Stray capacitance and inductance
Aside from power ratings and power
losses, transformers often harbor other undesirable limitations which
circuit designers must be made aware of. Like their simpler counterparts
-- inductors -- transformers exhibit capacitance due to the insulation
dielectric between conductors: from winding to winding, turn to turn (in
a single winding), and winding to core. Usually this capacitance is of
no concern in a power application, but small signal applications
(especially those of high frequency) may not tolerate this quirk well.
Also, the effect of having capacitance along with the windings' designed
inductance gives transformers the ability to resonate at a
particular frequency, definitely a design concern in signal applications
where the applied frequency may reach this point (usually the resonant
frequency of a power transformer is well beyond the frequency of the AC
power it was designed to operate on).
Flux containment (making sure a
transformer's magnetic flux doesn't escape so as to interfere with
another device, and making sure other devices' magnetic flux is shielded
from the transformer core) is another concern shared both by inductors
and transformers.
Closely related to the issue of flux
containment is leakage inductance. We've already seen the detrimental
effects of leakage inductance on voltage regulation with SPICE
simulations early in this chapter. Because leakage inductance is
equivalent to an inductance connected in series with the transformer's
winding, it manifests itself as a series impedance with the load. Thus,
the more current drawn by the load, the less voltage available at the
secondary winding terminals. Usually, good voltage regulation is desired
in transformer design, but there are exceptional applications. As was
stated before, discharge lighting circuits require a step-up transformer
with "loose" (poor) voltage regulation to ensure reduced voltage after
the establishment of an arc through the lamp. One way to meet this
design criterion is to engineer the transformer with flux leakage paths
for magnetic flux to bypass the secondary winding(s). The resulting
leakage flux will produce leakage inductance, which will in turn produce
the poor regulation needed for discharge lighting.
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