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

bad regulation or bad power factor (PFC aside).

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