite accurately. The art is in simulating the ACTUAL circuit including > parasitic elements that are not visible in the pile of parts. > > Even the best-intentioned reference designs from parts vendors have > errors. All it takes is a sleepy typesetter and an incompetent > proofreader. Most of the stuff you find on the web was "designed" > by people without a clue, but some luck. > > The aspects of the design that you question and FIX are not the > things that will cause failure. It's the things you didn't think > about or test for. > Since you disclosed little, we can't help with that. > > For example, nasty things can happen if the inductor saturates. > > Anything you intend to plug into a car electrical system requires > SIGNIFICANTLY more care. > > What does the system do when a peak load > transient exceeds the current limit? I've seen USB hard drives > go into a limit-cycle oscillation when they don't get enough peak > current and thrash themselves to death. > > Power supply design is "system design" and we often have no clue > what's inside the load end of the system. > > FWIW, I've seen that chip used in car cigarette lighter adapters. > They always chose to use a heat sink glued to the chip. > But, just cause you can't feel the package get hot doesn't > mean that the chip temperature isn't making wild transient swings. > Heat sinking a cool chip won't help that. > > Are we having fun yet? >
MC34063 Step-down Converter Heating Problem
Started by ●July 26, 2011
Reply by ●July 27, 20112011-07-27
Reply by ●July 27, 20112011-07-27
Anand P. Paralkar wrote:> Dear Mike, > > Thanks a lot for your reply. I inserted the URL so that it serves as a > ready reference for readers to see the diagram of my circuit. As such, > the circuit I have constructed is from the Motorola/ON/TI datasheet and > the ON Semi design worksheet. > > There a few things I can pick-up from your reply: > > * This chip seems to be used quite a lot (like some other people tell > me). I had feared that this chip is not so popular for standard products. > > * This chip should probably be used with a heat sink.If the temperature is acceptable without it, you don't need a heat sink. BUT You have to make sure you're testing under the conditions that cause the most heating.> > There is a thing with the inductor though. I have to use higher values > than that are worked out in the calculations. (I use inductors that are > wire wound on a powder iron torroidal core which is what quite a few > people seem to be using.)What's the symptom of needing higher values? Toroidal cores come in a WIDE range of materials. Inductance goes down as flux/current goes up. If the rate of change of current goes up as a function of current, current can quickly get out of hand. If the transistor comes out of saturation, it can melt before you can shut it off. This is particularly troublesome at high voltages, like the 30V in your original spec. Google "safe operating area". In general, the more the inductance/turn, the more likely it'll saturate. You really need to look carefully at the curves and stay out of the saturation region for the exact core you're using. Picking a random core and measuring only the inductance is risky.> > As for the testing, I am not testing for peak/transient loads currently. > I would first like to ascertain that the heating issue is addressed for > steady state load (Vin: 10V, Vout: 5.3V, Iout: 250mA).I don't think you'll have any problems at that operating point. It's the 30V and transient/short-circuit load conditions that may give you problems. Assumes your inductor still looks like an inductor under all operating conditions...not saturated. Another thing newbies do is not pay attention to where the currents go. The wiring has parasitic inductance and resistance. So, connecting to different places on the same wire can have different effects on the stability/response of the system. It's very easy to get source transients or load transients coupled into the loop that controls the switch. Typically, not a serious problem on a simple buck converter, but can be disasterous on a push-pull forward converter that depends on an EXACT symmetrical drive to keep the core out of saturation. Even though your switching frequency may be only 20-100KHz., you have to use design practices for a much higher frequency.> > This power supply design will not plug into a $1000 design but then > whatever it plugs into is worth more than that to me! > > Thanks, > Anand > > On 27-07-2011 01:58, mike wrote: >> You asked an open-ended question. Here's an open-ended answer that I >> often give fledgeling power supply designers. >> >> What's the cost of failure? >> I've seen people save $20 by copying a random schematic off the web, >> spending >> a week building it with not-quite-the-same parts, not testing >> the result competently, then plugging it into a $1000 device. >> Often, it works. One of the significant failure modes is >> that the part melts and shorts input to output. How lucky do you feel >> today? >> Got a spare $1000 device? >> >> IF you can afford to fail, put on your boots and wade on into the >> power supply swamp. >> >> Power supply design is an art. Yes, you can simulate the schematic >> quite accurately. The art is in simulating the ACTUAL circuit including >> parasitic elements that are not visible in the pile of parts. >> >> Even the best-intentioned reference designs from parts vendors have >> errors. All it takes is a sleepy typesetter and an incompetent >> proofreader. Most of the stuff you find on the web was "designed" >> by people without a clue, but some luck. >> >> The aspects of the design that you question and FIX are not the >> things that will cause failure. It's the things you didn't think >> about or test for. >> Since you disclosed little, we can't help with that. >> >> For example, nasty things can happen if the inductor saturates. >> >> Anything you intend to plug into a car electrical system requires >> SIGNIFICANTLY more care. >> >> What does the system do when a peak load >> transient exceeds the current limit? I've seen USB hard drives >> go into a limit-cycle oscillation when they don't get enough peak >> current and thrash themselves to death. >> >> Power supply design is "system design" and we often have no clue >> what's inside the load end of the system. >> >> FWIW, I've seen that chip used in car cigarette lighter adapters. >> They always chose to use a heat sink glued to the chip. >> But, just cause you can't feel the package get hot doesn't >> mean that the chip temperature isn't making wild transient swings. >> Heat sinking a cool chip won't help that. >> >> Are we having fun yet? >> >
Reply by ●July 27, 20112011-07-27
On Tue, 26 Jul 2011 22:59:16 -0400, "Michael A. Terrell" <mike.terrell@earthlink.net> wrote:> >Jim Thompson wrote: >> >> I had a CD ignition system breadboard throw me up and over the back of a stool >> without me knocking the stool over (at Philco-Ford Santa Clara, ~1968). Again >> there was much cheering from the technicians... they just love seeing the boss >> getting hit ;-) > > > Did they help you up off the floor, or go on break?They just stood around, laughing their asses off :-( ...Jim Thompson [On the Road, in New York] -- | 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 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●July 27, 20112011-07-27
Jim Thompson wrote:> > On Tue, 26 Jul 2011 22:59:16 -0400, "Michael A. Terrell" > <mike.terrell@earthlink.net> wrote: > > > > >Jim Thompson wrote: > >> > >> I had a CD ignition system breadboard throw me up and over the back of a stool > >> without me knocking the stool over (at Philco-Ford Santa Clara, ~1968). Again > >> there was much cheering from the technicians... they just love seeing the boss > >> getting hit ;-) > > > > > > Did they help you up off the floor, or go on break? > > They just stood around, laughing their asses off :-(Did you remind them of that, during their annual reviews? -- It's easy to think outside the box, when you have a cutting torch.
Reply by ●July 27, 20112011-07-27
On 27 Jul., 19:26, "Michael A. Terrell" <mike.terr...@earthlink.net> wrote:> Jim Thompson wrote: > > > On Tue, 26 Jul 2011 22:59:16 -0400, "Michael A. Terrell" > > <mike.terr...@earthlink.net> wrote: > > > >Jim Thompson wrote: > > > >> I had a CD ignition system breadboard throw me up and over the back =of a stool> > >> without me knocking the stool over (at Philco-Ford Santa Clara, ~196=8). =A0Again> > >> there was much cheering from the technicians... they just love seein=g the boss> > >> getting hit ;-) > > > > =A0 Did they help you up off the floor, or go on break? > > > They just stood around, laughing their asses off :-( > > =A0 =A0Did you remind them of that, during their annual reviews? >a guy I know got knocked over while repairing a defibrillator, no one noticed and he didn't say anything until talking about it a few days later, and all his coworkers went crazy, standard procedure when getting shocked by one of those were 24hour observation because the shock might trigger a heart attack after some time -Lasse
Reply by ●July 27, 20112011-07-27
On Wed, 27 Jul 2011 13:26:44 -0400, "Michael A. Terrell" <mike.terrell@earthlink.net> wrote:> >Jim Thompson wrote: >> >> On Tue, 26 Jul 2011 22:59:16 -0400, "Michael A. Terrell" >> <mike.terrell@earthlink.net> wrote: >> >> > >> >Jim Thompson wrote: >> >> >> >> I had a CD ignition system breadboard throw me up and over the back of a stool >> >> without me knocking the stool over (at Philco-Ford Santa Clara, ~1968). Again >> >> there was much cheering from the technicians... they just love seeing the boss >> >> getting hit ;-) >> > >> > >> > Did they help you up off the floor, or go on break? >> >> They just stood around, laughing their asses off :-( > > > Did you remind them of that, during their annual reviews?Naaah! Good bunch of guys! They just need some entertainment once in a while. I aim to please ;-) ...Jim Thompson [On the Road, in New York] -- | 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 | I love to cook with wine. Sometimes I even put it in the food.
Reply by ●July 27, 20112011-07-27
"langwadt@fonz.dk" wrote:> > On 27 Jul., 19:26, "Michael A. Terrell" <mike.terr...@earthlink.net> > wrote: > > Jim Thompson wrote: > > > > > On Tue, 26 Jul 2011 22:59:16 -0400, "Michael A. Terrell" > > > <mike.terr...@earthlink.net> wrote: > > > > > >Jim Thompson wrote: > > > > > >> I had a CD ignition system breadboard throw me up and over the back of a stool > > > >> without me knocking the stool over (at Philco-Ford Santa Clara, ~1968). Again > > > >> there was much cheering from the technicians... they just love seeing the boss > > > >> getting hit ;-) > > > > > > Did they help you up off the floor, or go on break? > > > > > They just stood around, laughing their asses off :-( > > > > Did you remind them of that, during their annual reviews? > > > > a guy I know got knocked over while repairing a defibrillator, no one > noticed and he > didn't say anything until talking about it a few days later, and all > his coworkers went > crazy, standard procedure when getting shocked by one of those were > 24hour observation > because the shock might trigger a heart attack after some time.Some people are fools. Others are too 'macho' for their own good. -- It's easy to think outside the box, when you have a cutting torch.
Reply by ●July 27, 20112011-07-27
Jim Thompson wrote:> > Michael A. Terrell wrote: > > >Jim Thompson wrote: > >> > >> Michael A. Terrell wrote: > >> > >> >Jim Thompson wrote: > >> >> > >> >> I had a CD ignition system breadboard throw me up and over the back of a stool > >> >> without me knocking the stool over (at Philco-Ford Santa Clara, ~1968). Again > >> >> there was much cheering from the technicians... they just love seeing the boss > >> >> getting hit ;-) > >> > > >> > Did they help you up off the floor, or go on break? > >> > >> They just stood around, laughing their asses off :-( > > > > Did you remind them of that, during their annual reviews? > > Naaah! Good bunch of guys! They just need some entertainment once in a > while. I aim to please ;-)Sounds like you aimed WAY too low. :) -- It's easy to think outside the box, when you have a cutting torch.
Reply by ●July 28, 20112011-07-28
Jim Thompson wrote:> On Tue, 26 Jul 2011 23:47:58 +0530, "Anand P. Paralkar" > <anand.paralkar@gnospammale.com> wrote: > >> Well yes, I can hold my finger on it for 20 seconds? Can I take that as >> a "thumb" rule? :) >> >> Forgot to mention, the package is 8 pin PDIP. >> >> Thanks a lot for the prompt reply. >> >> Anand >> >>> >>> Can you hold your finger on it for 20 seconds? >>> >>> Which package are you using? >>> >>> John S >>> > > One has to be cautious with the "thumb" rule... around 1980 I was so proud of > of an off-line switcher that was so well snubbed it had a load line that > virtually followed the axes. So I took the heatsinks off. Ran just fine. > Wondering how hot it was I touched the flag of a TO-220, forgetting that, > tough it was thermally cool, it has 340V P-P on it. The technicians were > over-joyed ;-) >I designed a board for fast switching of optical switches. The only available supply was a 12V rail so I made the needed HV. Had large font warnings on there in English and Spanish. "Peligro" and all that. Guess who was the guy the board bit? A Chinese engineer was laughing, said he didn't get bitten even though he was not the designer and the warning was not on there in Chinese. -- Regards, Joerg http://www.analogconsultants.com/
