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TYCO ELECTRONICS MEDIA DIGEST



TYCO ELECTRONICS MEDIA DIGEST

A series of technology briefs spanning Tyco Electronics product portfolio


Brief #12 March, 2006

Technology Focus:
Flyback Transformer: Function
and Design



Editors note: This series of technology briefs
was created to acquaint you with Tyco
Electronics current capabilities and new
developments in electronic components and
systems. We welcome your comments and
questions, and are ready to assist should you
wish to explore a topic in more detail.




The low cost, simplicity of design and intrinsic efficiency of flyback transformers have
made them a popular solution for power supply designs of below 100W to 150W. Other
advantages of the flyback transformer over circuits with similar topology include isolation
between primary and secondary and the ability to provide multiple outputs and a choice
of positive or negative voltage for the output.

This article discusses design parameters for the flyback or winging choke type of
transformer used in flyback converters. The latter has been used for many years and its
topology is unique within the transformer-isolated family of regulators.


Flyback transformer function

When the switch is turned on, energy is stored in the primary (within the core material).
As shown in Figure 1, the polarity dots on the transformer and the diode are arranged
such that there is no energy transferred to the load when the switch is on. When the
switch is turned off, the polarity of the transformer winding reverses due to the collapsing
magnetic field, the output rectifier conducts and the energy stored in the core material is
transferred to the load. This activity continues until the core is depleted of energy or the
power switch is once again turned on.








Figure 1. Typical flyback transformer circuit.

The flyback regulator can operate in either discontinuous or continuous mode. In the
discontinuous mode

(see Figure 2), the energy stored in the core when the FET is
on/off is completely emptied from the core during the flyback period. In the continuous
mode, (see Figure 3) the FET is turned on before the core empties of flyback energy. A
typical flyback transformer may operate in both modes depending on the load and input
voltage.

Designers should consider the maximum load at low voltage, including all conditions
within the operating range of the flyback, as it will simply shut down (discontinuous
mode) between cycles and wait for the load demand to catch up with the power-delivery
capability. This is one of the most dynamic characteristics of the flyback, regulated over
a wide range of input voltage and load.



Figure 2. Flyback transformer in discontinuous mode.



Figure 3. Flyback transformer in continuous mode.


Design parameters for a flyback transformer

The following equations are frequently used to specify a flyback transformer. They are
followed by a typical design example.

Let V= L d
i
/d
t
, and V
in,min
= (L
p
I
pp
f)/(
max
)
Where V
in
= input voltage, V

L
p
= primary inductance, mH

I
pp
= peak current, A
max
= maximum duty cycle, us

f = operating switching frequency, Khz

For discontinuous mode,

Power out = �*L
p
*I�
pp
*f

I
pp
= (2P
out
)/(V
in
,
min
*
max
)

In the flyback transformer, as mentioned above, regulation is accomplished by PWM. If
the transformer V
in
varies from V
in
,
min
to V
in
,
max,
then

min = (max) / ((1- max)C + max)
where C = V
in
,
max
/ V
in
,
min

Since I
pp
is known,
L
p
= (V
in
,
min
*
max
) / (I
pp
*f)

Although designers may rely on experience for core selection, that can only result in an
approximation. The following formula is recommended for a better estimate.

A
c
*A
e
= (((6.33*4)*L
p
*I
pp
*D�)*10^8) / B
max
.
Where A
c

= winding area, cm�
A
e

= core effective area, cm�
B
max
= B
sat
/2, Gauss. Consult core manufacturers for material and loss vs.
frequency
D
= diameter of wire, inch

Air gap must be calculated for the flyback transformer, since it is operating single-ended
and uses only half the flux capacity. This may create potential for driving the core into
saturation.


Gap (cm) = l
g
= ((0.4**L
p
*I�
pp
)*10^8) / (A
e
*B�
max
)

After the air gap length is determined, the primary and secondary number of turns can
be found.


N
pri
= (B
max
*l
g
) / (.4**I
pp
)

N
sec
= (N
p
(V
p
+ V
d
) (1-
max
)) / (V
in
,
mi
n*
max
)
The following example demonstrates the design of a flyback transformer in the
discontinuous mode. Modern designs utilize PFC (power factor correction), positioned
immediately after rectification. Boost topology is frequently used for its dynamic
characteristic and wide range of input voltage. PFC will not be covered in this example.


Design Parameters

V
input
= 85 to 132 V
ac

V
output
= 5Vdc @ 10A = 50Watts
Frequency = 100Khz
Assume
max
= .45

Discontinuous Mode

1) Calculate the peak I
pp

Since V
in
,
min
= 85V
ac
, then V
in
,
min
= 85*1.4-20V for ripple and diode drop gives
about 100V
dc
.
Thus, I
pp
= 2P
out
/ (V
in
,
min
*
max
) = 100 / (100*.45) = 2.22 A

2) Calculate the
min

V
in
,
max
= 132Vac*1.4 = 185Vdc
Allowing a 10% margin, V
in
,
max
= 203Vdc, say 200Vdc
Allowing a 10% margin for V
in
,
min
= 90Vdc
This gives us an input voltage ratio C = 200/90 = 2.22
Therefore
min
= .45 / ((1-.45)*2.22+ .45) = .27
As is evident from these results, the transformer will operate over the duty ratio of
0.27 to 0.45 for the V
in
range of 200Vdc to 90Vdc.

3) Calculate primary inductance
L
p
= 90*.45 / (2.22*100Khz) = .18mH

4) Select core
In this example we will use a current density of about 300 c.m/A. Since I
pp
=
2.22A, a total c.m will be 300*2.22 = 666 c.m. From wire chart, 22 AWG has
diameter of .028 inch. We chose Magnetics, Inc material type P and from their
catalog selected B
max
= 500 Gauss. This will give us about 100mW/cm�.
Therefore,
A
c
A
e
= (6.33*4) *(.00018Hy)*2.22*(.028)�*(10^8) / (500) = 1.59 cm^4.
From the catalog PQ43230 (PQ3230) size has A
c
A
e
= 1.60 cm^4.

5) Calculate the air gap length
l
g
= (.4**.00018*(2.22)�*10^8) / (1.37*(500)�) = .30 cm (approx) at center leg of
the core.

6) Calculate primary and secondary number of turns N
pri
= 500*.30 / (.4*3.14*2.22) = 54 turns
N
sec
= 54*(5 + 1) (1 - .45) / (90*.45) = 4.4 turns.
We will use 5 turns because there will be losses from winding, PCB and other
parasitic losses which we did not include. Next select wire for the output. For 10A, a
wire of 10*300 = 3000cm, we choose 16AWG for the secondary. To minimize
copper losses due to skin effect, we propose using multiple strands of thinner wires (4
strands of 22 AWG is equivalent of single 16 AWG).

Design engineers must also check for bobbin fill factor and temperature rise calculations,
as implementation of safety requirements will increase the size of the flyback
transformer.


Sidebar: Custom flyback transformers

CoEv Magnetics offers a wide range of SMPS transformers to meet customer needs and
requirements. Designed to optimize size, cost and performance for specific applications,
custom transformer packages are developed using a wide range of parameters,
including:
- Turns ratio


- Current Handling Capability
-
Drive
Levels
-
Inductance
- Leakage inductance

- Self Resonant Frequency
- DC Resistance


- Mounting Configuration
-
Isolation
Voltage



About the Authors:
Steve Chyo is Senior Design Engineer, and Vithi Singh is product manager for the CoEv
line of products at Tyco Electronics Magnetics.