24 OCTOBER 2017
winding. Using Ampere’s Law, the ampere turns across
the interwinding region is the same as the ampere turns in
either winding. Therefore, the flyback transformer’s winding
width was maximized in order to reduce the field strength in
Also, the interwinding region volume needed to be as
small as possible to reduce leakage. Leakage inductance
was further reduced by increasing the winding width, which
was accomplished by interleaving the layers. This design
had four turns on the secondary implemented using a
single layer insulated wire, so it can eliminate creepage and
clearance space requirements between the primary and
secondary windings. Doubling the winding breadth of the
primary by interleaving it with secondary effectively cut the
leakage inductance value in half.
The interwinding volume is better controlled if the
secondary is also implemented in PCB material. However,
on the downside, interwinding capacitance is adversely
affected if the distances between layers are decreased,
and is serious for high voltage applications as it worsens
the coupling of AC power line noise through to the power
The leakage inductance test is carried out by shorting the
secondary winding and measuring the primary inductance.
The leakage inductance on this custom planar flyback
transformer design was measured as 14 μH at 130 kHz.
AC Loss Control
In a flyback transformer, the AC resistance doesn’t
improve by interleaving the primary and secondary windings
as these windings are out of phase. To control the AC
At maximum load, the secondary AC current is 7. 3 A, due
to the pulsating nature of current on the secondary side that
causes copper losses of 1.5 W due to the thin diameter
(0.8 mm) of the wire. Using Finite Element Analysis (FEA)
analysis revealed that the conductor closest to the gap
experienced a hot spot as a result of the high circulating
AC and eddy currents induced by fringing effects.
It was found that reducing copper losses at high
frequencies required the insulated secondary wire to be
replaced with a helical winding. The designers also realized
that FR4-type PCB material is not considered safe at
high frequencies, so a barrier, as Mylar or polyimide tape,
needed to be bonded to the PCB substrate.
AC/DC Adaptor Application Example
The design of the custom planar flyback transformer
is based on a Discontinuous Conduction Mode (DCM)
Flyback Converter topology with valley switching and
synchronous rectification. Both the valley switching and
synchronous rectification reduce power losses in the
external MOSFET and rectifier, respectively. Operating in
DCM mode meant that there would be zero ampere turns in
the transformer for a period every switching cycle.
It was tested on an AC/DC adaptor where the operating
mode of the transformer is shown by the voltage across the
drain of the MOSFET (Figure 2).
For purposes of this example, the AC/DC power supply
operated at 115 V AC input with a 5V output. Bourns
measured the efficiency at different output powers. The
Figure 2: The planar transformer was tested in an AC/DC adaptor with the bulk
voltage across the primary plus the reflected output voltage from the secondary.