inductor sizing

[Tom Bruhns]

Tom,

Do you have a copy of Randy Rhea's (of Eagleware Genesys fame) paper, "A
Multimode High-Frequency Inductor Model?" He does a pretty good job
demonstrating how most real inductors are a lot closer to a transmission line
model as you're discussing than the ECE 101 "fields and waves" model that's
often proffered.

---Joel

Hi Joel,

Well, I'll be the first to admit that I'd get overloaded very quickly
if I looked up every reference I come across -- and that particular
paper is not one I have. I did read John Mezak's paper that was
published in "RF Design" sometime in the early to mid 90's, describing
his work on a computer model for the helical transmission line coil
model, and if you go to Serge Stroobandt's wonderful website I
referenced yesterday, you'll find a reasonable amount of discussion
there about where the model came from, with links to some other good
sites. I really like that Serge has provided all that background info
and the links; so many sites with things like that just present a
calculator and you have little or no way to build confidence that it
was done right or to learn about what went into it. That's one of the
reasons I trust Mezak's program--plus by this time I've had lots of
opportunities to wind coils based on it and to measure them and see
for myself that they agree with the predicted results plenty close
enough for my engineering work. Mezak's program and the calculator at
Serge's site give results that agree close enough for my purposes; as
with ALL modeling, especially if you're pushing the limits, plan to
test an actual circuit and make adjustments based on actual
performance.

Cheers,
Tom

 

[Joel Koltner]

See http://www.coilcraft.com/bcl-s.cfm. You don't have to wind your
own. ;-)

We got some of those as samples... they are quite good, albeit quite spendy
(but of course cheaper than making your own if someone's paying for your
time...). Someone mentioned that initially low yields were one of the reasons
the prices were so steep, so perhaps they've come down in the year or so since
they introduced them as their yield has improved?

 

[Tom Bruhns]

Hi Tom, sorry for the confusion.
The ferrite inductor is 25uH in a 2.75" cube. It is designed for 35Apk,
20.0Aac and 2.0Adc in a larger core than necessary because the OP will
probably have to drop down to 100KHz in switching frequency and he needs a
lot of wiggle room at his skills level. That said, a 2.25" cube would get
the job done. My Rdc = 8.0mR is real but the Rac is a optimistic guestimate.
Though I used Litz wire, the core gaps will cause havoc with the Rac. With
the large Aac component, Rac is a dominate player and reducing it is an
ongoing task.
In winding ferrite cores with high Aac levels, we normally can not use many
layers nor fill the available window because of proximity losses, air cores
should exhibit the same or worse effects due to the flux being outside the
core. Do you have a way of measuring your Rac at 200KHz? Your #24 Litz wire
sounds too fat, maybe >#28 would serve you better. Tell us about your
dimensions and layering.

Regards,
Harry

Hi Harry,

Thanks for the further explanations! I'm not shy about saying it's a
breath of fresh air to find someone willing to actually discuss
alternate ways of doing things--so many of the discussions get mired
in "is so/is not" rhetoric. Thanks for helping keep things sane.

As I've mentioned before, my work normally is with RF filters. Almost
all the Litz wire I have is based on 44 and 46 AWG stranding, and is
dear enough that I'm not about to use a whole bunch of it (e.g. about
200 feet of 175/46) in a test coil I'm not going to be actually using
for anything other than a test. So that left me last night digging
out a coil of heavier Litz wire that a friend had given me, and
discovering why he had given it to me: it's about 100 feet of 20/32,
so equivalent to 19AWG, and that would be almost reasonable for 200kHz
work--though really optimum for down around 10-20kHz. I fully agree
that 24AWG would be way too big for the stranding! But what looked
like a reasonably well-ordered coil turned out to be impossible to
just unwind. So I spent an hour or so teasing it out (trying to keep
it from kinking), and have it probably 3/4 done now. No coil yet. I
also decided that I'm probably NOT going to cut it to do the proposed
test--I'd want to cut it into something like 7 lengths and put them in
parallel, and it's just worth too much to do that. In fact, for
200kHz, even 32AWG stranding is way too big. The Litz manufacturers
suggest 40AWG stranding, typically. Anyway, that all leaves me in a
quandary about how to make a test coil. For sure I'll wind all the
20/32 I have into one multi-layer coil, ID about 1.33" and length
about 1.5" (I made a bobbin for it...), and I can measure that, but
I'm not sure it will be all that representative of the rather
different coil we'd really want: similar size, but fewer turns of
heavier wire. I do expect the Q to be similar, though, if the form
factor is similar, and the SRF probably scales in some reasonably nice
way with inductance.

Measuring Rac is easy enough for me: I measure the Q and inductance,
resonated with good polypropylene caps to the desired frequency.
Assuming the caps have infinite Q puts all the loss in the coil and
therefore puts an upper bound on Rac. Oh, and for a 25uH coil, Rac at
200kHz of around 15 milliohms would be a Q in excess of 2000! That
one I'm _really_ not going to believe. ;-) I've measured the loss
(through Q measurements) of a variety of power inductors I've salvaged
from commercial supplies. Some of them have very disappointingly low
Q (presumably were used at a place where the DC current was much
greater than the AC through them), and I'm always happy when I find
one with a Q at 100 or a bit more. I don't think I've ever found one
with a Q very much above 100 in the 100kHz-200kHz region. Based on
some other measurements made the same way on higher Q coils, I know
it's not a limit of the test method. But this may not be a fair test,
since the excitation is at a very low level compared with the level at
which the part is used in the supply. On the other hand, I've seen a
rule of thumb that core losses in high frequency transformers
(including gapped flybacks??) are almost always several times the
copper losses. If that's right, Rac in those cases must necessarily
likewise be in an even larger ratio to Rdc.

'Nuff for now...

Cheers,
Tom

 

[Tom Bruhns]

Hi Joel,

Well, I'll be the first to admit that I'd get overloaded very quickly
if I looked up every reference I come across -- and that particular
paper is not one I have. I did read John Mezak's paper that was
published in "RF Design" sometime in the early to mid 90's, describing
his work on a computer model for the helical transmission line coil
model, and if you go to Serge Stroobandt's wonderful website I
referenced yesterday, you'll find a reasonable amount of discussion
there about where the model came from, with links to some other good
sites. I really like that Serge has provided all that background info
and the links; so many sites with things like that just present a
calculator and you have little or no way to build confidence that it
was done right or to learn about what went into it. That's one of the
reasons I trust Mezak's program--plus by this time I've had lots of
opportunities to wind coils based on it and to measure them and see
for myself that they agree with the predicted results plenty close
enough for my engineering work. Mezak's program and the calculator at
Serge's site give results that agree close enough for my purposes; as
with ALL modeling, especially if you're pushing the limits, plan to
test an actual circuit and make adjustments based on actual
performance.

Cheers,
Tom

I mentioned Serge's website, and how it has good links. One of the
links that I had not previously looked into is
http://www.g3ynh.info/zdocs/magnetics/part_1.html (and parts 2 and 3
of the same article). I think David does a great job of giving an
overview about the need for different inductor models at different
frequencies, and then diving into a discussion of the various
calculation methods that have been developed, how accurate they are,
where they're useful and so forth. But I should also point out that
this is getting to be pretty bad basenote drift, since we've gotten
into what normally would be considered "RF" coils, single-layer
solenoid coils, rather far from the discussion about high-current
inductors for 200kHz switching supplies!

Cheers,
Tom

 

[Joel Koltner]

Thanks for the links, Tom, I'll take a look!

 

[Tom Bruhns]

... So that left me last night digging
out a coil of heavier Litz wire that a friend had given me, and
discovering why he had given it to me: it's about 100 feet of 20/32,
so equivalent to 19AWG, and that would be almost reasonable for 200kHz
work--though really optimum for down around 10-20kHz. I fully agree
that 24AWG would be way too big for the stranding! But what looked
like a reasonably well-ordered coil turned out to be impossible to
just unwind. ...

Got it untangled and wound onto the bobbin. At 200kHz, it's about 1.5
millihenries at a Q of about 60, not terribly good. I was hoping for
something a bit higher. I suppose something closer to the recommended
Litz stranding would help, and possibly also a different layup. I
wouldn't expect Q to change much with a change of wire gauge/
inductance if the coil occupies the same volume. This was just layers
back and forth. 1.33" ID, 2.03" OD, about 1.7" long. I didn't count
the turns as I was untangling things. ;-) SRF is only about 500kHz,
but assuming the effective shunt capacitance stays constant, the SRF
of a 15uH coil would be about 5MHz. The effective capacitance may go
down a bit with larger wire, and also with a different layup.

Cheers,
Tom

 

[Terry Given]

See http://www.coilcraft.com/bcl-s.cfm. You don't have to wind your
own. ;-)

Cheers,
Tom

Cool! I guess a variable-pitch spiral will behave similarly! I want
slightly bigger inductors though*

Cheers
Terry


* spiral of 20mm x 5mm Al strip

 

[John Larkin]

See http://www.coilcraft.com/bcl-s.cfm. You don't have to wind your
own. ;-)

Cheers,
Tom

These are copies of the Piconics design; I assume the patents have
expired. The Piconics parts are better and cheaper. These are commonly
used in super-wideband bias tees, like the PSPL boxes.

John

 

[Harry Dellamano]

Got it untangled and wound onto the bobbin. At 200kHz, it's about 1.5
millihenries at a Q of about 60, not terribly good. I was hoping for
something a bit higher. I suppose something closer to the recommended
Litz stranding would help, and possibly also a different layup. I
wouldn't expect Q to change much with a change of wire gauge/
inductance if the coil occupies the same volume. This was just layers
back and forth. 1.33" ID, 2.03" OD, about 1.7" long. I didn't count
the turns as I was untangling things. ;-) SRF is only about 500kHz,
but assuming the effective shunt capacitance stays constant, the SRF
of a 15uH coil would be about 5MHz. The effective capacitance may go
down a bit with larger wire, and also with a different layup.

Cheers,
Tom

Ok, so its my turn to do some measurements on a ferrite EC core
(2.10"x1.10"x0.75") that yields about a 2.20" cube. I started with one layer
of 7 turns Litz, (330x36 = 12AWG) and setting the gap to yield 25.0uH. The
measured "Q" at 50KHz, 100KHz and 200KHz resulted in 190, 153 and 125
respectfully (?). I added another 7 turn layer, set the gap to yield 25uH
and got readings of Qs at 114, 107 and 95. A final layer (3) was added of 7
turns, reset the gap and measured Qs at 89, 74 and 63.
IMHO the only acceptable readings above are a single layer at 100KHz or
less. The Q of 153 means that Rac = 0.103R which is 20X greater than Rdc.
Now if I am using Litz in only one layer, what happens if I just use plain
stranded, for flexibility, wire? I wound one layer of #14 AWG and got a Q of
138 at 100KHz. compared to 153 for Litz but that was #12AWG. Not much gain
for Litz.
So the OP should be using only one layer of non Litz winding at 100KHz or
less.
The air core's Q should be lower because of flux fields flying all over the
place and inducing localized current loops (proximity effect) and shrinking
the actual wires cross sectional area.
Cheers,
Harry

 

[John Fields]

On Wed, 12 Mar 2008 13:10:53 -0700, "Harry Dellamano"
..
..
..

 

[Tom Bruhns]

Ok, so its my turn to do some measurements on a ferrite EC core
(2.10"x1.10"x0.75") that yields about a 2.20" cube. I started with one layer
of 7 turns Litz, (330x36 = 12AWG) and setting the gap to yield 25.0uH. The
measured "Q" at 50KHz, 100KHz and 200KHz resulted in 190, 153 and 125
respectfully (?). I added another 7 turn layer, set the gap to yield 25uH
and got readings of Qs at 114, 107 and 95. A final layer (3) was added of 7
turns, reset the gap and measured Qs at 89, 74 and 63.
IMHO the only acceptable readings above are a single layer at 100KHz or
less. The Q of 153 means that Rac = 0.103R which is 20X greater than Rdc.
Now if I am using Litz in only one layer, what happens if I just use plain
stranded, for flexibility, wire? I wound one layer of #14 AWG and got a Q of
138 at 100KHz. compared to 153 for Litz but that was #12AWG. Not much gain
for Litz.
So the OP should be using only one layer of non Litz winding at 100KHz or
less.
The air core's Q should be lower because of flux fields flying all over the
place and inducing localized current loops (proximity effect) and shrinking
the actual wires cross sectional area.
Cheers,
Harry

So the Q for the air-core guy of 7 layers (I think that's what it
turned out to be) at 200kHz is pretty similar to the Q you got for 3
layers. -- For air core solenoid coils at 1MHz, I'm commonly getting
close to 3 times improvement in Q for 175/46 Litz versus similar
diameter solid, but these are single-layer with the turns spaced
slightly--around 1.2* wire diameter. Interesting that the Litz and
non-Litz in the case of your coils makes so little difference. It
illustrates to me, once again, the need to try these things before
jumping to confusions.

I wonder what kind of core materials they're using in the supplies
that run at a few MHz. They seem to be able to get good efficiency.

Thanks for sharing your measurements.

Cheers,
Tom

 

[Terry Given]

So the Q for the air-core guy of 7 layers (I think that's what it
turned out to be) at 200kHz is pretty similar to the Q you got for 3
layers. -- For air core solenoid coils at 1MHz, I'm commonly getting
close to 3 times improvement in Q for 175/46 Litz versus similar
diameter solid, but these are single-layer with the turns spaced
slightly--around 1.2* wire diameter. Interesting that the Litz and
non-Litz in the case of your coils makes so little difference. It
illustrates to me, once again, the need to try these things before
jumping to confusions.

Gorgeous! Jumping to confusions.... I'll have to steal that.

I wonder what kind of core materials they're using in the supplies
that run at a few MHz. They seem to be able to get good efficiency.

Thanks for sharing your measurements.

Cheers,
Tom

and yeah, thanks for sharing. This has been a great thread, and helped
me solve a problem. I had taken a T50-26 toroid with 450T of AWG34, and
made it AWG44 instead, as I needed the series R. And the choke all of a
sudden had an anti-resonance (at 3.6MHz) which needless to say made it
less than useful.

I had figured it was multi-resonant becuase of the distributed
inductance & high capacitance - it went from 8 layers to 1 layer. So I
had some universal-wound (kinda) and halved the end-to-end capacitance,
which increased the first resonance by 1.4x, and likewise with the 2nd
resonance.

The magnetics vendor didnt believe me when I showed them an impedance
plot, until I showed them a photo of the network analyser setup. but
they couldnt explain it. And my sidekick inthe US (an EMC guy) hadnt
come across this before either. Hurrah, I learned something new.

I had already concluded that the low perm single-layer winding would
have distributed coupling, and the fix was pretty simple - go back to
AWG34 and add a series R - but Toms reference sorted out exactly what
was going on.

one of the best threads!

now all I have to do is figure out how I can exploit shell helix
resonance. I must be able to do something useful with it....

Cheers
Terry

 

[Jamie Morken]

Ok Jamie,
You sound like a nice person so here is a filter design that will meet
your needs with a bottle of tequila to be sent upon the design's
approval. This is the first LC section for one phase and will survive
the 200KHz/440VDC stress and present a nice 60Hz sine wave to the second
LC section input which you can design using high flux 60Hz material to
meet you unspecified output requirements. As stated earlier the THD
depends on the control loop and not the HF output filter.
For the 25uH/30Apk inductor use a Ferroxcube or similar EC-70 (3F3
material) core set and bobbin with 21 turns of #10 AWG LITZ wire wound
in 3 layers of 7turns/layer. Insulate all for 3500VDC. The LITZ wire
should be made up with 256/#34 AWG wires. This will require a 0.21" gap
across all three legs of the core. Do not grind the center leg for a
0.42" gap because this will cause increased Rac losses. The total
dissipation will be <7.0W so be aware.
For the shunt caps use 5X EPCOS 2.2uF/400VDC MetalPoly at DigiKey for
$1USD/100 PN B32594C6225J008. Irms in each cap is ~=1A at 200KHz which
MetalPoly types of this size can handle.

Hi Harry,

Sorry for the late reply I was out of province for the last week.

For the feedback of an LCLC filter like this, are only one of the
inductor currents and capacitor voltages required, or are both
currents and voltage required?

For the LC filter version I use this algorithm for the feedback loop:

if inductor current > current_limit, pulse off

Duty cycle = ((desired voltage - current voltage) * gain) - (Inductor
current * gain2)

I am not sure if I can use this for the LCLC filter as well.

Also do you have part#'s for the high flux inductor for this LCLC filter
and the output capacitor? The output is 120VAC 60Hz at 3.6kW by the
way.

I plan on using planar inductors instead of E cores and litz wire, but
I will still send you the tequila for the design

cheers,
Jamie

 

[Harry Dellamano]

Hi Harry,

Sorry for the late reply I was out of province for the last week.

For the feedback of an LCLC filter like this, are only one of the
inductor currents and capacitor voltages required, or are both
currents and voltage required?

*The second LC stage is not required in the feedback loop and may be
detrimental due to added lag. It is only needed to meet your output high
frequency attenuation requirements which you have not yet revealed.

For the LC filter version I use this algorithm for the feedback loop:

if inductor current > current_limit, pulse off

Duty cycle = ((desired voltage - current voltage) * gain) - (Inductor
current * gain2)

I am not sure if I can use this for the LCLC filter as well.

*As we have stated before, the LCLC filter is a separate issue from the
feedback loop. To simplify the feedback you may chose to feedback the output
of the first LC and let the second LC run open loop. The FB loop must
operate with the first LC but you can close the loop in any way your heart's
desire as long as it stable.

Also do you have part#'s for the high flux inductor for this LCLC filter
and the output capacitor? The output is 120VAC 60Hz at 3.6kW by the
way.

* So, asked but never answered, what is your output noise requirement? Are
you trying to meet FCC, DO-160, MS-461, CE....? How much must the 100KHz and
harmonics be attenuated? Notice I said 100KHz because at 200KHz the AC
resistance of the first inductor will eat you up unless you know of a way to
reduce it. The best I could do was using a single layer winding, at 100KHz,
and got a Q of 475. Earlier in this thread I reported a Q of 150 to PH but
cleaned up my measurement methods and attained a true 475. The AC resistance
was still 5X the Rdc and the biggest obstacle. The second LC will see little
100KHz current and can be any high flux core. I like the MPP-HF or a gapped
MET-GLAS. Size, cost, availability and familiarity come into play here so
there is no simple answer.

I plan on using planar inductors instead of E cores and Litz wire, but

*Sounds interesting, are these planar inductors something you will
purchase from the boys in Florida or your in house design? What kind of Rac
at 100KHz and 200KHz is attainable?

I will still send you the tequila for the design

*Tequila is only necessary for the closing ceremony.

cheers,
Jamie

Cheers,
Harry

 

[Jamie Morken]

*The second LC stage is not required in the feedback loop and may be
detrimental due to added lag. It is only needed to meet your output high
frequency attenuation requirements which you have not yet revealed.

The output has to remain below 2.5% THD + noise.

*As we have stated before, the LCLC filter is a separate issue from the
feedback loop. To simplify the feedback you may chose to feedback the
output of the first LC and let the second LC run open loop. The FB loop
must operate with the first LC but you can close the loop in any way
your heart's desire as long as it stable.

I tried this in the simulator but never got the LCLC to regulate as well
as a simple LC does.

* So, asked but never answered, what is your output noise requirement?
Are you trying to meet FCC, DO-160, MS-461, CE....? How much must the
100KHz and harmonics be attenuated? Notice I said 100KHz because at
200KHz the AC resistance of the first inductor will eat you up unless
you know of a way to reduce it. The best I could do was using a single
layer winding, at 100KHz, and got a Q of 475. Earlier in this thread I
reported a Q of 150 to PH but cleaned up my measurement methods and
attained a true 475. The AC resistance was still 5X the Rdc and the
biggest obstacle. The second LC will see little 100KHz current and can
be any high flux core. I like the MPP-HF or a gapped MET-GLAS. Size,
cost, availability and familiarity come into play here so there is no
simple answer.


*Sounds interesting, are these planar inductors something you will
purchase from the boys in Florida or your in house design? What kind of
Rac at 100KHz and 200KHz is attainable?

Payton (http://www.paytongroup.com/) or Himag (http://www.himag.co.uk/)
both make these planar inductors. I got a quote for a 25uH 82Amp
inductor from Payton for the 200kHz filter with 15Amp peak to peak
ripple rating, and it has a specified 30watts loss. Its dimensions are
116mm by 66mm by 26mm high (big rectangular block).

I was way off with the original 250uH inductor, that would be huge at
these currents and frequency, 25uH seems to be more than enough
inductance (I think 15uH would work actually)

cheers,
Jamie

 

[[email protected]]

I am absolutely in awe to the welth of information found here. If I
may? I am working on a similar power supply, with similar requirements
as the origional poster save the 60hz final output. The supply I am
working on will be directly controlling the current with little focus
on the actual voltage.

The Basics, 208Vac 3phase or 220Vac single phase input rectified and
filtered.
I would like to use a bank of mosfets at a switching frequency around
200-250 Khz
Or another possibility would be a large IGBT at around 10-12 Khz.
Drive and controll considerations asside, I am looking for 2% or less
of THD if possible
output current will average 20-35A with a sustained 50A possible. The
output voltage will be anywhere from 95V to as much as 160V DC. The
full voltage of the rectified mains will be on the output untill the
sustained arc is started and at which point the current regulation
will be put to work. The voltage will vary and that is expected but
the current must stay stable. I have a pretty good design on the
controlls and feedback, But...
My question is weather to use a bank of mosfets with there own LC
network each, such as 5 or 6 in parallel with all outputs tied
together, or1 bank of mosfets\IGBT and a single filter?

Much Thanks
Cory

 

[Harry Dellamano]

I am absolutely in awe to the welth of information found here. If I
may? I am working on a similar power supply, with similar requirements
as the origional poster save the 60hz final output. The supply I am
working on will be directly controlling the current with little focus
on the actual voltage.

The Basics, 208Vac 3phase or 220Vac single phase input rectified and
filtered.
I would like to use a bank of mosfets at a switching frequency around
200-250 Khz
Or another possibility would be a large IGBT at around 10-12 Khz.
Drive and controll considerations asside, I am looking for 2% or less
of THD if possible
output current will average 20-35A with a sustained 50A possible. The
output voltage will be anywhere from 95V to as much as 160V DC. The
full voltage of the rectified mains will be on the output untill the
sustained arc is started and at which point the current regulation
will be put to work. The voltage will vary and that is expected but
the current must stay stable. I have a pretty good design on the
controlls and feedback, But...
My question is weather to use a bank of mosfets with there own LC
network each, such as 5 or 6 in parallel with all outputs tied
together, or1 bank of mosfets\IGBT and a single filter?

Much Thanks
Cory

Hi Cory, let me take a shot at restating your requirements. AC to DC
converter. Input, 220Vac or 208V-3phase, 50 or 60 Hz with THD of the input
current <2% . So you need a PFC stage that will output about 550Vdc at
5.0Kws. The PFC is followed by a Dc/Dc converter that will output 160Vdc and
current limit at 20A to 50A, externally control. The 550Vdc to 160Vdc
converter sounds like a phase shifted bridge, maybe 2 bridges, driven 180d
out of phase, driving into two transformers with output rectifiers driving
one or two output inductors summing into a array of filter capacitors. The
output inductors are operating at a high DC current and lower AC current,
much different from our previous design.
As far as parallel or single MOSFETS, a lot of trade off must be done
before a final topology can be decided. Take a look at two or three phase
shifted bridges, operating at ZVS to reduce the output stress on the LC
filter.
Hope this helps,
Harry

 

[Turbo]

Ummm... Well I had thought of that approach, however... I was
looking at something considerably less complex, I hope. What I am
looking at doing for the moment, I do not care about input ripple or
noise. I was planning on using simple 60hz input rfi filter to a
bridge rectifier, capacitors and maybe a inductor to help with ripple
on the dc bus.
Here is where my question starts.
Similar to the original question, he had a DC supply to a mosfet
bridge then an inductor out. My output needs to be very clean with
<2% THD+N. I however am not looking to recreate AC. Just clean
filtered DC between 20-50A. The mosfets will be PWM controlled
200-250khz and act as current control \ limiting. I will try to post
an elementary schematic this evening. I was wondering if multiple
lower current LC filters and mosfets in parallel would be better than
a bank of mosfets feeding 1 LC or LCLC Circuit and what values for the
LC should I look for.

Thank You in advance
Cory

 

[Harry Dellamano]

Ummm... Well I had thought of that approach, however... I was
looking at something considerably less complex, I hope. What I am
looking at doing for the moment, I do not care about input ripple or
noise. I was planning on using simple 60hz input rfi filter to a
bridge rectifier, capacitors and maybe a inductor to help with ripple
on the dc bus.
Here is where my question starts.
Similar to the original question, he had a DC supply to a mosfet
bridge then an inductor out. My output needs to be very clean with
<2% THD+N. I however am not looking to recreate AC. Just clean
filtered DC between 20-50A. The mosfets will be PWM controlled
200-250khz and act as current control \ limiting. I will try to post
an elementary schematic this evening. I was wondering if multiple
lower current LC filters and mosfets in parallel would be better than
a bank of mosfets feeding 1 LC or LCLC Circuit and what values for the
LC should I look for.

Thank You in advance
Cory

I still have a problem with your requirements, so this is a one of a kind
with no AC input EMI requirements. When we state THD, we normally mean input
current harmonics. The Vdc output normally has very low AC input harmonics,
reduced by the loop gain, which are ignored but does have harmonics of the
switching frequency (200KHz) which we call noise. So <2% output noise means
at 160Vdc we can have 3.2Vrms or 9.0Vpp noise, not very clean but workable.
There are many solutions, depending on the parts availability, size, cost
and circuits that you are comfortable with. Please post your circuit and we
will critique.
Cheers,
Harry