Accurately Test Magnetics Carrying DC Bias Current

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

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In power supplies, dc-dc converters and many other power electronics circuits, chokes and transformers form an essential part of the filter network that provides low-ripple dc power. In many designs, such as the forward converter, the main output smoothing choke is also an essential part of the complete switching converter design topology. For this reason, it is vital to the overall performance, reliability and efficiency of the converter that the smoothing choke behaves predictably at all possible operating points of the switching converter.

During the manufacture of this critical component, it is possible that manufacturing errors or a combination of design tolerances may produce a choke that will not perform satisfactorily in-circuit. It is common practice then to test the choke before it is fitted to the power circuit.

During design, the power-circuit design engineer will confirm design calculations and margins by measuring inductance (Q), ac resistance and turns ratio over a range of frequencies, and will confirm overload capacity (or margin) by raising dc bias current to the point at which the transformer or choke saturates as detected by a fall in inductance. In addition, during manufacture, spot measurements are made to confirm that the component has been correctly assembled using the right core material and air gap, as well as winding the right number of turns of the specified wire.

Whether during component development or power-supply development, there are certain pitfalls engineers must avoid when testing power inductors. These include the use of common test methods that may produce wildly inaccurate results or that require cumbersome test setups. A new versatile method described here helps designers avoid such pitfalls and achieve highly accurate inductance measurements.

Tests for a DC Choke

A common example of an inductor that carries dc current is the output storage choke of a forward converter (Fig. 1). In this example, the current in the output choke (I_L) consists of two parts: the dc load current and a saw-tooth current (i_L) that is determined by the voltage across L, V_l and the value of inductance:

The dc current is often the most significant part of I_L.

In terms of the B-H loop of the choke's core, this means that the magnetic flux density (B) is also shifted from its normal loop around zero (Fig. 2). The magnetic design of the choke must ensure that there is a sufficient flux density margin to avoid saturation with the dc bias current being applied.

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