# LTspice question...Inductifiers

Discussion in 'Electronic Design' started by Martin Griffith, Oct 13, 2007.

1. ### Martin GriffithGuest

Just mucking around with a LT1533 low noise smps simulation, and I
realised how little I know (what's new?) about spice, and it's too
damn windy to walk up the hill for a beer (spanish, yuck)

Each inductor has a value called Parallel Resistance, and I really
don't have a clue how to give it a nominal value/guess

Any rules of thumb?
Nominal L's are from 470u to 8mH

Martin

2. ### D from BCGuest

http://www.intusoft.com/articles/inductor.pdf

"
The magnetic loss can be modelled reasonably well as a
parallel resistor (Rp) across the existing model. The value
can again be calculated from data sheet parameters, using
the quality factor (Q). In a parallel RLC3 circuit the relationship
between the quality factor and inductance is given
by; Q = Rp/2piFoLo "

So...I suppose.. Since V^2/Rp = power then
Hot core from losses ..low Rp
Cool core from gentle use...high Rp

I think Rp generally drops in value with increasing B swing or
increasing frequency.
It's gonna be core material dependent too..

Come to think of it...I think Rp is fake in spice.. It's not really
there.. I think it's just included to explain where the power is
disappearing.

D from BC

3. ### Martin GriffithGuest

Ha
I'm just going to send the ltspice file to my tranny maker/guru and

Martin

4. ### James ArthurGuest

Martin, if you're just poking around you don't have to enter any value
at all--the simulation will just run with a 'perfect' inductor.

In real life, a decent guess is to omit the parallel value, and assume
a series resistance for the inductor such that (iL.average)^2 * R =
2-3% of total power. With equal copper and core losses, that's about
5% of efficiency loss budgeted for the inductor. Adjust per
application.

HTH,
James Arthur

5. ### Martin GriffithGuest

great stuff, LTspice is excellent, but when doing a transformer,there
are too many thingummies to make sense of, unfortuantely LTspice on
yahoo groups is a bit like snowboarding through mud

Martin

6. ### Tam/WB2TTGuest

Won't this cause a problem in some applications? A series resistor will have
a DC drop, whereas a parallel will not. If you know the Q, you can make the
parallel resistance 2*PI*f*L*Q, or a series resistor of 2*PI*f*L/Q; or,
split the R between series and parallel. For an air core, you can make the
Rp=10E10. A stab in the dark guess for inductors wound with decent size wire
might be Q=100 for the 470u and Q=30 for the 8mH.

Tam

7. ### WinfieldGuest

I've had good results simulating my transformers in spice.
Most of of time I design them to avoid core saturation, or
even excessive core losses, if I can. I also like to see
the important transformer parameters right on my circuit,
so I use a perfect transformer with its turns ratio, add a
parallel input inductor for the core magnetizing inductance
Lm = AL N^2, where you get AL from the core's datasheet.
Getting the right value for Lm usually isn't very important.

Then, very important, I add input and output resistors for
the two copper winding resistances (these do a better job of
dealing with the high peak currents from rectifiers driving
storage caps, etc., than a general parallel loss parameter),
an ohmmeter is handy for this.

Series leakage inductance is important. This can be measured
(while shorting the other side), or it can be calculated (see
posts by Tony Williams) or you can try Lell = 1% or 2% of Lm
as a starting estimate. You only add the measured leakage
inductance once, on either the primary or secondary side - its
value includes both sides. If you have multiple secondaries,
measure Lell between them and place half that on each one,
then measure the primary and subtract the turns-ratio-squared-

This overall scheme gives spice results that are pretty close
to the bench results, unless you're at high frequencies and
high ac voltages, where core loss is an issue. You can try
estimating the power loss from core-material curves, and add
a parallel resistor to implement it. But generally I'm less
concerned with an accurate primary power consumption estimate
than I am with knowing an accurate secondary waveform result.

8. ### WinfieldGuest

Let me hasten to add, my statement was for forward converters,
or for push-pull, etc, as used with the LT1533. OTOH, flyback
converters, etc., depend rigorously upon the Lm value, with the
core's air gap closely determining AL.

9. ### JosephKKGuest

D from BC posted to sci.electronics.design:
That is not too bad D. It shows a willingness to do a little work to
learn some more.

10. ### Fred BloggsGuest

For someone who makes sweeping generalizations like RC-timers and
discrete logic is for newbies, you sure don't know a heck of whole lot

11. ### JosephKKGuest

Winfield posted to sci.electronics.design:
If you do your measurements correctly, you can derive the parallel
resistance like components of core losses this way. And by extension
for any specific core with similar windings by scaling as an
approximation.

12. ### Martin GriffithGuest

Can you add me to that list

Martin

13. ### Winfield HillGuest

Pleased tell us more. The core loss goes as something
like f^1.8 and B^2.5, etc., so you'd have to test at
the right frequency and amplitude, or be confidant of
the scaling factors. Is there an easy trick?

14. ### JosephKKGuest

Winfield Hill posted to sci.electronics.design:
Not in the least, i specified the same core with similar windings. I
clarify, not more than 50 percent difference in turns, peak and
average ampere turns, and volt-seconds, and maximum MMF (assuming
that the difference does not approach core saturation and fairly
linear cores). The idea is that it is measurable and that the
measurements can be used in SPICE simulations.

Almost sorry to have to back off so much, but it is useful for single
frequency of operation designs.

Alas, this poor butterfly is pinned.

15. ### James ArthurGuest

I don't see how it would cause problems. Real inductors really do
have series resistances, and they have an honest d.c. drop if there's
a d.c. flow. Putting in a realistic series resistance, then, makes
the simulation more realistic (and grounded more closely on actual
physical parameters).

Real inductors do not have parallel resistances carrying any
meaningful current. Rp, as used in Q and filter calculations, is a
mathematical convenience to model certain losses. It *is* convenient,
especially for low-level signals where the core isn't being pushed,
but it's not a real resistor.
That's handy stuff for RF filter work. For my occasional switch-mode
power supply work, cores are loss so variable with load and frequency--
and small to begin with, by careful choice of core and windings--that
they aren't that critical. I usually just estimate, and don't bother
simulating them. If I'm really interested, I measure (in real life).

Cheers,
James Arthur

16. ### colinGuest

tbh I usually just use series resistance.
typically taken from available data sheets of suitable sized inductors.

so might be anything from say 0.1 ohms to 20 ohms.

theres usually a defualt series resistance, of some milliohms,
it often goes crazy if this is zero.

Colin =^.^=

17. ### Winfield HillGuest

Hmm, it should be pointed out that the actual
ac resistance is higher than the dc resistance,
sometimes considerably, due to proximity effect.

18. ### Phil HobbsGuest

This is pretty well quantified in a series of old NBS Circulars from the
1920s...I have C22, which is sort of a summary of the more detailed
studies.

Cheers,

Phil Hobbs

19. ### Winfield HillGuest

I don't have any proximity-effect papers from that era,
although I know a lot of work was done on inductance
and it's subtleties at the NBS, as they developed the
theory for and created various inductance standards,
etc.,. and I have copies of some mid-30's stuff.

I wonder if you could scan that gentle beast for us?

20. ### Phil HobbsGuest

It's about 300 pages long, and now quite fragile because it's printed on
toilet paper and bound with staples like raffle tickets--from the front
cover through the whole thickness to the back cover. They decently
covered this offence with tape. I'll see if I can make a couple of
readable digital photos of the curves. IIRC they talk in terms of
frequency dependent resistance for various winding styles, but don't
specify how it arises.

Cheers,

Phil Hobbs