# Line filtering

T

#### Tim Williams

Jan 1, 1970
0
So what kind of filtering does an induction heater need?

Potentially salient points...
10kW (240V, 50A USA supply) ultimate goal
~20kHz resonance (may go as low as 5kHz in the future)

Currently, DC filtering is 44uF (2 x (10 x 2.2uF film), filtering +V/GND/-V)
after inverter, then electrolytics at FWB (2 x 8 x 470uF 200V). Measured HF
ripple of 4% at 50V supply, 15A peak output IIRC.

I'm supposing a chunky inductor of some uH (or mH!) suitable to put a few
ohms in the way of >2kHz, at enough current that it doesn't saturate due to
line current, which is ZERO, for a common mode choke... The wire at least
will have to be 6 or 8AWG.

And as a wider question, what is the *point* of a common mode choke? I can
see differential mode, which is how I'm drawing the current for pete's
sakes, but I don't particularly intend to be swinging around wildly in
space, leveraging my chassis' potential against--air? I mean, how does a
grounded box like a computer produce HF hash when the case isn't also
radiating in some obvious manner? I don't see how to calculate the
inductance for a common-mode choke, out of the blue. And yet I've seen them
in switchers with two- and three-wire cords.

Tim

J

#### John Popelish

Jan 1, 1970
0
Tim Williams wrote:
(snip)
And as a wider question, what is the *point* of a common mode choke?
(snip)

They are there to put an impedance in series with a signal
that is in common with both sides of the line. For
instance, assume you have a rectifier and capacitor down
stream of the filter, and the DC supply feeds a switching
mechanism that produces rapid voltage swings that feed
capacitive current into ground (through a transformer inter
winding shield, perhaps). That current completes a loop
through ground, back to the power lines and through the
common mode inductor back to the rectifiers and capacitor,
every time the rectifiers conduct. This causes a pulse of
high frequency in common on both sides of the power line
twice per cycle. The common mode choke gets in the way of
that current completing the loop. Without it, the radio
noise radiated by the pair of power lines because of the
current they each carry half of, acting as a single
conductor at RF, would be much worse.

L

#### Lionel

Jan 1, 1970
0
And as a wider question, what is the *point* of a common mode choke? I can
see differential mode, which is how I'm drawing the current for pete's
sakes, but I don't particularly intend to be swinging around wildly in
space, leveraging my chassis' potential against--air? I mean, how does a
grounded box like a computer produce HF hash when the case isn't also

Harmonic leakage from the cables & any gaps on the metalwork, mostly.

J

#### John Popelish

Jan 1, 1970
0
Tim said:
So what kind of filtering does an induction heater need?

Potentially salient points...
10kW (240V, 50A USA supply) ultimate goal
~20kHz resonance (may go as low as 5kHz in the future)

Currently, DC filtering is 44uF (2 x (10 x 2.2uF film), filtering +V/GND/-V)
after inverter, then electrolytics at FWB (2 x 8 x 470uF 200V). Measured HF
ripple of 4% at 50V supply, 15A peak output IIRC.

I'm supposing a chunky inductor of some uH (or mH!) suitable to put a few
ohms in the way of >2kHz, at enough current that it doesn't saturate due to
line current, which is ZERO, for a common mode choke... The wire at least
will have to be 6 or 8AWG.

I would be thinking in terms of using a choke input DC
filter. That way, you not only clean up the differential
noise on the line, you improve the power factor by
eliminating the high current peaks each time the rectifiers
turn on. Think in terms of the choke used on the output of
a simple DC welder. Small shoe box size.

T

#### Tim Williams

Jan 1, 1970
0
So why don't I just put a big fat choke on the ground line? It's not like
it has to carry any current! <g,dr>

How about the other points: two-line supplies and my situation?

Tim

T

#### Tim Williams

Jan 1, 1970
0
John Popelish said:
I would be thinking in terms of using a choke input DC
filter. That way, you not only clean up the differential
noise on the line, you improve the power factor by
eliminating the high current peaks each time the rectifiers
turn on. Think in terms of the choke used on the output of
a simple DC welder. Small shoe box size.

Ouch, there goes rail regulation. Seems kind of a waste to run 600V IGBTs
from +/-100V. I don't even have much capacitance, at least yet.

Besides, that's gotta be what, 0.1H at 50A? Yech! Could buy the IGBTs to
build a PFC for the price of the *wire* for that sucker!

Tim

T

#### Terry Given

Jan 1, 1970
0
Tim said:
Ouch, there goes rail regulation. Seems kind of a waste to run 600V IGBTs
from +/-100V. I don't even have much capacitance, at least yet.

Besides, that's gotta be what, 0.1H at 50A? Yech! Could buy the IGBTs to
build a PFC for the price of the *wire* for that sucker!

Tim

PI-filter on your DC bus then. But without some line inductance (before
or after rectifier, no major diff) then as JJ points out, current peaks
will be HIGH. A Pi-filter also means the cap lifetime calcs are easier -
rectifier-side caps see BRUTAL 100/120Hz ripple, but no HF. vice-versa
for SMPS-side caps.

Anecdote:

testing a range of 230V and 400V AC motor controllers. Tech measures RMS
input current. 4.7kW drive has 30% *more* Irms than 12kW drive. WTF says
I, then goes and does calcs. large drive has 5% line chokes, small
drives none. Voila, math = measurement. ouch.

Cheers
Terry

T

#### Terry Given

Jan 1, 1970
0
Tim said:

no worries. thanks for the pics. hairy-faced gits flock together?
So I should say, split my electrolytic cap bank with a few microhenries (or
maybe as much as mH, I forget) to decouple the HF from one side? Or just
toss on more film caps (maybe even one of those swanky "film-lytics") for
the HF side of the filter?

rather than a WAG, use a scientific WAG (SWAG). this is for the DM filter.

you ought to know what the HF ripple in the DC bus capacitance is. its
usually easier to set up a crude simulation than an analytic expression.
Then do a reasonabole (include ESL,ESR) model of the DC bus caps, so
you have a reasonable idea of the ripple voltage.

now you can look at a roughly accurate spectrum of said ripple voltage.

I typically do the design with a copy of the relevant EMI spec - that
way you know what your allowable DM current into 50R is. Because EMC is
all about parasitics, I pick the lowest allowable level, and use that to
design the filter. but I include reasonable values of parasitics for the
DM filter components.

I usually have 2 models - a switching-level model, so I can get fairly
accurate RMS currents, and a cruder model for the filter, using some
sort of a vandalised current source to represent the current into the
filtering network.

the rectifier complicates things somewhat. but it tends to look pretty
capacitive, so is a short at EMC frequencies. which is what I use.

then fiddle with your components (AC sweep) until you have something
that looks like it will meet the requisite EMC standards. inductor
end-to-end capacitance is hugely important - a massive inductor does
naff all at EMC frequencies because of this capacitance, which is
dominated by the start wire and the final layer of turns + the finish
wire. Adding more L does naff all, the C doesnt really change but the
corner frequency (at which the L stops helping) gets lower...... if you
make the inductors, you can control this - eg bank winding, with N+0.5
turns so the start and finish stay the hell away from each other.....

with reasonably large caps that work at HF (ESL, ESL, ESL) and a
moderate switching frequency, you can probably use a fairly small L, <<
1mH in your PI filter. the sims/calcs will show this.

CM noise occurs because of parasitic capacitance to anything that is
earthed. like say the metal plate upon which the LISN sits at the EMC
lab, if you have no earth wire. or all the metalwork if you do. so plan
on having a CM choke, and Y caps.
So, at the line, I take it there's no way to get away from high ripple or

Tim

for a reasonable AC (or DC) inductance, use 5% chokes. 3% is the
critical value below which not much happens; 5% gives a huge reduction
in peak current, for 5% drop in AC volts across said chokes at 100%
rated current (do the calc ignoring the nasty(ish) current waveshape) so
the line regulation is ~ 5%.

5% for a 10kW 240V single-phase supply is:

Vbase = 240Vrms

Pbase = 10kW

Ibase = 41.7Arms

Zbase = 5.8R

Wbase = 120*pi

Lbase = 15.3mH

5% L = 763uH

Epeak = 1.32J - and that DOESNT INCLUDE the evil current peaks. its a
~2J inductor.

again its easy to simulate the overall rectifier behaviour for AC line
current, but hard to calculate. try stepping L, have a look. then use
the actual peak current to calculate the required energy, so your L
doesnt saturate at the current peaks (thereby defeating its purpose)

HTH

Cheers
Terry

F

#### Fred Bartoli

Jan 1, 1970
0
Tim Williams a écrit :

So I should say, split my electrolytic cap bank with a few microhenries (or
maybe as much as mH, I forget) to decouple the HF from one side? Or just
toss on more film caps (maybe even one of those swanky "film-lytics") for
the HF side of the filter?

So, at the line, I take it there's no way to get away from high ripple or

Sure there is. A PFC will handle all that nicely

T

#### Tim Williams

Jan 1, 1970
0

So I should say, split my electrolytic cap bank with a few microhenries (or
maybe as much as mH, I forget) to decouple the HF from one side? Or just
toss on more film caps (maybe even one of those swanky "film-lytics") for
the HF side of the filter?

So, at the line, I take it there's no way to get away from high ripple or

Tim

T

#### Tim Williams

Jan 1, 1970
0
Terry Given said:
its usually easier to set up a crude simulation than an analytic expression
....
again its easy to simulate the overall rectifier behaviour for AC line
current
....

Bugger... I don't like simulation. I don't have a simulator on hand,
either.
I typically do the design with a copy of the relevant EMI spec - that
way you know what your allowable DM current into 50R is. Because EMC is
all about parasitics, I pick the lowest allowable level, and use that to
design the filter. but I include reasonable values of parasitics for the
DM filter components.

Don't have any idea what kind of EMI spec I want. I mean, I'm building this
for myself, is the FCC going to give a damn about a couple kilowatts of
radiated hash running for at most a couple hours?

Tim

T

#### Terry Given

Jan 1, 1970
0
Tim said:
...

Bugger... I don't like simulation. I don't have a simulator on hand,
either.

LTspice. I dont use it, but its free and good (a rare combination)
Don't have any idea what kind of EMI spec I want. I mean, I'm building this
for myself, is the FCC going to give a damn about a couple kilowatts of
radiated hash running for at most a couple hours?

Tim

In that case, size your rectifier caps for the 120Hz ripple current, and
the caps on the HF side of the Pi filter to prevent bus overshoot and
device destruction. then stick as much inductance as you can be bothered
making between the two; 20dB is a good start wrt attenuation. remember
to put something HF in parallel with the LF electros.

then smack in as much CM inductance in the AC line as you can be
bothered winding. an X2 cap across the AC of the rectifier, and a pair
of Y caps from P,N to E at the same point - so any capacitive coupling
to chassis flows thru these caps and back into your smps.

or do nothing. after all, your work coil is air-cored so spews flux
everywhere (although its pretty sinusoidal). this is the quickest option.

Cheers
Terry

T

#### Tim Williams

Jan 1, 1970
0
Terry Given said:
In that case, size your rectifier caps for the 120Hz ripple current, and
the caps on the HF side of the Pi filter to prevent bus overshoot and
device destruction. then stick as much inductance as you can be bothered
making between the two; 20dB is a good start wrt attenuation. remember
to put something HF in parallel with the LF electros.

If you don't recall, I have electrolytics from computer supplies, 16 x 470uF
200V. Any bearing on the capacity of them? I recall typical specs for
similar product are around 2-4A ripple, and I have no recollection of ESR,
ESL...

Any recommendations on film caps? It's looking like I'll have to buy some
more... I've got Cornell Dubilier DMEs (2 x 10 x 2.2uF 400V) on there right
now. Seems to be the cheapest, densest mylar line available, at least at
the time I checked. No good for current (I have a clip of some 0.1uF's on a
tank circuit and one starts oozing smoke) and notable ESL (series resonant
circa 300kHz for...0.47uF I think?), so basically bulk nonpolar capacitance.

With some nice polypropylenes on the inverter itself, HF crud should be low
enough by the time the ripple gets to the film cap bank, huh?

Oh...do I have links to any recent pictures? I forget...
This is about the most recent I have online...
http://webpages.charter.net/dawill/Images/Induction806sm.jpg
Except the big coupling capacitor is now split in two, so it makes a fake CT
off the supply voltage.
then smack in as much CM inductance in the AC line as you can be
bothered winding. an X2 cap across the AC of the rectifier, and a pair
of Y caps from P,N to E at the same point - so any capacitive coupling
to chassis flows thru these caps and back into your smps.

Got it. Common mode should be pretty easy, although I don't really have a
core big enough to loop all that 6AWG through. Or the #6, for that
matter...Maybe another pi-wound dealie from 1/2 x 0.04" copper (like the
Lmatch)...
or do nothing. after all, your work coil is air-cored so spews flux
everywhere (although its pretty sinusoidal). this is the quickest option.

Yeah, but that's cheating. The power line conducts crud, while the coil
drops off as what, inverse cubed distance?

Tim

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