Using Standard
Transformers inMultiple Applications
Creative Use of Alternate Winding Connections
Introduction

DC Resistance

Many Coilcraft transformers were designed for optimal
performance in a specific application. Catalog information
presents the transformers based on the specifications
for that original application. However with a little creative
thinking, most transformers can be adapted to a varietyof circuit uses just by considering different connections
for the windings.

When connecting two equal windings in parallel, the combined DC resistance is half that of each winding.

What to Consider
A transformer winding might be constructed of multiplewires intended to be connected in parallel. The usermight instead consider connecting those wires in series
to increase the resulting inductance or turns ratio. Careful consideration of all the resulting variables should bemade before attempting alternate configurations. Thosevariables include:
••
•
•
•
•
•

inductance value (L)dc resistance (DCR)
rms current (Irms)
temperature rise (due to current and resistance)
peak / saturation current (Ipk or Isat)
self resonant frequency (SRF)
voltage / isolation between wires / safety

For example, assume a coil winding, either primary orsecondary side, is made with two parallel (bifilar) wires.

Inductance (L)Inductance is proportional to the square of the number ofturns (N2) when wound on the same core. So two equalcoils, wound on the same core and connected in series
have about four times the inductance of the parallelconfiguration.
When connecting two equal-turn windings in parallel on
the same core, the inductance does not change, but theDC resistance is half of that of each single winding. This
is often confused with connecting separate inductors inparallel, for which the inductance is halved. When the twowindings are in parallel on the same core, it is effectively
replacing a single wire by two wires, and therefore theinductance remains the same.

When connecting two equal windings in series, the combined DC resistance is two times that of each winding. When equal windings are connected in series, theDC resistance is fourfold (4×) compared to the samewindings in a parallel combination.

RMS Current
The rms current rating is based on temperature rise, whichis related to power loss.
The amount of current flowing through the series combination that causes the same temperature rise as theparallel combination is calculated as follows:

For equal windings, Irms for the series connection ishalf that of the parallel connection and yields the sametemperature rise.

Saturation Current
Saturation of ferrite or powdered iron cores is related tothe energy storage capability of the inductor as indicatedby the volt-time product (V × t). An approximation of thevolt-time product can be calculated from the equation:
V = L × di/dt
rearranged in the form V × T = L × Isat.
Where:
V × T is the volt-time product in volt-µs
L is the inductance in µH
and it is assumed that:
dt is approximated by T, anddi is approximated by Isat (the rated saturation current
shown on the data sheet)With two equal windings in parallel, Isat is the same asthat of one winding. With two equal windings in series,
the number of turns is doubled, which doubles V × T,
quadruples L, and therefore results in an Isat that is halfof the single-winding and (equal) parallel winding Isat.
Document 857-1

Revised 07/19/23

Self Resonant Frequency (SRF)
The self resonant frequency of two bifilar-wound wires
depends on the inductance and capacitance. Whenconnected in series, the interwinding capacitance andwinding capacitance to ground may change. The effective
capacitance change may be small compared to inductance but should be considered. Since inductance is 4 ×,the self resonant frequency is proportional to 1/  L × C so
the series SRF will be about 1/  4 , approximately halfthat of the parallel configuration.

Voltage / Isolation / Safety
Coilcraft typically specifies isolation between windingsthat are intended to be used separately, such as withcoupled inductors or transformers. We do not normallyspecify isolation between wires on multi-wire (bifilar, trifilar, etc.) windings designed to be connected in parallel.

When wires are bifilar wound, the isolation between themwill be quite small, and not routinely tested. Therefore, it
is recommended that sufficient testing of samples be performed to verify a safe isolation voltage for the application.

Conclusion
The example discussed is a best-case simple two-wirecoil, yet it illustrates many of the variables to consider
when using our products in connection configurations forwhich they were not designed. More complicated wiringconfigurations could require additional considerations
that are not discussed here.

Document 857-2

Revised 07/19/23