APPLICATION INFORMATION (cont.)
the operating frequencies of a particular design are
within the synchronizable frequencies of the controller.
UCC1972/3
UCC3972/3
half cycles (i.e. only �½ of the primary winding sees the
buck current depending upon which transistor is on). Maxi-
Component Selection for the Resonant Tank and Out
mum resonant current is equal to:
-
V
820
put Circuit
(6)
I
RES
=
PRIMARY
=
=
600
mA
L
PRIMARY
44
Since high efficiency is a primary goal of the backlight
67
·
0. 1
C
RES
converter design, the selection of each component
must be carefully evaluated. Losses in the ballast ca-
pacitor are usually insignificant, however, its value de- Buck inductor current is calculated in the next section.
termines the tank voltage which influences the losses in Secondary current is simply the lamp current, the second-
the resonant capacitor and transformer. Since the reso- ary winding has 176W of resistance.
nant capacitor has high circulating currents, a capacitor Core losses are a function of core material, cross sectional
with low dissipation factor should be selected. Power area of the core, operating frequency, and transformer
loss in the resonant tank capacitor will be:
voltage. For ferrite material, the hysteresis core losses in-
C
RES
_
LOSS
(
watts
)
=
2
(5)
(
V
TANK
)
·
2p
·
F
RESONANT
·
C
RES
·
Dissipation Factor
Polypropylene foil film capacitors give the lowest loss;
metalized polypropylene or even NPO ceramic may
give acceptable performance in a lower cost surface
mount (SMT) package. Table 2 gives possible choices
for the resonant and high voltage ballast capacitors.
The transformer is physically the largest component in
the converter, making the tradeoff of transformer size
and efficiency a critical choice. The transformer’s effi-
ciency will be determined by a combination of wire and
core losses. A Coiltronics transformer (CTX110600)
was chosen for this application because of its small
size, low profile, and overall losses of about 5% at 1W.
Low profile CCFL transformers are also available from
Toko (847)-297-0070 in Mt. Prospect, IL or Sumida
(408)-982-9660 in Santa Clara, CA.
Wire losses are determined by the RMS current and
the ESR of the windings. The primary winding resis-
tance for the Coiltronics transformer is 0.16W. The RMS
current of the primary winding includes the sinusoidal
resonant current and the DC buck current on alternate
crease with voltage by a cubed factor; for a given core
cross sectional area, doubling the tank voltage will cause
the losses to increase by a factor of 8. This makes the se-
lection of the ballast capacitor a critical decision for effi-
ciency.
Other elements influencing the resonant tank and output
circuit efficiency include the push-pull transistors, the base
drive and sense resistors, as well as the lamp. High gain
low V
CESAT
bipolar transistor such as Zetek’s FZT849 al-
low high efficiency operation of the push-pull stage. These
SOT223 package parts have a typical current transfer ratio
(h
FE
) of 200 and a forward drop (V
CESAT
) of just 35mV at
500mA. Rohm’s 2SC5001 transistors provide similar per-
formance. For low power, size sensitive applications, a
SOT23 transistor is available from Zetek (FFMT619) with
approximately twice the forward drop at 500mA. The base
drive resistor R
B
is sized to provide full V
CE
saturation for
all operating conditions assuming a worst case h
FE
. For ef-
ficiency reasons, the base resistor should be selected to
have the highest possible value. A 1kW resistor was se-
lected in this application. Losses scale with buck voltage
as:
R
B
(
LOSS
)
V
2
BUCK
=
R
B
(7)
Table 2. Capacitor selection
Manufacturer
Ballast Capacitor
Cera-Mite (414) 377-3500
NOVA-CAP (805) 295-5920
Murata Electronics
Resonant Capacitor
Wima (914)347-2474
Capacitance Type
Series
Dissipation Factor
(1kHz)
High Voltage Disk Capacitor (3kV)
SMT 1808 (3kV)
SMT 1808 (3kV)
Polypropylene foil film FKP02
Metalized Polypropylene
SMT Metalized polyphenylene-sulfide
SMT Metalized polyphenylene-sulfide
SMT Ceramic
564C
COG
GHM
FKP02
MKP2
MKI
CHE
COG
0.0003
0.0005
0.0015
0.0006
0.001
Paccom (800)426-6254
NOVA-CAP
8
TI [ TEXAS INSTRUMENTS ]
