SMPS mains circuit

[John Woodgate]

If this 4:1 variation occurs at harmonics higher than n=11, then I
don't find it very surprising. The occasional higher harmonic can
unexpectedly null in a unit, for purely serendipidous combinations of
components and load.

For an SMPS without PFC, there are nulls or quasi-nulls all through the
spectrum, above a specific odd-order. I thought that they could occur in
practice as low as the 9th order, but some recent data indicates that
they can occur at lower orders.

There isn't a corresponding unexpected peaking, as the stray is
undocumented series leakage inductance elements in CMFilter. This
depends on winding method, orientation, turns accuracy and just
sometimes just plain sloppiness. There is an upper limit achievable
without turning the choke into a recieving antenna at other
frequencies.

The nulls I'm talking about are inherent in the circuit design, not due
to strays. The rectifier takes short pulses of current and the spectrum
of those pulses has the nulls or quasi-nulls, depending on the
conduction angle..

This stray leakage begins to look like 100mOhms or larger at the
+decade harmonic, and appears twice in every common-mode position (not
neccessarily with identical measurable values). Even low impedances
strongly influence the sharpness of the impulse input current
waveform, and the higher harmonics that this represents.

Your comments that an LISN is not required in the test procedure is a
little ingenuous.

I wrote 'not required or allowed'. 'Allowed' matters. I was certainly
not intending to be either ingenuous or disingenuous. Just the facts,
ma'am!

The networks were developed to allow predictable and
reproducible high frequency EMI performance. The fact that a similar
line source impedance is not specified in the testing of low frequency
harmonics, when 100mOhm impedances are capable of producing errors
(that may be of concern), is laughable. At the very least, the test
method should indicate that any LISNs present for any other test
purposes should be removed, if their LF impedance is likely to affect
producable harmonic levels.

The standard, the famous IEC/EN 61000-3-2, is extremely specific about
the requirements for voltage distortion of the test supply at the EUT
input. Effectively, this means that the supply source impedance has to
be very low. No competent person would allow an LISN to be included in
the test set-up. We do suspect that one set of results may have had the
'benefit' of an LISN.

 

[legg]

The nulls I'm talking about are inherent in the circuit design, not due
to strays. The rectifier takes short pulses of current and the spectrum
of those pulses has the nulls or quasi-nulls, depending on the
conduction angle..

I had thought that nulls and phase reversals were more likely to be
expected from duty-variant square waves, rather than from haversines
and triangular waveshapes. The latter have less 'artificially' steep
wavefronts, resulting in lower harmonics that reduce more quickly with
order.
........................................
off your topic -

An unexpected property of Genome's elementary constant power model in
LTSpice (from abse) is the generation of increased average value DC
outputs when a series DM inductor is introduced. A 470uH part produces
average DC outputs that are 2% higher than CM-only circuits.

The simulation also shows this DC average voltage's peak ripple to
exceed the AC absolute input peak voltage, whether the discrete DM
part is there or not, even with more specific diode models in the
rectifier.

This goes against most practical experience.

RL

 

[legg]

For an SMPS without PFC, there are nulls or quasi-nulls all through the
spectrum, above a specific odd-order. I thought that they could occur in
practice as low as the 9th order, but some recent data indicates that
they can occur at lower orders.

For straight square waves, the harmonic absolute content vs duty is
plotted for us by Wenzel Associates.

http://www.wenzel.com/pdffiles/choose.pdf

What isn't obvious from this chart is that the sign of the amplitude
has been discarded. The minima actually represent crossings of the
zero axis, going into increasingly inverted ampitudes.

But, like I said, I'd thought that trapezoids, triangles and
haversines showed less evidence of minima, reversals and (signifigant)
higher harmonics generally.

RL

 

[Ken Smith]

[.. about SMPS ...]

Another thought just popped into my head.

A few years back I did a design using some Coilcraft surface mount
inductors. Inductors like the DO5022 don't have a polarity mark. There
were 2 of these on the board. I discovered that the EMI could be 4
different values depending on how the inductors happened to be rotated.

In my case it was a radiated field I was concerned with but I wonder if
there may be some interaction between the fields of the various inductive
elements that is making more or less harmonic noise come out on the power
supply lines.

 

[John Woodgate]

But, like I said, I'd thought that trapezoids, triangles and
haversines showed less evidence of minima, reversals and (signifigant)
higher harmonics generally.

No doubt, but for single-phase capacitor-filtered rectifiers, the
current is a short, quasi-sinusoidal pulse, not any of the waveforms you
mention.

 

[John Woodgate]

I read in sci.electronics.design that Ken Smith <[email protected]>

I wonder if
there may be some interaction between the fields of the various inductive
elements that is making more or less harmonic noise come out on the power
supply lines.

It's a possibility, and it would be the responsibility of the
manufacturer to eliminate that.

 

[Ken Smith]

I read in sci.electronics.design that Ken Smith <[email protected]>


It's a possibility, and it would be the responsibility of the
manufacturer to eliminate that.

Perhaps not:

I don't think it needs to be eliminated so long as the worst case meets
the specs.

In my case the quantity was low enough that manually identifying the ends
of the inductor was an option. Others may just try all 4 versions and
then qualify the worst.


On another subject:

The common mode chokes often used in the input section may be adding a
large variation in performance with regard to harmonics. The leakage
inductance of a common mode choke is a smallish fraction of the chokes
common mode inductance. As a result I would expect the leakage inductance
to vary by a much larger percentage than the common mode inductance. This
leakage inductance will be seen as an impedance on the input side of the
bridge.


Also:

The current in the diodes of the bridge contains a reverse recovery
current. I'd expect this to increase the higher harmonics and to vary a
lot from unit to unit.

 

[John Woodgate]

I read in sci.electronics.design that Ken Smith <[email protected]>

Perhaps not:

I don't think it needs to be eliminated so long as the worst case meets
the specs.

It's not quite as simple as that, which is technically correct. The
point is that the legal responsibility for the causes of these
variations has to be allocated, and causes under the control of the
manufacturer, such as the orientation of inductors, has to be allocated
to the manufacturer. It's the manufacturer's responsibility to decide
whether to control the orientation or accept the greater variability.

In my case the quantity was low enough that manually identifying the ends
of the inductor was an option. Others may just try all 4 versions and
then qualify the worst.

That depends on whether variability is simply accepted or whether some
form of non-arbitrary limitation can be devised (which I doubt at this
point).

On another subject:

The common mode chokes often used in the input section may be adding a
large variation in performance with regard to harmonics. The leakage
inductance of a common mode choke is a smallish fraction of the chokes
common mode inductance. As a result I would expect the leakage inductance
to vary by a much larger percentage than the common mode inductance. This
leakage inductance will be seen as an impedance on the input side of the
bridge.

Yes, although it may not be big enough to matter. Do you have any
typical maximum values?

Also:

The current in the diodes of the bridge contains a reverse recovery
current. I'd expect this to increase the higher harmonics and to vary a
lot from unit to unit.

That's worth investigating, too, just to try to deal with 'nuisance'
failures to conform at just one high-order harmonic frequency. Maybe
100 k resistors across all diodes to swamp the variation?

 

[Ken Smith]

I read in sci.electronics.design that Ken Smith <[email protected]>
wrote (in <[email protected]>) about 'SMPS mains circuit', on [..]

The common mode chokes often used in the input section may be adding a
large variation in performance with regard to harmonics. The leakage
inductance of a common mode choke is a smallish fraction of the chokes
common mode inductance. As a result I would expect the leakage inductance
to vary by a much larger percentage than the common mode inductance. This
leakage inductance will be seen as an impedance on the input side of the
bridge.

Yes, although it may not be big enough to matter. Do you have any
typical maximum values?

For toroidal common mode chokes, it will have a leakage of about 1%. The
other types can run as much as 20%

That's worth investigating, too, just to try to deal with 'nuisance'
failures to conform at just one high-order harmonic frequency. Maybe
100 k resistors across all diodes to swamp the variation?

If it was me, I's spend the extra money for fast, soft recovery
rectifiers. Installing 4 extra resistors would cost more than the
increase in the cost of the rectifiers.

I don't see how the 100Ks would change much of anything. The diode has a
quite low impedance until it is well into recovering.