Planar Transformer Measurement Using the Bode100 · 3.3.5 Other Coil Defects ... In our application note we use an AC measurement to compare defect and non-defect planar coils by

Bode 100 - Application Note Planar transformer measurements

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Smart Measurement Solutions

Planar Transformer Measurement Using the Bode100

by Lukas Heinzle ©2008 Omicron Lab – V0.09 Visit www.omicron-lab.com for more information.

Contact [email protected] for technical support.

http://www.omicron-lab.com/

mailto:[email protected]


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Table of Contents

1 Executive Summary ....................................................................................................3

2 Measurement Tasks ....................................................................................................3

3 Measurement Setup & Results ...................................................................................3

3.1 Used Equipment ......................................................................................................3

3.1.1 Planar Coils........................................................................................................4 3.1.2 Probe .................................................................................................................6

3.2 DC-Resistance of the Planar Coil ............................................................................7 3.2.1 Measurement Setup ...........................................................................................7

3.2.2 Measurement Results ........................................................................................8 3.3 AC-Impedance of Planar Coils ................................................................................9

3.3.1 Bode100 Setup ..................................................................................................9 3.3.2 Outer Coil Measurement ..................................................................................10

3.3.3 Measurement of Inner Coil B ...........................................................................13 3.3.4 Measurement of Inner Coil C ...........................................................................15

3.3.5 Other Coil Defects ............................................................................................17

4 Conclusion .................................................................................................................17

Note: Basic procedures like setting-up, adjusting and calibrating Bode 100 are described in the operational manual of Bode 100. Therefore these procedures are not described in detail in this application note.


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1 Executive Summary

This application note explains how to verify the function of planar transformer coils. These coils are usually tested with a flying probe test. A flying probe test only measures the DC-Impedance and so it is difficult to determine if the planar transformer will work or not. In our application note we use an AC measurement to compare defect and non-defect planar coils by their resonance shift and impedance deviation.

2 Measurement Tasks

Using the Bode100 it is possible to measure the impedance characteristics of planar coils over a frequency range from 1Hz to 40 MHz. By comparing the measurement results of planar coils we can detect defective planar transformers.

3 Measurement Setup & Results

3.1 Used Equipment

The following equipment was used to perform the measurements described in this application note: • One Vector Network Analyzer Bode 100 (with its measurement accessories) • One defective and two fully functional planar transformers • One measurement probe with tips


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3.1.1 Planar Coils

In our application note we use an old PCB-Board to demonstrate the measurement task. This demo board contains 3 planar coils. Two of them are well working and one is invisible damaged.


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Each planar transformer has one primary and two secondary coils. The primary coil is called the "outer coil" (visible) and the secondary coils are called "inner coils". The following picture shows the equivalent circuit model of the planar transformer.

Before using this demo board we have to remove the ferrite cores and some other parts and devices of the board. This step is necessary to get similar test conditions to the flying probe test which is applied before the PCB assembly.


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The following picture shows the connection points used for the measurements. The numbering of the points equals the numbering in the equivalent circuit model (see picture on the previous page).

3.1.2 Probe

To measure the impedance of the planar coil we recommend using a probe in 1:1 mode. We add a tip to the probe ground connector to ensure good contacting during the measurement. With these two tips we can measure the impedance between two pins on the evaluation board.


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3.2 DC-Resistance of the Planar Coil

As mentioned before, PCB-Boards are usually verified using a flying probe test. A flying probe test generally measures the DC-Impedance (resistance). The results of the following measurements show that planar coils which passed the DC test can be defective.

3.2.1 Measurement Setup

We measure the DC-Impedance (resistance) of the coils using a digital multimeter and two probe tips. The impedance of each coil is measured. To do so, the probes are hold to the corresponding pins as shown in the following pictures.


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3.2.2 Measurement Results

The following table shows the measurement results of the planar transformers. Each transformer consists of 3 coils.

Transformer Number

Outer Coil Resistance

Inner Coil B Resistance

Inner Coil C Resistance

1 (ok) 4,06 Ω 2,85 Ω 2,80 Ω

2 (defective) 4,18 Ω 2,69 Ω 2,90 Ω

3 (ok) 4,04 Ω 2,90 Ω 2,95 Ω

The resistance of a defective or working coil only differs in some 100mΩ. This difference could also be caused by bad connection respectively high contact resistance when probing. Measuring the DC-resistance of the planar transformer does not seem applicable to decide if the transformer is working or not. Therefore we try to find another measurement method to detect defective coils.


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3.3 AC-Impedance of Planar Coils

When measuring the impedance of the planar coils in a high frequency range up to 40MHz the planar coils show resonance behavior. By measuring deviations of the coil resonance frequency it is possible to distinguish between defective and non defective coils.

3.3.1 Bode100 Setup

Select the "Frequency Sweep" mode

Configure the Bode 100 as shown below:

Set the following sweep configuration:


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To ensure exact measurement results, calibrate the Bode 100 using the Probe- Calibration/Impedance. Details thereof can be found in the Bode 100-User Manual in section "Calibrating Bode 100". The impedance probe calibration was performed as follows. open probe tips are not connected together short probe tips are put together during calibration load a 50Ω resistor is connected between the probe tips

After the basic setup of Bode100 you can place the probes as shown in the next paragraph and start the measurement.

3.3.2 Outer Coil Measurement


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If you have a good connection between the PCB-Board and the probes start the measurement. The following drawing shows the results for the frequency sweep measurement of the outer coil A.

Outer coils A

Memory Track

Data Track

Impedance (Magnitude & Phase)

Tra

nsfo

rmer

3

(ok

)

(da

sh

ed

lin

e)

Tra

nsfo

rmer

2

(de

fect)

(s

oli

d lin

e) 0

5000

10000

15000

20000

5M 10M 15M 20M 25M 30M 35M 40M

TR

1/O

hm

f/HzTR1: Mag(Impedance) TR1(Memory): Mag(Impedance)

-80

-60

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-20

0

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60

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5M 10M 15M 20M 25M 30M 35M 40M

TR

2/°

f/HzTR2: Phase(Impedance) TR2(Memory): Phase(Impedance)


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Outer coils A

Memory Track

Data Track


Tra

nsfo

rmer

3

(ok

)

(da

sh

ed

lin

e)

Tra

nsfo

rmer

1

(ok

) (s

oli

d lin

e) 0

5000

10000

15000

20000

5M 10M 15M 20M 25M 30M 35M 40M

TR

1/O

hm


-80

-60

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60

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5M 10M 15M 20M 25M 30M 35M 40M

TR

2/°


The measurement results show that the resonance frequency and the impedance at resonance of a planar coil are very sensitive to defects. This shows that the measurement is suitable to detect defective planar coils.

Transformer Number

Center Frequency

(Peak)

Frequency Shift (compared to coil 3)

Impedance Amplitude

(Center Freq.)

Impedance Shift (compared

to coil 3)

1 (ok) 13,57 MHz - 1,27 MHz 17,2 kΩ - 3,3 kΩ

2 (def.) 18,29 MHz + 3,43 MHz 10,2 kΩ - 10,3 kΩ

3 (ok) 14,85 MHz - 20,5 kΩ -

In the following two sections the same measurements are applied to the inner coils B and C to check if this method can be used for error detection.


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3.3.3 Measurement of Inner Coil B

Measurement of the inner coil B:

Inner Coils B

Memory Track

Data Track


Tra

nsfo

rmer

3

(ok

)

(da

sh

ed

lin

e)

Tra

nsfo

rmer

2

(de

fect)

(s

oli

d lin

e) 0

1000

2000

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4000

5000

6000

7000

5M 10M 15M 20M 25M 30M 35M 40M

TR

1/O

hm


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5M 10M 15M 20M 25M 30M 35M 40M

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2/°



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Inner coils B

Memory Track

Data Track


Tra

nsfo

rmer

3

(ok

)

(da

sh

ed

lin

e)

Tra

nsfo

rmer

1

(ok

) (s

oli

d lin

e) 0

1000

2000

3000

4000

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6000

7000

5M 10M 15M 20M 25M 30M 35M 40M

TR

1/O

hm


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5M 10M 15M 20M 25M 30M 35M 40M

TR

2/°


Measurement results for the secondary coil B:

Transformer Number

Center Frequency

(Peak)

Frequency Shift (compared to coil 3)

Impedance Amplitude

(Center Freq.)


to coil 3)

1 (ok) 13,16 MHz - 1,69 MHz 5,38 kΩ - 1,39 kΩ

2 (def.) 18,35 MHz + 3,5 MHz 1,96 kΩ - 4,81 kΩ

3 (ok) 14,85 MHz - 6,77 kΩ -


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3.3.4 Measurement of Inner Coil C

Measurement of the inner coil C:

Inner Coils C

Memory Track

Data Track


Tra

nsfo

rmer

3

(ok

)

(da

sh

ed

lin

e)

Tra

nsfo

rmer

2

(de

fect)

(s

oli

d lin

e) 0

1000

2000

3000

4000

5000

6000

5M 10M 15M 20M 25M 30M 35M 40M

TR

1/O

hm

f/HzTR1(Memory): Mag(Impedance) TR1: Mag(Impedance)

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2/°



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Inner coils C

Memory Track

Data Track


Tra

nsfo

rmer

3

(ok

)

(da

sh

ed

lin

e)

Tra

nsfo

rmer

1

(ok

) (s

oli

d lin

e) 0

1000

2000

3000

4000

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6000

5M 10M 15M 20M 25M 30M 35M 40M

TR

1/O

hm


-80

-60

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-20

0

20

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60

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5M 10M 15M 20M 25M 30M 35M 40M

TR

2/°


Measurement results for the secondary coil C:

Transformer Number

Center Frequency

(Peak)

Frequency Shift (compared to

coil 3)

Impedance Amplitude

(Center Freq.)


to coil 3)

1 (ok) 13,57 MHz - 1,29 MHz 4,64 kΩ - 1,39 kΩ

2 (def.) 18,45 MHz + 3,59 MHz 3,3 kΩ - 2,73, kΩ

3 (ok) 14,86 MHz - 6,03 kΩ -

Comparing the results from the outer coil A and the inner coils B and C it can be seen, that in each measurement the results of the defective coil deviate from the non defective coils. Therefore the measurements shown seem to be suitable for the error detection of planar transformer coils.


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3.3.5 Other Coil Defects

If a planar transformer coil has a connection failure it is also possible to detect it. The measurement result will be similar to the results shown in the following picture.

0

2000

4000

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8000

10000

5M 10M 15M 20M 25M 30M 35M 40M

TR

1/O

hm

f/HzTR1: Mag(Impedance)

-80

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-40

-20

0

20

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5M 10M 15M 20M 25M 30M 35M 40M

TR

2/°

f/HzTR2: Phase(Impedance)

Note: This measurement result might also appear if you have a problem with your connection between the PCB-Board and the probe.

4 Conclusion

In this application note it has been shown how the Bode 100 can be used to measure the impedance of planar transformer coils. The AC-Impedance measurement of the Bode 100 offers an easy and reliable possibility detect failures in planar transformer coils.

Documents

Planar Transformer Measurement Using the Bode100 · 3.3.5 Other Coil Defects ... In our application note we use an AC measurement to compare defect and non-defect planar coils by