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From: Tim Williams on 10 Jan 2010 20:59
"Jon Kirwan" <jonk(a)infinitefactors.org> wrote in message
news:k4gkk51ls3eqo6bv6pkth2f4ebu9gnqonm(a)4ax.com...
> You mentioned 300uH measured and 24mA load and 12 turns on
> the inductor.

That implies A_L = 2.08 uH/T^2 (unbiased), which is pretty high, typical for
an ungapped high-mu ferrite toroid. They're typically around 1-10 amp-turns
(AT) saturation, pretty easy to saturate. If the peaks are four times
higher than average current, that's easily 0.024A * 12T * 4 = 1.15AT, which
might be enough to saturate it. If it's highly discontinuous, the peaks
could be much higher, bringing it into saturation. And if it is
discontinuous, that could explain the unusually low frequency.

> 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.
>
> <snip>

Bsat ~ 0.4T is more typical, though you might want to drop it to 0.2 or 0.1
for better linearity, or for high frequency use (transformer duty only, DC
choke is different). I usually go with ~0.2T, which is a practical
factor-of-2 headroom, just so I can say I've made the allowance.

Notice that assuming peak B is equivalent to a certain amount of applied
voltseconds. This is simply because EMF = -dB/dt * A_e, or Vs = -B * A_e.
In terms of easy-to-measure parameters, you can get AT(sat) and A_L from the
inductor's V-I curve, which gets you V*s = AT(sat) * A_L / N.

I like to work with Vs because it's more useful to circuit analysis. How
many turns do I need? Integrate voltage over a quarter wave period to get
V.s (it's usually a square wave, so that's just Vpk * 4/f), and I've already
measured the core's A_L and AT(sat), so just divide and you get turns.

In this case, if AT(sat) = 1AT and A_L = 2uH/T^2, then Vs = 2*1 * 12 =
24uVs. At 9V, that's only 2.7us on-time. Pawihte, are you seeing ~3us wide
pulses?

> ...
> That can be stuffed into L, as:
>
> L = N * A_e * Bsat / I

Notice that mu_r disappears -- that means it doesn't matter how much
magnetization you apply, it's the volts and time that gets it up to
saturation. (You could do the same to a powdered iron core (let's say
0.1uH/T^2 and 300AT(sat)), although magnetization current gets absurdly
large for transformer duty!)

> 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.

L2 has to carry the same current, so it'll need the same characteristics,
although it has less delta B, which basically means it can be lossier =
cheaper (one of those ugly yellow/white toroids?).

Powdered iron cores that size are in the 80nH/T^2 range, so you need about
sqrt(300 / 0.08) = 61 turns to get there. Saturation is in the >200AT
range, so you can safely dump over 3A though it -- more likely you won't
even be able to get enough turns inside it to see it saturate before I^2*R
losses overwhelm it.

Tim

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


From: Tim Williams on 10 Jan 2010 21:21
"Hammy" <spam(a)spam.com> wrote in message
news:ravkk510qu6ilvsgkl6lalecgq9t9onk91(a)4ax.com...
> The advice mook gave is good particularly when measuring with your
> scope.Your probe acts like an antenna and picks up switching noise.

Oh, I remembered something else about probes and grounding:

If you put the probe itself through a ferrite bead, you can observe its
effect, if any. Ideally, this won't change anything. If it changes, then
you have current going somewhere it shouldn't, either up through the mains,
or between probes (if you're using more than one).

Last time I did this and saw the effect, I got:
- No bead: fast ringing, medium amplitude, medium decay
- With bead: slow ringing, same amplitude, medium decay
- Damped bead: slow ringing, low amplitude, fast decay

Bead setup: three turns through large ferrite bead. Last test: 10 ohm
resistor soldered in place through the ferrite bead (so it's acting like a
leaky shorted turn).

If you are getting probe current, adding a ferrite bead doesn't really help
your measurement, but it does change it, so you can at least guess what
hides behind the "probe cable resonance" jigglies.

BTW, I happen to have some 3" high-mu ferrite toroids, which are *beautiful*
for putting a couple turns around a big fat probe. (They're supposedly
12uH/T^2, so 3T will get over 100uH easily!)

Tim

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


From: pawihte on 11 Jan 2010 04:05
Mark wrote:
>
> 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.
>

You're right. I touched the probe tip to the ground-clip and the
spikes remained almost the same. In fact, I should have thought
of this myself. Although it must be obvious that I have large
gaps in my knowledge, I've certainly been aware of such induced
pickups for a long time - decades actually. My only excuse is
that it was already the small hours of the morning when I ran the
trial. Thanks for pointing it out.

As a quick test, I placed the whole thing inside a tincan. I
wrapped the whole thing, including the mains transformer, in
bubble plastic wrapping without even grounding the circuit to the
can. Only the 9V output lines (7" of flex) and the mains wire
were outside. The spikes dropped from about +/- 2V to +/- 20mV !

I'll check out the suggestions made by others and report back.


From: pawihte on 11 Jan 2010 04:31
pawihte wrote:
> Mark wrote:
>>
>> 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.
>>
>
> You're right. I touched the probe tip to the ground-clip and
> the
> spikes remained almost the same. In fact, I should have thought
> of this myself. Although it must be obvious that I have large
> gaps in my knowledge, I've certainly been aware of such induced
> pickups for a long time - decades actually. My only excuse is
> that it was already the small hours of the morning when I ran
> the
> trial. Thanks for pointing it out.
>
> As a quick test, I placed the whole thing inside a tincan. I
> wrapped the whole thing, including the mains transformer, in
> bubble plastic wrapping without even grounding the circuit to
> the
> can. Only the 9V output lines (7" of flex) and the mains wire
> were outside. The spikes dropped from about +/- 2V to +/- 20mV
> !
>
> I'll check out the suggestions made by others and report back.

Correction: I was too quick to draw a conclusion. I took out the
whole thing from the tincan again, but the spikes are still only
+/-20mV. (This time I left the scope probe and ground clip
attached to the ends of the output wires). I further observed
that even slightly moving the setup (millimeters) caused the
spikes to shoot up again, and they remain high until I turn it
off and then on again. Even just touching the wires have an
effect. It seems something's unstable.


From: pawihte on 11 Jan 2010 05:51
pawihte wrote:
> pawihte wrote:
>> Mark wrote:
>>>
>>> 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.
>>>
>>
>> You're right. I touched the probe tip to the ground-clip and
>> the
>> spikes remained almost the same. In fact, I should have
>> thought
>> of this myself. Although it must be obvious that I have large
>> gaps in my knowledge, I've certainly been aware of such
>> induced
>> pickups for a long time - decades actually. My only excuse is
>> that it was already the small hours of the morning when I ran
>> the
>> trial. Thanks for pointing it out.
>>
>> As a quick test, I placed the whole thing inside a tincan. I
>> wrapped the whole thing, including the mains transformer, in
>> bubble plastic wrapping without even grounding the circuit to
>> the
>> can. Only the 9V output lines (7" of flex) and the mains wire
>> were outside. The spikes dropped from about +/- 2V to +/- 20mV
>> !
>>
>> I'll check out the suggestions made by others and report back.
>
> Correction: I was too quick to draw a conclusion. I took out
> the
> whole thing from the tincan again, but the spikes are still
> only
> +/-20mV. (This time I left the scope probe and ground clip
> attached to the ends of the output wires). I further observed
> that even slightly moving the setup (millimeters) caused the
> spikes to shoot up again, and they remain high until I turn it
> off and then on again. Even just touching the wires have an
> effect. It seems something's unstable.

I increased the load to ~0.1A, then to 0.6A. The spikes remained
fairly stable at approx +/-10-20mV until I momentarily touch the
output +, at which point the spikes shoot up again until I turn
it off and on. Normal ripple, which was effectively nil at light
load, was about 5mVp-p at 0.6A.

Frequency is now about 45kHz. The ~15kHz I reported earlier was
probably an observation error on my part. There was a lot of
jitter. I think it's time to work on the inductors.


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