TDK-Lamda PFE700S - 2 to power audio amp

[mr coffee]

I see from the application notes that these PFE700S switching units can put out 51 to 55vdc each (isolated) at plenty of amps for a bipolar supply to power an audio amp. The app notes show the need for a mains filter using multiple common-mode chokes and safety caps.

Do I need to have two complete mains filters or can I use one filter for the input to both modules? I'm fairly new to high power switching supplies, and I don't know how critical the 11mh value for the common mode chokes, and I'm looking to use some toroid cores I have laying around. I figure I need to add some additional series filtering (non-common mode) for the 10 to 200 Khz range to make it play nice with other audio equipment, and I'm assuming that one such filter would be sufficient for both modules.

Any help you may be able to give would be appreciated. If you would like to reference the app sheet for the TDK-Lambda PFE700S, they call it an "Instruction Manual" on their site, and it pops right up on Google with a search for "PFE700S datasheet".

mr coffee

 

[infinia]

kinda pricy for a pair
pretty much follow their guide lines to a T iE esp connecting multiple modules.
I would plan on using separate CM chokes. Sometimes the control loops go crazy with crosstalk, the CM chokes help prevent that by isolating / floating "module grounds". Then you could choose a single Pri. EMI filter once the modules are isolated. buy a dozen or so high perm ferrite toroid's.

 

[mr coffee]

They are REAL pricey to buy new, but I picked up a pair surplus and figured I could save weight over a huge toroid power transformer.

I found some reasonably priced common mode chokes, same size core as their app note, but higher permeability core as it would take much less that 25 turns per winding to reach the 12 mh. inductance as the spec sheet for their specified CMC call for. Probably 10 turns or less per winding to get 12mh. The spec'ed inductors are 13 bucks each, so I'm trying to improvise.

Do you have a sense of how critical the tuning of the common mode sections are to control of RFI? Or is it mostly just a non-critical low pass filter design with the cut-off frequency being pretty arbitrary? The cores I have would be more conducive to making higher inductance CMCs (with therefore a lower cutoff frequency, which should be good??)

Thanks.
mr coffee

 

[infinia]

good deal mr coffee

CMC value is not so important within range of 5-20 mH is good, keep it mains safe, build in creep margins etc . look at coilcraft.com etc. mainly just keeping AC circulating currents local ie leakage to other grounds is bad.

 

[mr coffee]

Infinia,
Sounds good - I should be able to wind 20mh using #18AWG on the cores - they have a divider down the middle of the donut hole, as they are meant to be CMC.

Keep the baseplate grounds separate and just join them at the mains EMI input filter then?

mr coffee

 

[infinia]

Keep the baseplate grounds separate and just join them at the mains EMI input filter then?

technically the base plates are electrically floating, but can serve as a huge common mode noise injection point. I would float for lowest EMI, but it's not very practical for many reasons. Safety wise they could develop a potential, requiring 2 heat sinks with warning stickers etc.. .