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MC34063 Step-down Converter Heating Problem

Discussion in 'Electronic Basics' started by Anand P. Paralkar, Jul 26, 2011.

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  1. 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:

    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.

  2. John S

    John S Guest

    Can you hold your finger on it for 20 seconds?

    Which package are you using?

    John S
  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. John S

    John S Guest

    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
  8. John S

    John S Guest

    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.
    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?
    You could try reducing the switching frequency along with increasing the
    Not for your current operating condition according to the "finger test."

    John S
  9. mike

    mike Guest

    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,
    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
    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?
  10. 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!

  11. mike

    mike Guest

    If the temperature is acceptable without it, you don't need a heat sink.
    You have to make sure you're testing under the conditions that cause
    the most heating.
    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.
    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
    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.
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