Toroid or EE Ferrite core, which is Best for 240V mains SMPS

[Workhorse]

Hi SMPS Masters,

In my country I have access to both Toroid as well as EE ferrite cores, which core could be best suited for Mains AC SMPS work....
I want to switch my SMPS at 25-45KHZ, using IGBT's...But Struck in choosing the right shape for core...

I Hope Eva as always must have the wonderful answer for this too...


K a n w a r

[rlg_200]

hi kanwar,

In my openion, toroid is the best for audio power supply but if u used it in switch mode power supply toroid is not suited. ferite core is the best use for high frequency because it reduce the magnetic interference.

rlg

[Eva]

Toroids in SMPS are not as wonderful as most people think. There is too much "repeat what you heard without worrying whether it's true or not".

With toroids, stray magnetic fields only cancel out in the far field, while in the near field they produce stray fields in all planes. On the other hand, E cores only produce fields in a single plane. Thus a toroid is likely to disturb a SMPS control circuit (or even a current sense resistor!!) placed in close proximity to it despite the plane in which it is placed, but an E core will be completely harmless if the plane of the windings is perpendicular to the plane of the sensitive PCB tracks (daughterboards come handy here).

Also, achieving 4KV primary-to-secondary safety insulation in a toroid is quite difficult, while with a E core it's a matter of adding the traditional 3 layers of mylar tape, leaving some (>3mm) margins between the windings and the sides of the coil former, and between primary and secondary side magnet wires themselves where they come out of the bobbin to the pins (>5mm).

I recommend E or ETD cores for the main transformer and iron powder for the output inductors.

The following picture shows the PFC converter in which I'm now working (450V output, 1.7KW@88V AC input, 4.6KW@230V AC input). I had to place the boost inductors that way in order to reduce the amount of leakage flux disturbing the 0.01ohm current sense resistor (that shiny thing with the shape of a bridge) and the control circuit daughterboard. I'm considering two current sense resistors with the current flowing through them in opposite directions in order to fully cancel the effects of stray flux from the toroids...

[rlg_200]

yeah, E ferrite core is most suited in smps.

rlg

[Workhorse]

Thanks Eva, for the wonderful and helpful Suggestions...

Could i also use Iron powder cores for Common-mode Inductors for Mains Filtering.......

So its clearly evident from your detailed description that Toroid cores as Transformers are a bad thing in Mains SMPS....

I am currently working on 5KW SMPS to power my 3.2KW Class-H amp...

regards,
K a n w a r

[Eva]

An iorn powder core is the last thing that you would want in a common mode filter. Permeability is low so resulting inductnce is low, and resulting impedance in the Mhz range is also low. Standard power or signal ferrites should produce far better results, particularly when winding capacitances (though which RF leaks) are low.

BTW: A common-mode choke wound on a 3E25 toroid core is shown behind the capacitors in the picture from my previous post.

[Workhorse]

Hi Eva,

So what you are saying is that Standard Power Ferrites are also much better than toroids when it comes to common-mode Filtering...is it right.....

Then again what suited best to the Secondary side after the rectification for the LC filter for the Rails...
I have seen QSC PLX amps uses Ferrite beads on secondary side after the diodes....Whereas Crest PRO series uses a power ferrite based common-mode inductor on the rails itself.....[rails of power amp]

K a n w a r

[Elvee]

Originally posted by Workhorse


Then again what suited best to the Secondary side after the rectification for the LC filter for the Rails...
I have seen QSC PLX amps uses Ferrite beads on secondary side after the diodes....Whereas Crest PRO series uses a power ferrite based common-mode inductor on the rails itself.....[rails of power amp]

K a n w a r

You have to make the distinction between the main power filter and the suppression components: for the power choke, a metal powder toroid (iron or molypermalloy) is quite suitable, as is a gapped ferrite core.
Ferrite beads on the diodes serve to remove the RF caused by the switching or the hard recovery.
And common mode inductors on the output rail are there to remove ...well, common mode interferences, or to decouple groups of outputs from one another.
LV

[Eva]

Originally posted by Workhorse
Hi Eva,

So what you are saying is that Standard Power Ferrites are also much better than toroids when it comes to common-mode Filtering...is it right.....

Then again what suited best to the Secondary side after the rectification for the LC filter for the Rails...
I have seen QSC PLX amps uses Ferrite beads on secondary side after the diodes....Whereas Crest PRO series uses a power ferrite based common-mode inductor on the rails itself.....[rails of power amp]

K a n w a r

You have to distinguish between regulated and unregulated supplies. In a regulated circuit, after the diodes you are going to find either a big iron powder toroid or a big gapped ferrite core, usually with two or more coupled windings (those are differential mode, not common moe). In an unregulated circuit you are going to find small smoothing pi filters made with ferrite beads, which sometimes are also present after the main inductor in regulated supplies.

Note that QSC uses a particular SMPS topology in their highest power models, which involves a PFC stage feeding directly the primary side of a transformer through a full bridge. Don't try to understand how that works at first because it's a bit complex.

[djQUAN]

it hasn't dawned on me before this thread but is it possible to get good enough regulation going with an unregulated design right after a PFC module (astec, 380VDC output at 950W)?

and also, I can't find locally a big enough EE core, will a 2.4" dia toroid (amidon FT 240-77 from what I recall)but to increase separation, I would wind half the circle with the primary and the other half of the circle the secondary? I guess that would result to EMI nightmare?