How to Measure Leakage Transformer Inductance

Transformers are simply coupled inductors and they appear to be simpler than they really are. Transformers are not exactly simple coupled inductors, instead they comprise two inductances: a magnetizing inductance that defines coupling, and a parasitic leakage inductance. The magnetizing inductance is the inductance responsible for coupling power to another coil in a transformer, while the leakage inductance is the excess inductance.

The excess leakage inductance is responsible for some types of noise in isolated switching regulators, and design of these systems requires knowledge of the leakage inductance. Fortunately, the leakage inductance can be easily measured with a simple LCR measurement. Here are some points to understand about leakage inductance measurements, specifically with an LCR meter.

Leakage Inductance Measurement Process

A transformer’s leakage inductance can be measured with two techniques. The first is to use an LCR meter, and the second is to compare the rated inductance with the actual inductance using a turns ratio measurement.

Direct Measurement With an LCR Meter

The process for measuring with an LCR meter is very simple:

1. Connect the LCR meter potential and current probes to the primary coil

2. Solder a wire to short the terminals of the secondary coil

3. Measure the inductance of the primary coil with the LCR meter

4. Repeat the process with the coils swapped

The value measured in Step 3 is the leakage inductance of the primary coil. To get the magnetizing inductance, perform the same process but with the secondary coil left open circuit.

There are some points to note here in this example:

• The measurement frequency should be close to the operating frequency of the transformer

• The transformer will have some interwinding capacitance which will start to create a coupling efficiency reduction at high frequencies

• The transformer windings will have some series DC resistance due to their finite conductivity

• High signal levels could potentially saturate the core in the transformer

The voltage and frequency do not need to exactly be at the operating values, it is important to get close to the operating values based on the transformer’s capabilities to get an accurate estimate of the leakage inductance for use in a design.

Turns Ratio Method

An alternative approach uses a measurement of the input and output voltages to estimate the leakage inductance. This only requires an oscilloscope and does not require an LCR meter. To perform this method, the turns ratio must be known:

N = Vp/Vs

Based on the nominal value of the turns ratio, a voltage applied to the primary should produce an exact value of coupling to the secondary. However, due to the leakage inductance, the actual inductance that couples power to the secondary side will be:

L = L(M) + L(P)

Where L(M) is the magnetizing inductance and L(P) is the parasitic leakage inductance. By measuring the output voltage and currents, the primary and secondary leakage inductances can be measured if the magnetizing inductances are known. This would be performed with the following two equations:

This should be performed with an oscilloscope that has waveform generation included. These waveform generators can have adjustable settings, such as adjustable current, voltage amplitude, and frequency. The output is then measured directly with the same scope and the two traces can be overlaid on each other. This gives a quick way to visually check that the turns ratio is within an order of magnitude of the correct value.

A Third Approach: In-Circuit Method

There is a third method for measuring leakage inductance that involves placement of the transformer into a certain circuit that forces it to begin ringing. For example, this could be done with an RC snubber, where the resistor in this circuit is an adjustable resistor. The leakage inductance, capacitor, and resistor form an RLC circuit, as shown below.

When the switcher in this system turns OFF, the leakage inductance creates ringing, which can be measured at the SW node with an oscilloscope; this is a well-known problem in isolated DC/DC converters. As the switcher turns off it will cause the primary coil’s magnetizing inductance to appear as a voltage source that creates ringing due to the fast transition between ON and OFF states. The resistor can be adjusted to eventually reduce and critically dampen the resulting ringing. Learn more about determining circuit values from ringing waveforms.

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