#### Shiladri

##### Newbie level 4

I am facing a peculiar problem in my board-integrated Planar transformer...

It is an 8-layer board where the primary and secondary windings are interleaved(P1 S1 P2 S2 P3 S3 P4 S4)..connections done by vias..

Each primary layer has 2 turns and each secondary layer has 8 turns..thus in total there are 8 primary turns and 32 secondary turns..

The core is a standard E-I core manufactured by Ferroxcube(EI 38/8/25)

I am observing something really strange while measuring the magnetizing inductance(I am measuring this by keeping the secondary open and measuring

the inductance at the primary side using an RLC meter)...The results are as follows

1.Without the core introduced the RLC meter reads 1.5 microH (expected as the reluctance is high because the flux path is through air)

2.With only the E core introduced the meter reads 3.8 microH (again expected as reluctance is slightly reduced..)

3.But the strange thing is that with the 'I' part also introduced(i.e now I have the complete E-I set in place)..the RLC meter shows the terminal

impedance is capacitive(something like 10 nF)..

This really baffles me!!

With the E-I core in place the reluctance is the least and hence the magnetizing inductance should be maximum... how this becomes capacitive is beyond me?

Can anyone plz help..??

Could there be some issue with the board which is causing this problem??But then without the core the board shows 1.5 microH as I said..

Also I have seen that it is working perfectly fine as a 1:4 transformer w/o any core (checked this by applying a sine wave of 1 unit from a function

generator..I get a sine wave of 4 unit at the secondary)...

With the core in place the input signal from the function generator gets distorted and is no longer a sine wave(becomes like flat topped sine wave)...

This problem really beats me!

Any inputs would be greatly appreciated..