Introduction to Testing Ferrite Transformers
As electronic products utilize higher frequency
techniques to reduce size and improve efficiency, ferrite cores are used
in an increasing proportion of transformer designs.
Transformer manufacturers must therefore meet a need for smaller
transformers designed to operate at higher frequencies, which introduces
additional demands on both manufacturing and testing methods.
These issues apply to a wide range of common applications
including switched mode power supplies, lighting ballasts, inverter
drives, audio and telecommunications equipment and many more.
Today's need for the proven performance of all components within a product
has resulted in a demand for each and every transformer to be more
thoroughly tested than traditionally expected.
In the following pages, we will consider the range of tests that are
appropriate for thorough testing of ferrite transformer designs and we
begin with a review of the components present in a common transformer.
FIGURE 1
Schematic of a simple two winding transformer connected
to the four wire Kelvin nodes of an AT series transformer tester.
From the schematic in figure 1, it can be seen that even
the most simple of transformers includes quite a complex combination of
resistive and reactive components.
In order to establish with confidence that a transformer
has been manufactured correctly, it is necessary to execute a range of
tests that combine to provide an assurance that the materials used and
manufacturing process executed results in transformers that meet the
design specification.
CTY: Continuity
Ensures that the transformer is correctly seated in its
fixture and that all winding termination integrity is good.
Unit of measurement, Ohms. Range from 10KOhms to 10MOhms
By selecting this test first, the operator can be alerted
if any connections are poor prior to executing the main tests, saving time
and avoiding incorrect transformer error reports in batch statistics.
R: Resistance
Ensures that the gauge of copper being used for each
winding is correct.
Unit of measurement, Ohms. Range 10mOhms to 10MOhms
All windings are tested individually ensuring that there
are no windings with an insufficient gauge of copper to carry the required
current.
Figure 2 Example test entry screen for resistance using
the Editor program.
LS: Series inductance
Ensures the correct core material has been used and that
the number of turns is correct.
Unit of measurement, Henries. Range 1nH to 1MH with
signal level from 1mV to 5V @ 20Hz to 3MHz
Different core materials exhibit different permeability
and therefore a different value of inductance for a particular number of
turns. With the correct number of turns, inductance provides a measure of
the core materials ability to maintain the required magnetic flux without
saturation.
Figure 3 Example test entry screen for inductance using
the Editor program.
QL: Quality Factor
Ensures that core material and its assembly is correct
Unit of measurement, Q. Range 0.001 to 1000 with signal
level from 1mV to 5V @ 20Hz to 3MHz
Quality factor represents the efficiency of an inductor
as the ratio of energy stored to energy wasted and is derived from the
equation L / (RÖLC). It can be seen that higher Q values are obtained when
the inductive component is large relative to the resistive and capacitive
components.
Figure 4 Example test entry screen for Q Factor using the
Editor program.
ANGL: Angle of impedance
Ensures that the core material, wire resistance, number
of turns and inter-winding capacitance combine to meet design
specifications.
Unit of measurement, Degrees. Range -360° to +360° with a
signal level from 1mV to 5V @ 20Hz to 3MHz
For transformers in applications that operate over a wide
frequency range, e.g. audio transformers, the designer or the production
department may have to measure the phase angle between the real impedance
(resistive (R)) and imaginary impedance (inductive or capacitive (jXs)).
The sum of R and jXs is commonly referred to as Z (total impedance). As
the applied frequency is increased on an inductor the impedance increases
and the impedance phase angle decreases up to the point of self-resonance,
at this point the impedance phase angle is zero (also the highest
impedance value).
Figure 5 Example test entry screen for Phase Angle using
the Editor program.
LL: Leakage inductance
Ensures that windings are positioned correctly on the
bobbin and that any air gap included in the core design is the correct
size.
Unit of measurement, Henries. Range 1nH to 1kH with
signal level from 1mV to 5V @ 20Hz to 3MHz
Leakage inductance is the inductive component
attributable to magnetic flux that does not link primary to secondary
windings. Designs may require a specific value of leakage inductance for
the correct operation of the circuit into which the transformer will be
fitted or it may be necessary to keep the value very low. Measurement of
leakage inductance requires the application of a short circuit to
secondary windings and this can often present problems in a production
environment. The AT series testers eliminate these problems with a unique
measurement technique that is described in detail in a separate technical
note VPN: 104-105.
Figure 6 Example test entry screen for leakage inductance
using the Editor program.
C: Inter-winding capacitance
Ensures that the insulation thickness between windings is
correct.
Unit of measurement, Farads. Range 100fF to 1mF with
signal level from 1mV to 5V @ 20Hz to 3MHz
Capacitance occurs in inductors and transformers due to
the physical proximity of electrostatic coupling between wire within a
winding. Capacitance also exists between separate windings from primary to
secondary or secondary-tosecondary.
Figure 7 Example test entry screen for capacitance using
the Editor program.
TR: Turns ratio
Ensures that the number of turns on each winding and the
winding polarity meet specification.
Unit of measurement, Decimal Ratio. 1:100k to 100k:1 with a signal level
from 1mV to 5V @ 20Hz to 3MHz
Turns ratio is measured to establish that the number of
turns on primary and secondary windings are correct and therefore the
required secondary voltages are achieved when the transformer is in use.
It is important to remember that the various transformer losses shown
figure 1 will result in a voltage ratio that does not correspond exactly
with the ratio of physical turns present on the windings. The AT series
testers include the ability to calculate turns from the ratio of
inductance (TRL) which overcomes errors attributable to core loss and
leakage inductance. This and other turn ratio considerations are described
in a separate technical note VPN: 104-113.
Figure 8 Example test entry screen for Turns Ratio using
the Editor program.
SURG: High voltage surge testing
Ensures that the insulation material around the copper
wire (usually lacquer) has not been damaged during manufacture introducing
the risk of an inter-winding short circuit.
Unit of measurement, mV Seconds. Range 1mVs to 1kVs with
an impulse signal level from 100V to 5kV.
Transformers with a high number of turns using fine wire
are vulnerable to insulation damage. Damage to the insulation material
during production is very difficult to detect as there may not be a total
short circuit and the voltage applied during turns testing will not be
sufficient to bridge this partial short. However, during operation within
the finished product, the transformer is exposed to much higher voltages
which can cause a corona arc at the point of damage or the heating effect
of normal use may cause a short circuit after a short period of time.
By connecting a charged capacitor within the AT3600 to a
transformer winding, the winding is exposed to an impulse voltage and by
measuring the area under the decaying oscillation, it is possible to
establish if a breakdown between turns of the winding has occurred. The
diagram below illustrates the decaying oscillation of a transformer
winding with no insulation damage versus the same winding with damaged
insulation.
Figure 9 Surge waveform examples
By computing the volt-second product under the curve, the
AT3600 provides a numeric quantity by which to establish good or bad
components. This gives the benefit of shorted turns detection using an
impulse voltage technique, while avoiding the potential errors inherent in
user interpretation of complex waveforms.
Figure 10 Example test entry screen for Surge Stress
using the Editor program.
IR Insulation Resistance
Ensures that the isolation between windings meets the required
specification
Unit of measurement, Ohms. Range 1MOhms to 100GOhms with a signal level
from 100V to 7kV (AT3600) or 500V (ATi).
Using a DC high voltage generator and DC current measurement system, the
value of resistance is calculated.
Figure 11 Example test entry screen for Insulation Resistance using the
Editor program.
HPAC High Voltage AC safety testing
Ensures that the windings are positioned correctly with
the correct materials to provide the required level of safety isolation.
Unit of measurement, Amps. Range 10uA to 10mA with a signal level from
100Vac to 5kVac.
All transformers that provide isolation from an AC power
system must be tested to confirm their ability to withstand safety-testing
voltages without breakdown. In order to meet testing regulations, it is
necessary to provide evidence that the test voltage is maintained during
the test period and the AT3600/AT5600 achieves this by measuring and
controlling the applied voltage throughout the complete duration of test.
Figure 12 Example test entry screen for HPAC using the
Editor program.
Conclusions
It can be seen that the appropriate range of tests will
provide complete assurance that all materials and production processes
within a transformer are correct.
This in turn will guarantee that each and every
transformer tested is known to fully meet the required specification.
Such thorough testing here has historically been to
costly, too difficult or too time consuming.
However the AT series testers provide a cost effective,
easy to use and fast solution.
The complete test shown above was executed by the AT
tester at a speed of 1.2 seconds, with the single touch of a button.