Application Note AN-1024
Flyback Transformer Design for the IRIS40xx Series
Table of Contents
Page
1. Introduction to Flyback Transformer Design ............................................... 1
2. Power Supply Design Criteria Required ....................................................... 2
3. Transformer Design Process ......................................................................... 2
4) Transformer Construction ............................................................................. 9
4.1) Transformer Materials.................................................................................. 10
4.2) Winding Styles ............................................................................................. 12
4.3) Winding Order.............................................................................................. 12
4.4) Multiple Outputs........................................................................................... 12
4.5) Leakage Inductance .................................................................................... 13
5) Transformer Core Types .............................................................................. 14
6) Wire Table ..................................................................................................... 16
7) References .................................................................................................... 17
8) Transformer Component Sources............................................................... 17
One of the most important factors in the design of a flyback converter power supply is
the design of the transformer. The main advantages of the flyback circuit are cost,
simplicity and the ease of adding multiple outputs. Flyback topologies are practical and
lowest cost for systems up to 100W. Flyback transformer design is a somewhat iterative
process, due to the number of variables involved, but it is not difficult, and with a little
experience can become a quick and simple process.
A
PPLICATION
N
OTE
By Jonathan Adams
AN-1024a
International Rectifier
•
233 Kansas Street El Segundo CA 90245 USA
Flyback Transformer Design For The IRIS40xx Series
TOPICS COVERED
Introduction To Flyback Transformer Design
Power Supply Design Criteria Required
Transformer Design Process
Transformer Construction
Core Types
Wire Table
References
Transformer Component Sources
1) INTRODUCTION TO FLYBACK TRANSFORMER DESIGN
One of the most important factors in the design of a flyback converter power supply is the design of the
transformer. Although we call it a transformer it is not actually a true transformer, but more an energy storage
device, where during the period of time when the primary switch is on energy is stored in the air gap of the
core, and during the off time of the primary switch, this energy is transferred to the outputs. Current flows in
either the primary or secondary winding, but not both at the same time. Therefore it can be thought of more as
an inductor with secondary windings added.
The main advantages of the flyback circuit are cost, simplicity and the ease of adding multiple outputs.
Flyback topologies are practical and lowest cost for systems up to power levels of 100W. Above this power
level other methods such as forward converters become more cost effective, due to reduced voltage and
current stresses on the devices.
Flyback transformer design is a somewhat iterative process, due to the number of variables involved, but it
is not difficult, and with a little experience can become a quick and simple process. Before starting the trans-
former design it is important to define the power supply parameters such as input voltage, power output,
minimum operating frequency, and maximum duty cycle. From there we can calculate the transformer param-
eters, and select an appropriate core. Iterations may be needed if the calculated parameters do not fall within
design guidelines. An Excel spreadsheet is available on the website to simplify the process.
The IRIS40xx series of Integrated Switchers are designed primarily to be used in the quasi-resonant mode
which means that the transformer will be operating in a discontinuous mode ( The magnetic field is not continu-
ous, it will return to zero when all energy in the transformer is transferred to the secondary side). In PRC mode
the transformer will also generally be operating in a discontinuous mode, unless the minimum operating fre-
quency is set very low ( about 20kHz which would not generally be practical as this would require a larger core
size). So this application note will cover the case for a discontinuous design only.
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AN1024a
2) POWER SUPPLY DESIGN CRITERIA REQUIRED
In order to start the design of the transformer some parameters must be defined from the power supply
specification. These are:-
1) Minimum operating frequency - f
min
2) Estimated power supply efficiency -
η
≈
0.85~0.9 (High Vout), 0.75~0.85 (Low Vout)
3) Minimum DC bus voltage - V
min
(e.g.110V for 85Vac minimum input assuming 10V ripple)
4) Maximum duty cycle - D
m
(recommended maximum is 0.5)
5) Value of series resonant capacitor - C
res
(recommended range is 100pf~1.5nF seen below in Fig1)
Vin
(AC/
DC)
Vout
(DC)
3
Drain
Vcc
4
IRIS4011(K)
FB
Source
Gnd
5
1
2
Fig 1) Typical Flyback Power Supply Circuit Using the IRIS40xx Series
3) TRANSFORMER DESIGN PROCESS
The starting point for the design is to calculate the total output power, which is calculated from all the
secondary outputs and the bias output including the voltage drop across the output diodes. It is common to
use a schottky diode for the main outputs, if the output current is above 1A, or a fast recovery diode if the
output is less than 1A, and the bias winding can usually use a simple 1n4148 for the rectifier as this is only a
low current output (recommended voltage for the bias is 18V and current is 30mA).
2
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AN1024a
P
O
= ((VO
1
+VD
1
)x IO
1
)+.....((VO
n
+VD
1
)xIO
1
) + ((VB+VD
B
)xI
B
)
The output power (P
O
) calculated is the total output power.
With this information the primary inductance of the transformer (Lp) can now be calculated from the follow-
ing equation.
L
P
=
(
(
V
min
×
D
m
)
2
2
×
Po
×
f
η
min
+
V
min
×
π
×
f
min
×
D
m
×
C
res
)
2
The next step is to calculate the required turns ratio for the primary, all secondaries and the bias winding.
The following equation will allow you to calculate the primary (N
P
) and secondary (N
S
) turns.
N
P
=
N
S
×
V
min
D
m
×
V
O
+
V
D
1
−
D
m
Where V
O
is the output voltage of the secondary, and V
D
is the forward voltage drop of the output rectifier for
this secondary. A good starting point is to work on the basis of 1V/turn for the secondary, and calculate the
number of primary turns from there. The number of turns for the bias winding N
B
is calculated from the follow-
ing:
N
B
=
N
S
×
V
B
V
O
+
V
D
In a power supply with multiple outputs a number of iterations may be needed to find an optimum turns
ratio, and some compromise may be needed on the output voltages to ensure the turns are integer values and
there are no 1/2 turns.
Now the effective inductance for the gapped core can be calculated. It may be possible to acquire gapped
cores with the required A
LG
value from a core vendor, or a standard core is used and then gapped in the centre
leg by grinding to achieve the required A
LG
. It is calculated from the Primary inductance L
P
(in
µH)
and the
number of primary turns (N
P
) in the following equation.
A
LG
=
1000
×
L
P
N
P
2
nH/turn
2
The average primary current (I
AV
) can be calculated from the efficiency estimate (η), the required total
output power (P
O
) and the minimum DC bus voltage (V
min
).
I
AV
=
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P
O
η
×
V
min
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The peak primary current (I
P
) is now needed and can be calculated from:
I
P
=
I
AV
×
2
D
m
Fig 2) to the left shows the primary current waveform for the discontinuous mode. It shows that during the
on time of the switch (t1) there is a current ramp with the rate controlled by the DC bus voltage and the primary
inductance (L
P
), ending at a peak cur-
rent value I
P
, which we have just calcu-
I
lated. During the off time (t2) there is
no primary current flowing. The peak
Ip
flux will occur at the point where I = I
P
.
Due to the quasi-resonant nature of the
IRIS40xx circuit, t1 and t2 will change
depending on the output load and the
input voltage. For calculation purposes
t1
t2
we use the case of lowest frequency,
lowest DC bus voltage, and maximum
t
load as the worst case for the trans-
Fig 2) Primary Current Waveform for a
former, and therefore the design crite-
Discontinuous Flyback Circuit
ria.
RMS primary current (I
rms
) is needed to be able to calculate the required wires size and is calculated from
the following:
I
rms
=
I
P
×
D
m
3
The next step is to calculate the required core size and air gap. First select a core size, you can use
Section 5 which gives an indication of cores types and sizes which could be used and their appropriate
wattage levels. Use the following equation to calculate the maximum flux density B
m
using the effective cross-
sectional area A
e
(in cm
2
) for the core selected (B
m
should be in the range of 2000 to 3000 gauss - below 2000
the core would be underused, and above 3000 there may be a possibility of saturation depending on the ferrite
material used).
B
m
=
N
P
×
I
P
×
A
LG
10
×
A
e
An alternative is to start with a value for B
m
(e.g. 2500) and calculate the minimum A
e
needed for the core as
below.
A
e
=
N
P
×
I
P
×
A
LG
10
×
B
m
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