Accurately Test Magnetics Carrying DC Bias Current
Sep 1, 2006 12:00 PM
By Jon Francis, Sales & Marketing Manager, Voltech Instruments, Oxfordshire, United Kingdom
A New DC Bias Supply Topology
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A better method of supplying the constant current from a power supply to the choke under test is to use a transistor (Fig. 6). The transistor is not employed as a constant-voltage device as you would expect in a power supply, but is positioned after the output smoothing capacitor (C) and operates as follows.
An ideal bipolar transistor exhibits the following characteristic for a given value of base current (Fig. 7). As long as the transistor has sufficient current gain (B = I
The impedance of the bias supply seen by the LCR meter and the choke is given by:
In other words, this circuit, using an ideal transistor, provides infinite impedance to the L
I
where s is the slope in amps per volt. For a typical power transistor, the slope could be 1 mA/V. This represents an impedance of 1/s = 1 kΩ for the basic circuit. By careful use of ac current feedback and compensation, this value can be further increased from 1 kΩ to 100 kΩ or more. Typically, this impedance will introduce a measurement error that is well below the error of the LCR meter itself. In practice, the resulting measurement error with this technique is just 0.1% or less.
In Voltech's DC1000 dc bias supply, the use of this topology provides users with small, robust and easy-to-use supply that avoids all of the common testing pitfalls described earlier. Inside the DC1000, microprocessor control provides users with either a simple manual rotary dial to set the current or comprehensive remote control via RS232. Sweep software is provided for measuring inductance at increasing levels of bias current to confirm the saturation point of the part under test.
Each DC1000 can supply up to 25 A, and 10 units may be paralleled to supply up to 250 A if required. A key feature of the DC1000's topology is that errors are well defined and independent of the type of LCR meter being used. This means that the power supply can be used with confidence with any manufacturer's LCR meter.
| Test | Assembly Problem Detected |
|---|---|
| DC resistance | Winding not present or badly terminated. Wrong wire gauge used. |
| Inductance with bias | Wrong number of turns. Wrong core material or incorrectly gapped. |
| Quality factor (Q) | Wrong wire gauge. Wrong core material or incorrectly gapped. |
| Turns ratio | For chokes with multiple windings, checks that they have been wound with the right number of turns. |
| Leakage inductance | For flyback-type transformers, this can be critical to in-circuit performance. Checks that windings and insulation are in the correct position. |
| Insulation resistance | Faulty insulation between windings or to core or screen. |
| Interwinding capacitance | Windings in the wrong order and position. |
| Surge | High-voltage ring test. Detects faulty insulation of wire within a winding. Often used if there are a large number of fine wire turns that can be easily damaged during manufacture. |
| Test | Problem Detected |
|---|---|
| Inductance and Q over a range of frequencies | Unwanted resonances that could cause catastrophic circuit failure. |
| Inductance versus dc bias | Saturation point of the core too low. This test will allow the designer to pin point saturation and make sure there is sufficient margin for manufacturing tolerances. |
| DC bias alone | With a constant dc current through a high-current winding, copper (I |
