Testing Common Mode Chokes

Worked Example of  Suitable Tests

Overview of CM Chokes

Common-mode chokes are useful for prevention of electromagnetic interference (EMI) and radio frequency interference (RFI) from power supply lines
They pass differential currents (equal but opposite), while blocking common-mode currents, and can pass high DC currents without affecting their noise blocking performance.

The simplest form of common mode choke has two coils wound on a single core.
The choke is placed in the transmission line, so that the current flows through one winding then back through the other.

Epcos B82731M2351A030 choke

If a differential mode current is present, then the two coils create equal and opposite magnetic fields which cancel then each other out.
The choke thus presents zero impedance to any differential mode signal, which passes through the choke without any attenuation.

Differential signal

In contrast, a common mode signal flows in the same direction on both lines, creating magnetic files that are in phase and do not cancel.
This will then present high impedance and hence attenuate any common mode signal.
The actual attenuation (or common mode rejection) depends on the relative magnitudes of the choke impedance and the load impedance.

Common mode signal

 

A good example of a classic design of common mode choke is the EPCOS series B82731, of which we will be discussing the Epcos B82731M2351A030.

B82731M2351A030 drawing and footprint

Suggested testing for CM Chokes

AT Editor Schematic for CM choke

The four-pin transformer is easily represented using the AT Editor software using two windings.

AT Editor Schematic

AT Fixturing for CM Choke

The conventional THP pin footprint makes the part suitable for a Kelvin-pin fixture, which will allow us to compensate out the contact resistances inherent in any fixturing before measurement and give excellent part swap over times of the order of seconds.

Fixturing

AT Test Program for CM Choke

The test program first checks DC resistance on each winding, and then that the turns ratio is 1:1 within 2%

This is followed by two inductance tests; First he standard series inductance on one winding is checked to be within the spec, and then leakage inductance (a measurement of how well the windings are coupled), which in the case of a common mode choke must be kept to a minimum)

Finally the line-to-line stand off voltage is confirmed by means of a AC Hipot test at 1500V.

#

Test

Description

Pins and Conditions

Reason

1 R DC resistance pin 1-4, limits <4.5 Ohms +- 10% as per spec To check the winding resistance is below a maximum. Also acts as a check of correct wire gauge and good termination.
2 R DC resistance pin 2-3, limits <4.5 Ohms +- 10% as per spec To check the winding resistance is below a maximum. Also acts as a check of correct wire gauge and good termination.
3 TR Turns Ratio Energize pins 1-4, 500mv 10khz, check Turns ratio (1-4:2-3) to be 1:1 -+ 2% To check correct ratio of windings
4 LS Series Inductance Pin 2-pin 3, 100uA, 10 Khz, nominal 100mH + 50% / -30% (as per published spec) To check the correct number of turns and correct operation of the core material
5 LL Leakage Inductance Pins -4 Hi, Pins 2-3 Low, 5mA, 10kHz , check below 1.05mH To check leakage inductance is below specified limit.
6 HPAC AC Hi-Pot 1kV AC, 1 second, Pins 1&4 High, Pins 2,3 Lo To check isolation as per datasheet.
        AT5600 Run time 2.72 sec
        (AT3600 Run time 5.21 sec)

NOTE:

The data sheet also specifies curves for Impedance (Z) response over frequency.
This is not usually tested in a production environment as it is embodied in the design choice of cores and windings. If you wish, the AT’s “Z” test could be used here periodically, by using the AUDIT function to occasionally validate this performance on 1 in 100 transformers.

CM Choke - AT Test Results