2.8 LL - Leakage Inductance
If a secondary winding of an ideal transformer is short circuited, the
transformer would present zero impedance to the supply, and an infinite
current would flow: -
In practice the actual current is not infinite, even if there is no
winding resistances, because it is limited by the fact that the coupling
between windings is not perfect: -
P = Primary winding
S = Secondary winding
As a result of imperfect coupling, a short-circuited transformer acts as
if there was an inductive impedance in series with a winding:
This impedance is known as the leakage inductance, and is a measure of
the coupling between windings.
Low leakage inductance implies good coupling; high leakage inductance
poor coupling.
Leakage inductance limits the flow of current when the transformer is
short circuited.
Like winding resistance, it also causes the output voltage to fall with
increasing load current, adding to the transformer regulation. In SMPS
transformers, leakage inductance causes transistor over voltage when the
transistor is turned off.
Most transformer designs require low leakage inductance but for some
designs (e.g. for electronic ballasts, constant voltage transformers and
resonant converter transformers), leakage inductance is deliberately
introduced as part of the overall circuit design.
Leakage inductance can be reduced by ensuring that windings are in close
physical proximity to each other, have long winding lengths or are
interleaved.
Low leakage designs: -
a) Close proximity-----------------------b) Toroid -
long----------------c) Interleaved winding length
Leakage inductance can be increased by separating windings, providing
short winding lengths or introducing alternate flux paths.
High leakage designs: -
 |
 |
 |
| a) Short winding |
b) Increase separation |
c) Alternate flux path lengths between winding |
Figure 21
Where Used
Leakage inductance is important in many applications. One example is
flyback designs for high frequency switched mode power supplies, where the
leakage inductance must be less than a specified critical value for proper
operation.
Measurement Conditions
Leakage inductance is tested by measuring the inductance of a 'primary'
winding when one or more 'secondary' windings are shorted out. In
performing the calculation at the end of the test to extract the
inductance value from the measured winding impedance, the tester uses a
series equivalent circuit.
In making the measurement, the tester automatically compensates for the
impedance of the wiring, the connections and the relays in the shorting
path.
Leakage inductance can be measured using a test current in the range 20μA
to 100mA at a frequency of 20Hz to 3MHz.
You may choose a suitable test current and frequency based on the
expected value of the leakage inductance using the following table: -
| Leakage Inductance range |
Preferred test signal |
| Frequency |
Current |
100nH → 1uH
1uH → 10uH
10uH → 100uH
100uH → 1mH
1mH → 10mH
10mH → 100mH
100mH → 1H
1H → 10H |
300kHz
100kHz
30kHz
10kHz
1kHz
100Hz
100Hz
50Hz |
50mA
20mA
10mA
5mA
5mA
5mA
1mA
500μA |
NOTE: Because leakage inductance is measured with a secondary winding
shorted out, be careful to choose a test signal that will not cause
excessive currents to flow. This is particularly significant in
transformers where the turns ratio is very high and the shorted winding
has only a few turns.
If, for example, the primary winding has 300 turns, and the secondary 3
turns, a test current of 10mA flowing through the leakage inductance on
the primary side will give rise to a current of 1 Amp flowing in the
shorted secondary winding.
In order to protect transformer windings, the test current when measuring
leakage inductance is limited in the table to 50mA maximum.
In addition, the problem of self-resonant frequency listed under the
primary inductance test also applies when measuring leakage inductance, so
always use the lower of the available band of frequencies.