MC34063 Step-down Converter Heating Problem

[Anand P. Paralkar]

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

 

[John S]

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

 

[Anand P. Paralkar]

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

 

[Anand P. Paralkar]

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

 

[Anand P. Paralkar]

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

 

[Anand P. Paralkar]

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

 

[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

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

 

[John S]

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

 

[mike]

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?

 

[Anand P. Paralkar]

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

 

[mike]

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.