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MC34063 Step-down Converter Heating Problem

Started by Anand P. Paralkar July 26, 2011
Hi,

I have constructed a DC-DC step-down converter using the MC34063.  The 
circuit I use is very similar to the one shown here:

http://www.eleccircuit.com/wp-content/uploads/2008/01/ipod-mp3-player-charger-by-mc34063.jpg

My circuit does not have the Zener, output side LED and input side 
protection diodes (TVS1 and D2 in the circuit).  The rest of the circuit 
is quite similar.

The operating conditions for my circuit are as follows:

Vin : 10V to 30V.
Vout : 5V
Iout: 250mA

I tested the circuit at:

Vin: 10V, Vout: 5.3V, Iout: 250mA

I observe that the MC34063 is heating.  I am unable to say whether this 
heating is acceptable.

You could help me by answering these questions:

   1. Is there an accurate/thumb-rule method of _measuring_ the amount 
of power dissipated in an IC (MC34063 in this case).

   2. Categorically confirm or deny that the MC34063 is likely to heat 
at the test conditions (Vin: 10V, Vout: 5.3V, Iout: 250mA).

   3. Methods to reduce the power dissipation in the MC34063 at these 
test conditions.

   4. Is the heating really a problem?

I realise that some of these questions are open-ended, but it would be 
great to get some tips from the experts.  Thanks a lot for your time.

Thanks,
Anand



On 7/26/2011 12:42 PM, Anand P. Paralkar wrote:
> Hi, > > I have constructed a DC-DC step-down converter using the MC34063. The > circuit I use is very similar to the one shown here: > > http://www.eleccircuit.com/wp-content/uploads/2008/01/ipod-mp3-player-charger-by-mc34063.jpg > > > My circuit does not have the Zener, output side LED and input side > protection diodes (TVS1 and D2 in the circuit). The rest of the circuit > is quite similar. > > The operating conditions for my circuit are as follows: > > Vin : 10V to 30V. > Vout : 5V > Iout: 250mA > > I tested the circuit at: > > Vin: 10V, Vout: 5.3V, Iout: 250mA > > I observe that the MC34063 is heating. I am unable to say whether this > heating is acceptable. > > You could help me by answering these questions: > > 1. Is there an accurate/thumb-rule method of _measuring_ the amount of > power dissipated in an IC (MC34063 in this case). > > 2. Categorically confirm or deny that the MC34063 is likely to heat at > the test conditions (Vin: 10V, Vout: 5.3V, Iout: 250mA). > > 3. Methods to reduce the power dissipation in the MC34063 at these test > conditions. > > 4. Is the heating really a problem? > > I realise that some of these questions are open-ended, but it would be > great to get some tips from the experts. Thanks a lot for your time. > > Thanks, > Anand
Can you hold your finger on it for 20 seconds? Which package are you using? John S
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 >
On 7/26/2011 1:23 PM, Anand P. Paralkar 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
I use the rule-of-thumb that if you can hold your finger on the device, then its temperature is not much more than about 140F. Of course, your finger cools the chip a little. Looks like you are safe. John S
On 7/26/2011 12:42 PM, Anand P. Paralkar wrote:
> Hi, > > I have constructed a DC-DC step-down converter using the MC34063. The > circuit I use is very similar to the one shown here: > > http://www.eleccircuit.com/wp-content/uploads/2008/01/ipod-mp3-player-charger-by-mc34063.jpg > > > My circuit does not have the Zener, output side LED and input side > protection diodes (TVS1 and D2 in the circuit). The rest of the circuit > is quite similar. > > The operating conditions for my circuit are as follows: > > Vin : 10V to 30V. > Vout : 5V > Iout: 250mA > > I tested the circuit at: > > Vin: 10V, Vout: 5.3V, Iout: 250mA > > I observe that the MC34063 is heating. I am unable to say whether this > heating is acceptable. > > You could help me by answering these questions: > > 1. Is there an accurate/thumb-rule method of _measuring_ the amount of > power dissipated in an IC (MC34063 in this case).
The most obvious way is to use a thermometer of some sort. Do you have thermocouples, RTDs, or thermistors? Aside from actually measuring the temperature, you could measure input power, output power, and ancillary component powers and then subtract the output power and ancillary component powers from the input power to get the chip's dissipation. You can then calculate the chip's approximate temperature from its thermal resistance.
> 2. Categorically confirm or deny that the MC34063 is likely to heat at > the test conditions (Vin: 10V, Vout: 5.3V, Iout: 250mA).
It is likely to heat under almost any loaded condition. It has no choice but to lose some power. The question is how much will it heat at your maximum load condition and is that too much?
> 3. Methods to reduce the power dissipation in the MC34063 at these test > conditions.
You could try reducing the switching frequency along with increasing the inductor.
> 4. Is the heating really a problem?
Not for your current operating condition according to the "finger test."
> I realise that some of these questions are open-ended, but it would be > great to get some tips from the experts. Thanks a lot for your time. > > Thanks, > Anand
Cheers, John S
Anand P. Paralkar wrote:
> Hi, > > I have constructed a DC-DC step-down converter using the MC34063. The > circuit I use is very similar to the one shown here: > > http://www.eleccircuit.com/wp-content/uploads/2008/01/ipod-mp3-player-charger-by-mc34063.jpg > > > My circuit does not have the Zener, output side LED and input side > protection diodes (TVS1 and D2 in the circuit). The rest of the circuit > is quite similar. > > The operating conditions for my circuit are as follows: > > Vin : 10V to 30V. > Vout : 5V > Iout: 250mA > > I tested the circuit at: > > Vin: 10V, Vout: 5.3V, Iout: 250mA > > I observe that the MC34063 is heating. I am unable to say whether this > heating is acceptable. > > You could help me by answering these questions: > > 1. Is there an accurate/thumb-rule method of _measuring_ the amount of > power dissipated in an IC (MC34063 in this case). > > 2. Categorically confirm or deny that the MC34063 is likely to heat at > the test conditions (Vin: 10V, Vout: 5.3V, Iout: 250mA). > > 3. Methods to reduce the power dissipation in the MC34063 at these > test conditions. > > 4. Is the heating really a problem? > > I realise that some of these questions are open-ended, but it would be > great to get some tips from the experts. Thanks a lot for your time. > > Thanks, > Anand > > >
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?
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 ;-) ...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.
Jim Thompson formulated on Wednesday :
> 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 ;-) > > ...Jim Thompson >
That sounds much the same as touching the grid caps af an ancient PA amplifier then touching the Cap of the output 807. The resulting sound as a pitch of some 4 or 5 hundred volts from the technician. :-[
On Wed, 27 Jul 2011 08:40:54 +1000, John G <greentest@ozemail.com.au> wrote:

>Jim Thompson formulated on Wednesday : >> 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 ;-) >> >> ...Jim Thompson >> >That sounds much the same as touching the grid caps af an ancient PA >amplifier then touching the Cap of the output 807. >The resulting sound as a pitch of some 4 or 5 hundred volts from the >technician. :-[ >
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 ;-) ...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.
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? -- It's easy to think outside the box, when you have a cutting torch.