Transformer Impedance Measurements
Making essential measurements on high frequency power transformers.
Dr. Ridley reminds designers of the crucial importance of measuring impedances of every magnetic element that is placed in your power electronics circuit, regardless of the power level. Three impedance measurements versus frequency will speed up your development process, improve the performance of your design, and ensure manufacturing quality.
I learned very early in my career as a power designer that transformers and inductors should be characterized versus frequency, regardless of their power level or frequency of operation. This has been part of design procedure for almost 100 years, ever since the use of transformers and motors became widespread.
I have always assumed that all engineers made impedance measurements, but at a recent trip to the PCIM conference in Nuremberg I discovered that most designers do not take the time for this Instead, they rely on just a few static component parameters from their magnetics vendors. Most of the manufacturers of magnetics components also do not make impedance versus frequency measurements since they are usually unaware of the value of such information or they lack the necessary training and equipment to make the measurements.
Transformers and inductors come in many shapes and sizes. Figure 1 shows two magnetic elements – one is a forward transformer for a 60-100 W application and it measures about 3 x 3 cm. This component is dwarfed by the 1500 A inductor for MW applications. Regardless of the power level, the same impedance measurements should be collected for these components.
Figure 1: There is a vast range of magnetic components used in power electronics. All of them should be measured across a full frequency range to extract useful design information and important component values.
The equivalent circuit for a two winding power transformer is shown in Figure 2. Notice that many of these components, denoted by a red symbol, are strongly nonlinear. The winding resistances Rp and Rs,vary widely with frequency, sometimes by two orders of magnitude for multilayer windings . The core loss is represented by Rc, and this is a strong function of frequency, excitation level, temperature, and for some materials and applications, the age of the core .
The leakage inductance Ll, has significant frequency dependence due to proximity effects. Magnetizing inductance, Lm, will vary depending on core excitation level, and its value can drop drastically during saturation of the core. For ungapped, high-permeability transformers it is also a function of the test excitation level at low frequencies.
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