Accurately Test Magnetics Carrying DC Bias Current

Sep 1, 2006 12:00 PM
By Jon Francis, Sales & Marketing Manager, Voltech Instruments, Oxfordshire, United Kingdom

To avoid core saturation caused by high levels of dc bias, it is common to design this type of choke by introducing an air gap into the core, either physically with a spacer for ferrite cores or by using powdered core type. The additional complexity of the design further increases the need for comprehensive testing at the operating point. At this point, the choke must provide the desired inductance to the operating frequency, and as low as possible an impedance to the dc current.

The inductance measured at the operating point is often different than the inductance measured without bias, because the slope of the B-H curve changes over the range of H. It is important to test for the correct inductance in the presence of the maximum-rated dc current (Table 1).

In the design environment, the design engineer is making sure that the design calculations were correct and that the part will work as intended. The ultimate test is to study the part's behavior in circuit — checking waveforms, power loss and overall circuit performance — but making measurements first will usually save time.

In addition to the production tests described in Table 1, the engineer may perform the tests outlined in Table 2.

Applying a DC Bias Current

To measure inductance, an LCR or impedance meter is connected across the choke (Fig. 3). The meter applies an ac voltage at the desired frequency and determines the imaginary part of the impedance across its terminals. The inductance (L) equals X_L/(2πf). If the dc bias current is applied using a conventional bench power supply, or even a battery, then it's impedance must be significantly higher than that of the choke being measured. Unfortunately, this is never the case for a simple supply (Fig. 4).

The output capacitance of the power supply will dominate the impedance measurement and give a wildly inaccurate result. For example, if the inductance of the choke is 100 µH and its operating frequency is 100 kHz, then it has an impedance of:

X_L = 2πfL = 2 × π × 100 × 10^-6 × 100 × 10³ = 62.83 Ω.

If the output capacitance of the power supply is 10,000 µF, then its impedance is:

The impedance measured by the LCR meter is that of the power-supply capacitor and choke impedance in parallel, and the impedance measurement is swamped by the capacitor. Consequently, the result is 100% inaccurate.

A common solution to this problem, as used by most commercial apparatus, is to feed the dc power-supply output to the choke via a coupling inductor (Fig. 5). A typical solution uses a coupling inductor up to 10 times the value of the choke under test to reduce errors due to the source appearing as a load to the meter. However, to measure different values of chokes, several different values of coupling inductors must be fitted inside the dc bias supply and relay switched into the power circuit. Still, the resulting measurement will be fairly accurate with a measurement error of ±10%.

This solution may provide a reasonable measurement at optimum conditions, but it is large, heavy and often un-reliable because of the relay switches. Practical considerations of size, and particularly self-resonant frequency, limit the performance of this method in terms of both accuracy and the highest frequency that can be measured.

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