FreeLists / si-list / [SI-LIST] Re: [SI-LIST]surge suppression

["Bart Bouma" <bart.bouma@xxxxxxxxx>]

Hi Sajith, 
I'm not a specialist on this, but here my thoughts:
I suppose that the CM-choke is a RFI CM-choke, and that by feeding it this 
energy-rich and relatively low frequency 1.2/50usec pulse in common mode 
the choke's core gets saturated and doesn't provide a high impedance to 
the CM-signal/pulse anymore (i.e no attenuation of CM-signal).

I don't understand why the output voltage is higher than the input voltage 
(your test-setup is not clear to me: how you measured it and whether the 
two lines are tied together or not), but all I can think off is :

- a possibility could be peaking of the signal caused by a high-Q  L-C 
circuit formed by the choke and the X-caps and/or stray capacitance..
- or that in one way or another the signals are summed ?

hopefully this helps you.

best regards, Bart
www.yageo.com




Hi all,

The New year at our lab has kicked off with an interesting observation

We are in the development of an SMPS, the RFI filter section is built with 
=
discrete components

a common mode choke + 2class X caps + 2class Y caps

L)------------------I           I---------------------(
            __      I   CM      I        __               INPUT TO SMPS
         X ---      I  choke    I      X--- 
N)------------------I  _____    I---------(
            ___                                ___
        Y  -----                            Y -----
PE--------------------------------------------------


Hope the above figure makes the way they are connected=20

From left    :  the X cap across phase and neutral, then the common mode =
choke, the second X cap across phase and neutral and one Y cap each from =
Phase to Earth and Neutral to Earth.


Coming to the real issue :

While carrying out the surge test we injected 1.2/50us signal in both =
Phase and Neutral simultaneously
keeping earth as the return=20

( ie making it a common mode signal in P and N with respect to earth )

We observed that when 1.5KV is injected at the input , We are able to see 
=
around 2KV peak after the common mode choke=20

the common mode choke, which is suppose to suppress the common mode signal 
=
was seen to be doing=20
the opposite. Probing the point before CM choke was showing 1.5KV and just 
=
after it was 2KV

We tried removing the Y caps but it did not make any difference

The common mod choke is a standard part from a reliable manufacturer.

Can the common mode choke behave this way???

Could any one please explain what the phenomenon is????

Thank you for your time

Best regards,

Sajith
Analog Designer
IN.





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