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From: pawihte on 10 Jan 2010 14:45
I tried my hand at making a 9V power supply with an MC34063A. I
get the correct DC voltage output but was disappointed with the
very dirty output. I know that, in general, simple switched-mode
PSes have poorer performance than linear types, but what I
observed was worse than I expected from the sample circuit given
in the datasheet. This is the schematic, along with the
single-sided pcb layout (in case it's due to poor layout):
http://img683.imageshack.us/img683/4302/9vsmps.png

My main scope is out of order and I used my backup 15MHz
single-trace analog scope. It shows narrow spikes of unsteady
amplitude, varying from roughly +1V/-0.5V to +2/-1V around the dc
level. Moreover, the frequency of about 15 kHz is much lower than
I expected.

The spike amplitudes were first observed without the second L-C
filter. Adding that made little difference at the output of the
first filter, and only a slight reduction at the output of the
second filter. The load was the LED plus a 470-ohm resistor
(total 24mA).

I used general-purpose caps (ESR unknown) for the output filters.
Paralleling them with non-electrolytic plastic and ceramic caps
of 0.1uF have no discernible effect. The timing cap is a ceramic
disc that shows 465pF on my LCR meter. I wound the inductors with
23 swg (~22 awg) enamelled Cu wire on ferrite ring cores.

What am I doing wrong? Is it the filter caps, poor PCB layout or
something else?



From: Archimedes' Lever on 10 Jan 2010 14:59
On Mon, 11 Jan 2010 01:15:29 +0530, "pawihte" <pawihte(a)fake.invalid>
wrote:

>I tried my hand at making a 9V power supply with an MC34063A. I
>get the correct DC voltage output but was disappointed with the
>very dirty output. I know that, in general, simple switched-mode
>PSes have poorer performance than linear types, but what I
>observed was worse than I expected from the sample circuit given
>in the datasheet. This is the schematic, along with the
>single-sided pcb layout (in case it's due to poor layout):
>http://img683.imageshack.us/img683/4302/9vsmps.png
>
>My main scope is out of order and I used my backup 15MHz
>single-trace analog scope. It shows narrow spikes of unsteady
>amplitude, varying from roughly +1V/-0.5V to +2/-1V around the dc
>level. Moreover, the frequency of about 15 kHz is much lower than
>I expected.
>
>The spike amplitudes were first observed without the second L-C
>filter. Adding that made little difference at the output of the
>first filter, and only a slight reduction at the output of the
>second filter. The load was the LED plus a 470-ohm resistor
>(total 24mA).
>
>I used general-purpose caps (ESR unknown) for the output filters.
>Paralleling them with non-electrolytic plastic and ceramic caps
>of 0.1uF have no discernible effect. The timing cap is a ceramic
>disc that shows 465pF on my LCR meter. I wound the inductors with
>23 swg (~22 awg) enamelled Cu wire on ferrite ring cores.
>
>What am I doing wrong? Is it the filter caps, poor PCB layout or
>something else?
>
>
Yes.
From: Martin Riddle on 10 Jan 2010 15:35


"pawihte" <pawihte(a)fake.invalid> wrote in message
news:hidaos$ajr$1(a)news.eternal-september.org...
> I tried my hand at making a 9V power supply with an MC34063A. I get
> the correct DC voltage output but was disappointed with the very dirty
> output. I know that, in general, simple switched-mode PSes have poorer
> performance than linear types, but what I observed was worse than I
> expected from the sample circuit given in the datasheet. This is the
> schematic, along with the single-sided pcb layout (in case it's due to
> poor layout):
> http://img683.imageshack.us/img683/4302/9vsmps.png
>
> My main scope is out of order and I used my backup 15MHz single-trace
> analog scope. It shows narrow spikes of unsteady amplitude, varying
> from roughly +1V/-0.5V to +2/-1V around the dc level. Moreover, the
> frequency of about 15 kHz is much lower than I expected.
>
> The spike amplitudes were first observed without the second L-C
> filter. Adding that made little difference at the output of the first
> filter, and only a slight reduction at the output of the second
> filter. The load was the LED plus a 470-ohm resistor (total 24mA).
>
> I used general-purpose caps (ESR unknown) for the output filters.
> Paralleling them with non-electrolytic plastic and ceramic caps of
> 0.1uF have no discernible effect. The timing cap is a ceramic disc
> that shows 465pF on my LCR meter. I wound the inductors with 23 swg
> (~22 awg) enamelled Cu wire on ferrite ring cores.
>
> What am I doing wrong? Is it the filter caps, poor PCB layout or
> something else?
>
>
>

You might want to experiment with Tantalums for C1, C3.
And looking at your layout there is potential of coupling the input
switching noise to the feed back pin. A small capacitance might help,
with out affecting loop response.
Also this type of switcher creates a lot of hash, so a input filter of
some type (LC) is usefull.

Cheers



From: pawihte on 10 Jan 2010 15:54
Martin Riddle wrote:
> "pawihte" <pawihte(a)fake.invalid> wrote in message
> news:hidaos$ajr$1(a)news.eternal-september.org...
>> I tried my hand at making a 9V power supply with an MC34063A.
>> I get
>> the correct DC voltage output but was disappointed with the
>> very
>> dirty output. I know that, in general, simple switched-mode
>> PSes
>> have poorer performance than linear types, but what I observed
>> was
>> worse than I expected from the sample circuit given in the
>> datasheet. This is the schematic, along with the single-sided
>> pcb
>> layout (in case it's due to poor layout):
>> http://img683.imageshack.us/img683/4302/9vsmps.png
>>
>> My main scope is out of order and I used my backup 15MHz
>> single-trace
>> analog scope. It shows narrow spikes of unsteady amplitude,
>> varying
>> from roughly +1V/-0.5V to +2/-1V around the dc level.
>> Moreover, the
>> frequency of about 15 kHz is much lower than I expected.
>>
>> The spike amplitudes were first observed without the second
>> L-C
>> filter. Adding that made little difference at the output of
>> the first
>> filter, and only a slight reduction at the output of the
>> second
>> filter. The load was the LED plus a 470-ohm resistor (total
>> 24mA).
>>
>> I used general-purpose caps (ESR unknown) for the output
>> filters.
>> Paralleling them with non-electrolytic plastic and ceramic
>> caps of
>> 0.1uF have no discernible effect. The timing cap is a ceramic
>> disc
>> that shows 465pF on my LCR meter. I wound the inductors with
>> 23 swg
>> (~22 awg) enamelled Cu wire on ferrite ring cores.
>>
>> What am I doing wrong? Is it the filter caps, poor PCB layout
>> or
>> something else?
>>
>>
>>
>
> You might want to experiment with Tantalums for C1, C3.
> And looking at your layout there is potential of coupling the
> input
> switching noise to the feed back pin. A small capacitance might
> help,
> with out affecting loop response.
> Also this type of switcher creates a lot of hash, so a input
> filter of
> some type (LC) is usefull.
>

Thanks for the reply. I'll try out your suggestions and see what
happens.


From: mook johnson on 10 Jan 2010 16:57

"pawihte" <pawihte(a)fake.invalid> wrote in message
news:hidaos$ajr$1(a)news.eternal-september.org...
>I tried my hand at making a 9V power supply with an MC34063A. I get the
>correct DC voltage output but was disappointed with the very dirty output.
>I know that, in general, simple switched-mode PSes have poorer performance
>than linear types, but what I observed was worse than I expected from the
>sample circuit given in the datasheet. This is the schematic, along with
>the single-sided pcb layout (in case it's due to poor layout):
> http://img683.imageshack.us/img683/4302/9vsmps.png
>
> My main scope is out of order and I used my backup 15MHz single-trace
> analog scope. It shows narrow spikes of unsteady amplitude, varying from
> roughly +1V/-0.5V to +2/-1V around the dc level. Moreover, the frequency
> of about 15 kHz is much lower than I expected.
>
> The spike amplitudes were first observed without the second L-C filter.
> Adding that made little difference at the output of the first filter, and
> only a slight reduction at the output of the second filter. The load was
> the LED plus a 470-ohm resistor (total 24mA).
>
> I used general-purpose caps (ESR unknown) for the output filters.
> Paralleling them with non-electrolytic plastic and ceramic caps of 0.1uF
> have no discernible effect. The timing cap is a ceramic disc that shows
> 465pF on my LCR meter. I wound the inductors with 23 swg (~22 awg)
> enamelled Cu wire on ferrite ring cores.
>
> What am I doing wrong? Is it the filter caps, poor PCB layout or something
> else?
>

Couple of observations and tests to try.

1) L1 has high dv/dt and is unshielded so it will radiate noise readily.

2) Have you designed L1 and L2 with DC load in mind? you have to gap a
ferrite core or use a powered core with a distributed air gap so the cores
doesn't saturate. Magnetics inc has a good little DC inductor design
program for this purpose. You might consider shielding that core as well.

3) use X5R/X7R ceramic caps in parallel with the large electrolytics. lower
quality ceramic disks (Z5U) are not as good at filtering high frequencies.
Even the input capacitor. 0.1uF shoudl be fine.

4) If your second stage is a thin spike filter the indictor is way too large
and you're using the wrong type of capacitor. Large inductors have high
parasitic capacitance so high frequencies shoot right through them and the
electrolytic caps have high ESL/ESR so high frequencies skip right across
them. (recommend 30uH with high self resonant frequency, DC current rated,
~1uF X7R)

5) C2 is in a high noise spot next to D5 (ringing) and L1 with the high
dV/dt. That trace sets the frequency of the device and is noise sensitive.
the current path from the anode of the diode is between C2 and pin 4 as well
opening hte potential for noise. I would connect C2 directly between pins 3
and 4.

6) With a light load the inductor goes "discontinuous" and is generally more
noisy than continuous mode. With 24mA I'm going to assume you're in
discontiuous mode. discontinuous mode isn't bad in itself but something to
be aware of.

7) (this is a biggie) When probing for noise connect the tip of the probe to
the ground clip and touch this to board ground.
If you're seeing a lot of noise there is a lot of common mode noise between
the board ground and earth ground.
you have to make sure the board ground and scope grounds are at very close
potentials so there is no noise so you can believe what you see whne you
measure. Also a long ground probe lead can be a problem with switching
power supplies. The loop of the wire can pickup noise that is otherwise not
there. I take the probe clip off and wrap a 24-26AWG bus wire around the
top of the probe and leave a tail just long enough the reach the nearest
ground form where I'm probing. I've seen measures noise reductions by a
factor of 10 or better using this method vs the wired ground probe.

Give those a try and let us know what you find.














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