Page 48 - Troubleshooting Analog Circuits

Equivalent Circuits Demystify Transformers 35

ferrites. I am not going to presume to tell you how to design an inductor or trans-
former or how to design circuits that use them, but I will discuss the kinds of trouble
you can have with these components. For example, you can have a good core mate-
rial; but, if there is an air gap in the core and you don't carefully control the gap's
width, the energy storage and the inductance of the component can vary wildly. If
someone has substituted a core of the wrong material, you may have trouble spotting
the change; an inductance meter or an impedance bridge can help. But even with one
of those tools. you're not home free.
For most inductors and transformers with cores composed of ferromagnetic mate-
rials, you had better make sure that the test conditions-the AC voltage and the fre-
quency that the measuring instrument applies to the device under test+losely ap-
proximate those the component will see in your real-life application. If you fail to
take such precautions, your inductance measurements stand a good chance of seri-
ously misleading you and making your troubleshooting task much more frustrating.
The phenomena you are likely to run into as a result of incorrect test conditions in-
clude saturation. which can make the inductance look too low. and core loss. which
can lower the Q of an inductor. For transformers, make sure you understand which of
the inductances in the device's equivalent circuit you are measuring.

Equivalent Circuits Demystify Transformers
You can represent a transformer with a turns ratio of N as a "T" network (Figure 3.k).
N equals NI/N2. where N, is the number of secondary turns and N, is the number of
primary turns. However. if you plan to make measurements on transformers, it's
helpful to keep the equivalent circuit shown in Figure 3.Sb in mind. For example. the
inductance you measure between terminals A and B is quite large if you leave termi-
nals C and D open, but the measured inductance will be quite small if you short ter-
minals C and D together. In the first case, you are measuring the mutual inductance
plus the leakage inductance of the primary. But because the leakage inductance is nor-
mally much. much smaller than the mutual inductance, you are measuring the leak-
age inductance of the primary plus the reflected secondary leakage in the second cas.
When you work with inductors or transformers, you have to think in terms ofcur-
rent: In any transformer or inductor. flux is directly proportional to the current. and
resistive losses are directly proportional to the current squared. Therefore, be sure to
have several current probes. so you can observe what the current waveforms are
doing. After all, some of the weirdest. ugliest, and most nonideal waveforms you'll
see are the waveforms associated with inductors. (Especially in a switch-mode
regulator. . . )
In the absence of an instrument designed to measure inductance, parallel the in-
ductor with a known capacitance to create a parallel resonant circuit. If you use a
high-impedance source to apply a current pulse to this circuit, you can determine - the
inductor's value from the resonant frequency and the capacitance: f = I/ (2 T V'LC).
If you look at the inductor's waveform on a scope, you can compare it to the wave-
form you get with a known good inductor. This technique is also good for spotting a
shorted turn, which reduces inductance nearly to zero. The L meter and the similar Q
meter can help you ensure that good inductors haven't been damaged by saturation.
Incredible as it may sound, you can permanently damage an inductor by saturating
it. Some femte toroids achieve their particular magnetic properties by means of oper-
ation at a particular point on the material's magnetization curve. Saturating the core
can move the operating point and drastically change the core's magnetic properties.
The likelihood of your being able to return the material to its original operating point
is small to nonexistent. In other cases, as a result of applying excessive current. the