**Document**

**Name**

# The Voltech Handbook of Transformer Testing

**Description**

1 Transformer Basics |

2 Available Tests |

2.1 CTY - Continuity |

2.2 R - Winding Resistance |

2.3 RLS or RLP - Equivalent Series or Parallel R |

2.4 LS_LP - Primary Inductance |

2.5 LSB_LPB - Inductance With Bias Current |

2.6 QL - Q factor |

2.7 D - Dissipation Factor |

2.8 LL - Leakage Inductance |

2.9 C - Inter-winding Capacitance |

2.10 TR - Turns Ratio and Phasing |

2.11 TRL - Turns Ratio by Inductance |

2.12 Z_ZB - Impedance_Impedance with Bias |

2.13 R2 - DC Resistance Match |

2.14 L2 - Inductance Match |

2.15 C2 - Capacitance Match |

2.16 GBAL - General Longitudinal Balance |

2.17 LBAL - Longitudinal Balance |

2.18 ILOS - Insertion Loss |

2.19 RESP - Frequency Response |

2.20 RLOS - Return Loss |

2.21 ANGL - Impedance Phase Angle |

2.22 PHAS - Inter-winding Phase Test |

2.23 TRIM - Trimming Adjustment |

2.24 OUT - Output To User Port |

2.25 IR - Insulation Resistance |

2.26 HPDC - DC HI-POT |

2.27 HPAC - AC HI-POT |

2.28 SURG - Surge Stress Test |

2.29 STRW - Stress Watts |

2.30 MAGI - Magnetizing Current |

2.31 VOC - Open Circuit Voltage |

2.32 WATX - Wattage (External Source) |

2.33 STRX - Stress Watts (External Source) |

2.34 MAGX - Magnetizing Current (Ext. Source) |

2.35 VOCX - O.C. Voltage (External Source) |

2.36 LVOC - Low Voltage Open Circuit |

2.37 ILK - Leakage Current |

2.38 LSBX - Inductance with External Bias (Series) |

2.39 LPBX - Inductance with External Bias (Parallel) |

2.40 ZBX - Impedance with External Bias |

2.41 ACRT - AC HI-POT Ramp |

2.42 DCAT - DC HI-POT Ramp |

2.43 ACVB - AC Voltage Break Down |

2.44 DCVB - DC Voltage Break Down |

2.45 WATT - Wattage |

2.0 Available Tests On The AT Series |

3 Examples of Different Transformer Types |

## 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.