2.9 C - Inter-winding Capacitance
Practical transformers, with windings in proximity to each other, exhibit
capacitance between those windings, the inter-winding capacitance.
The value of capacitance depends on factors such as the layout and the
thickness of the insulation tape.
For applications such as communication transformers the interwinding
capacitance has to be carefully controlled to guarantee the transformer
frequency response. In SMPS transformers the interwinding capacitance can
transmit common mode noise between windings.
Capacitance occurs in transformers due to the physical proximity of, and
electrostatic coupling between, different turns of wire. In general, the
capacitance is distributed between the different layers within a winding,
and between the outside layer of one winding and the inside layer of the
next. Inter-winding capacitance may be of interest in transformers used in
audio, medical and instrumentation applications, where isolation between
primary and secondary windings is important. It can also play an important
part in the circuit operation of switch-mode transformers where, for
example, too large a capacitance may give rise to a large amount of noise
at the switching frequency being coupled into sensitive circuits connected
to the secondary windings.
To measure capacitance, the tester applies an ac voltage between the
windings to be tested, usually with all taps on each winding shorted
together. It then measures the voltage between the windings, and the
resulting current using harmonic analysis. Dividing the voltage by the
current gives the interwinding impedance, from which the capacitance may
be calculated. The test voltage can be in the range of 1mV to 5V at a
frequency of 20Hz to 3MHz.
The table below gives the recommended test conditions for different
values of capacitance: -
||Preferred test signal
|1pF → 10pF
10pF → 100pF
100pF → 1nF
1nF → 10nF
10nF → 100nF
The Test Conditions for Capacitance Measurement
When choosing the test conditions, the following potential problems
should be considered: -
a) Current levels
For larger capacitance, particularly at higher frequencies, the current
flowing during the measurement can be very high, and similarly the
measured current could also be very small for small capacitance at lower
frequencies and voltages.
Where possible, you should use the recommended test signal levels in the
table above to ensure that the currents which flow can be measured
b) Non-linear Capacitance
Normally non-linearity in the stray capacitance of transformers are not a
problem, and therefore capacitance is measured with as large a voltage as
c) Equivalent Circuit
As with inductance, capacitance is actually measured as a complex
impedance, and therefore the result can be expressed in terms of either a
series or a parallel equivalent circuit.
It was explained in section 1.5 of this chapter that parallel and series
equivalent inductance do not necessarily have the same values. The same is
true for capacitance; parallel and series equivalents can also be
different. The tester always uses a parallel equivalent circuit for