7/30/2019 Eeiol 2006apr24 Pow Ta 01
1/2
By Steve Chyo
Senior Design EngineerVithi Singh
Product Manager
Tyco Electronics Magnetics
The low cost, simplicity o design
and intrinsic eciency o yback
transormers have made them a
popular solution or power sup-
ply designs o below 100W to
150W. Other advantages o the
yback transormer over circuits
with similar topology include iso-lation between primary and
sec-
ondary and the ability to provide
multiple outputs and a choice o
positive or negative voltage orthe output.
This article discusses design
parameters or the yback or
swinging choke type o trans-
ormer used in yback convert-
ers. The latter has been used or
many years and its topology is
unique within the transormer-
isolated amily o regulators.
Flyback transormer unction
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 transormer and the
diode are arranged such that
there is no energy transerred to
the load when the switch is on.
When the switch is turned o,
the polarity o the transormerwinding reverses due to the
collapsing magnetic feld, the
output rectifer conducts and theenergy stored in the core
mate-
rial is transerred to the load. This
activity continues until the core is
depleted o energy or the power
switch is once again turned on.
The yback regulator can
operate in either discontinu-
ous or continuous mode. In the
discontinuous mode (see Figure
2), the energy stored in the core
when the FET is on/o is com-
pletely emptied rom the coreduring the yback period. In the
continuous mode, (see Figure 3)the FET is turned on beore
the
core empties o yback energy.
A typical yback transormer
may operate in both modes de-
pending on the load and input
voltage.
Designers should considerthe maximum load at low volt-
age, including all conditions
within the operating range o
the yback, as it will simply shut
down (discontinuous mode)
between cycles and wait or the
load demand to catch up withthe power-delivery capability.
This is one o the most dynamic
characteristics o the yback,
regulated over a wide range o
input voltage and load.
Figure 1. Typical yback transormer circuit
Flyback transformer tutorial:function and design
Flyback transFormer
Figure 2. Flyback transormer in discontinuous mode
EE Times-India | April 2006 | eetindia.com
http://www.eetindia.co.in/http://www.eetindia.co.in/
7/30/2019 Eeiol 2006apr24 Pow Ta 01
2/2
Design parameters or a
fyback transormer
The ollowing equations are re-
quently used to speciy a yback
transormer. They are ollowed by
a typical design example.
Let V= L dj/dt, and Vin,min =
( L p I p p ) / ( m a x )Where Vin = input voltage, V
Lp = primary inductance, mH
Ipp = peak current, A
max = maximum duty cycle, s = operating switching requen-
cy, kHz
For discontinuous mode,
Power out = *Lp*Ipp*
Ipp = (2Pout)/(Vin,min* max)
In the yback transormer,as mentioned above, regula-
tion is accomplished by PWM.
I the transormer Vin varies
rom Vin,min to Vin,max, thenmin = (max) /
((1- max)C + max)
where C = Vin,max / Vin,min
Since Ipp is known,
Lp = (Vin,min* max) / (Ipp*)
Although designers may rely
on experience or core selec-tion, that can only result in an
approximation. The ollowing
ormula is recommended or abetter estimate.
Ac*Ae = (((6.33*4)*L
p*Ipp*D)*108) / Bmax.
Where Ac = winding area, cm
Ae = core eective area, cm
Bmax = Bsat/2, Gauss. Consult
core manuacturers or ma-
terial and loss vs. requencyD = diameter o wire, inch
Air gap must be calculatedor the yback transormer, since
it is operating single-ended and
uses only hal the ux capac-
ity. This may create potential or
driving the core into saturation.
Gap (cm) = lg =
((0.4**Lp*Ipp)*108) /
(Ae*Bmax)
Ater the air gap length is
determined, the primary andsecondary number o turns can
be ound.
Npri = (Bmax*lg) / (.4**Ipp)
Nsec = (Np(Vp + Vd) (1-max)) /
(Vin,min* max)
The ollowing example dem-
onstrates the design o a yback
transormer in the discontinuous
mode. Modern designs utilizePFC (power actor correction),
positioned immediately ater
rectifcation. Boost topology is
requently used or its dynamic
characteristic and wide range
o input voltage. PFC will not be
covered in this example.
Design parameters
Vinput = 85 to 132 VAC
Voutput = 5VDC @ 10A = 50Watts
Frequency = 100kHzAssume max = .45
Discontinuous Mode
1) Calculate the peak Ipp
Since Vin,min = 85VAC,
then Vin,min = 85*1.4-
20V or ripple and diode
drop gives about 100VDC.
Thus, Ipp = 2Pout / (Vin,min*max) = 100 / (100*.45) =
2.22 A
2) Calculate the min Vin,max
= 132VAC*1.4 = 185VDC
Allowing a 10% margin,
Vin,max = 203VDC, say 200VDC
Allowing a 10% mar-
gin or Vin,min = 90VDC
This gives us an input volt-
age ratio C = 200/90 = 2.22
Thereore min = .45 /((1-.45)*2.22+ .45) = .27
As is evident rom these
results, the transormer willoperate over the duty ratio o
0.27 to 0.45 or the Vin range
o 200VDC to 90VDC.
3) Calculate primary inductance
Lp = 90*.45 / (2.22*100kHz) =
.18mH
4) Select core
In this example we will use
a current density o about
300 cm/A. Since Ipp =
2.22A, a total centimetres
will be 300*2.22 = 666 cm.From wire chart, 22 AWG
has diameter o 0.028 inch.
We chose Magnetics, Inc
material type P and rom
their catalogue selected
Bmax = 500 Gauss. This will
give us about 100mW/cm.
T h e r e o r e ,
AcAe = (6.33*4)*( .00018Hy)*2 .22*( .028)*
(108) / (500) = 1.59 cm4.
From the catalogue PQ43230(PQ3230) size has AcAe =
1.60 cm4.
5) Calculate the air gap length
Lg = (.4**.00018*(2.22)*108)
/ (1.37*(500)) = .30 cm (ap-
prox) at centre leg o the
core.
6) Calculate primary and sec-
ondary number o turns
Npri = 500*.30 /(.4*3.14*2.22) = 54 turns
Nsec = 54*(5 + 1) (1 - .45)
/ (90*.45) = 4.4 turns.
We will use 5 turns because
there will be losses rom
winding, PCB and other para-
sitic losses which we did not
include. Next select wire or
the output. For 10A, a wire o
10*300 = 3000cm, we choose
16AWG or the secondary. To
minimize copper losses due
to skin eect, we propose
using multiple strands othinner wires (4 strands o 22
AWG is equivalent o single
16 AWG).
Design engineers must also
check or bobbin fll actor and
temperature raise calculations,
as implementation o saety re-
quirements will increase the sizeo the yback transormer.
Custom fyback transormers
CoEv Magnetics oers a wide
range o SMPS transormers
to meet customer needs and
requirements. Designed to opti-
mize size, cost and perormance
or specifc applications, custom
transormer packages are de-
veloped using a wide range o
parameters, including:
- Turns ratio
- Current Handling Capability- Drive Levels
- Inductance
- Leakage inductance
- Sel Resonant Frequency
- DC Resistance
- Mounting Confguration
- Isolation Voltage
Figure 3. Flyback transormer in continuous mode
eetindia.com | April 2006 | EE Times-India
http://www.eetindia.co.in/http://www.eetindia.co.in/
