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From: Tim Williams on 10 Jan 2010 17:29
"mook johnson" <mook(a)mook.net> wrote in message
news:j3s2n.19852$0U1.2204(a)newsfe16.iad...
> 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.

The ol 'shorted' probe test?

This is more about RFI in general than common mode specifically. When it
switches, current loops and voltage loops throw off electromagnetic
radiation. With the probe anywhere in the near field, you'll pick up a
delicious burst of switching noise. The solution is a coax probe, where
possible. For instance,
http://webpages.charter.net/dawill/Images/RegBO1.jpg
The red/black twisted pair running up left of center goes to a 0.47 ohm
current sense resistor. It sees this waveform:
http://webpages.charter.net/dawill/Images/RegBO2.jpg
regardless of how the 10x probe is connected (notice the clip with wire
above the heatsink, and its ground clip coming across above the
transformer).

If I tried watching the same current with the 10x probe, it would be
unrecognizable from all the trash. Notice the complete absence of ringing
and hash when the transistor switches, or when the secondary diode turns off
(discontinuous mode).

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms


From: Jon Kirwan on 10 Jan 2010 18:04
On Mon, 11 Jan 2010 02:24:13 +0530, "pawihte"
<pawihte(a)fake.invalid> wrote:

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

I'm especially interested in the inductor core situation. If
you do change it, and it helps, let us know. There are some
other good points made but I thought I'd focus on the core,
because I'm still learning about these details and wanted to
consider it a little.

You mentioned 300uH measured and 24mA load and 12 turns on
the inductor. With ferrite, let's use a Bsat = 0.1T to be
safer. We assume not to know mu_r, for now. That still
tells a lot.

L = mu_0 * mu_r * N^2 * A_e / l_m

but also,

Bsat = mu_0 * mu_r * N * I / l_m

This second equation can be solved to place the unknowns on
one side and the knowns on the other:

(mu_r / l_m) = Bsat / (mu_0 * N * I)

That can be stuffed into L, as:

L = N * A_e * Bsat / I

But we know L, so solve for the unknown A_e as:

A_e = L * I / (N * Bsat)

Stuffing in L=300e-6, I=24e-3, N=12, and Bsat=0.1 yields a
value for A_e of 6e-6 m^2. Assuming a circular profile, this
is about a radius of 1.4mm or a diameter of 2.8mm. However,
I don't imagine that the average load current is the peak
inductor current. So this is actually too small to avoid
saturation -- the diameter will need to be greater, I think.

L goes proportional to N^2 while B goes proportional to N, so
if you reduce the mu_r of your core you will only have to
wind more turns by the ratio of change in mu_r and reduces B
allowing more Ipeak. So you might consider finding a
significantly lower mu_r core and winding a few more turns to
get back to your L value. Here, I'm talking about L1, I
think. L2 looks like part of a filter to me so I have little
comment about that, now.

Jon
From: Mark on 10 Jan 2010 18:18
On Jan 10, 6:04 pm, Jon Kirwan <j...(a)infinitefactors.org> wrote:
> On Mon, 11 Jan 2010 02:24:13 +0530, "pawihte"
>
> <pawi...(a)fake.invalid> wrote:
> >Thanks for the reply. I'll try out your suggestions and see what
> >happens.
>
> I'm especially interested in the inductor core situation.  If
> you do change it, and it helps, let us know.  There are some
> other good points made but I thought I'd focus on the core,
> because I'm still learning about these details and wanted to
> consider it a little.
>
> You mentioned 300uH measured and 24mA load and 12 turns on
> the inductor.  With ferrite, let's use a Bsat = 0.1T to be
> safer.  We assume not to know mu_r, for now.  That still
> tells a lot.
>
>     L = mu_0 * mu_r * N^2 * A_e / l_m
>
> but also,
>
>     Bsat = mu_0 * mu_r * N * I / l_m
>
> This second equation can be solved to place the unknowns on
> one side and the knowns on the other:
>
>     (mu_r / l_m) = Bsat / (mu_0 * N * I)
>
> That can be stuffed into L, as:
>
>     L = N * A_e * Bsat / I
>
> But we know L, so solve for the unknown A_e as:
>
>     A_e = L * I / (N * Bsat)
>
> Stuffing in L=300e-6, I=24e-3, N=12, and Bsat=0.1 yields a
> value for A_e of 6e-6 m^2.  Assuming a circular profile, this
> is about a radius of 1.4mm or a diameter of 2.8mm.  However,
> I don't imagine that the average load current is the peak
> inductor current.  So this is actually too small to avoid
> saturation -- the diameter will need to be greater, I think.
>
> L goes proportional to N^2 while B goes proportional to N, so
> if you reduce the mu_r of your core you will only have to
> wind more turns by the ratio of change in mu_r and reduces B
> allowing more Ipeak.  So you might consider finding a
> significantly lower mu_r core and winding a few more turns to
> get back to your L value.  Here, I'm talking about L1, I
> think.  L2 looks like part of a filter to me so I have little
> comment about that, now.
>
> Jon

accurately measuring the spikes at the output of a switchers is an art
and a science.

The loop formed by the scope probe itself and the ground clip can even
pick up spikes that are not really on the output.

Connect the scope probe to the ground where the ground clip is
connected and you should see nothing but you will probably still see
the spikes. This indicates that you need to improve the measurement.

I'm too lazy to search the web for you but you can look around the web
yourself for techniques to measure ripple of switches and see the
various techniques of filtering or common mode rejection that are
suggested to get a more accurate measurement.

To be clear, I am saying that the spikes you are seeing on the scope
are worse then are really there due to measurment technique issues.

Mark





From: Jon Slaughter on 10 Jan 2010 18:49
pawihte 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?

You did attach a load to the output? Compare the input to the regulator to
the output at no load and then try adding a load of maybe 10% of max and see
if it does any better. I don't see why you need a second L-C stage. The led
may not be a large enough load.

Also, your circuit seems a bit different than the buck in the datasheet.
Some of your resistor values are different(not sure if thats intentional or
not). Other than the the circuit looks correct.

On page 7 they give the characteristics of that circuit so you should be
seeing approximately the same(again, some of your component values are
different(R2 and R3).

Try a larger load of at least 100mA or even shorted and see what you get. As
has been mentioned, in an smps design, the output depends on the duty cycle.
For small loads it is non-linear and the duty is significant. For large
loads the regulation is almost independent of the duty. Hence the first
thing is try a larger load and see if that improves anything.


From: Hammy on 10 Jan 2010 20:48
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?
>
>
I have a small 0.5W NCP3063 pcb inverting. Heres a shot of the input
and output ripple below.

Is this what your output looks like?

http://i50.tinypic.com/2l9l0zl.png

That's just using a bead and a cheap 100uf AL cap for my post LC
filter. Yellow is input ripple; blue is output ripple 38mVpp full
load. Are you talking about those skinny spikes riding the ripple on
the blue trace? I'm measuring this with a short ground pin on my probe
and right over a 0.1uf X7R 0603 cap right at the output.

You're always going to have weird ripple on Hysteric controllers but
yours does sound really high. The inductance on your post LC filter
seems unnecessarily large as well.

The advice mook gave is good particularly when measuring with your
scope.Your probe acts like an antenna and picks up switching noise.

Try holding your probe in the air and keep moving it closer and you
will see waveforms on your scope without even touching the circuit.
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