UCC1972/3
UCC2972/3
UCC3972/3
APPLICATION INFORMATION (cont.)
the operating frequencies of a particular design are half cycles (i.e. only ½ of the primary winding sees the
within the synchronizable frequencies of the controller.
buck current depending upon which transistor is on). Maxi-
mum resonant current is equal to:
Component Selection for the Resonant Tank and O- ut
put Circuit
VPRIMARY
LPRIMARY
CRES
820
44
(6)
IRES
=
=
= 600mA
Since high efficiency is a primary goal of the backlight
converter design, the selection of each component
must be carefully evaluated. Losses in the ballast ca-
pacitor are usually insignificant, however, its value de-
termines the tank voltage which influences the losses in
the resonant capacitor and transformer. Since the reso-
nant capacitor has high circulating currents, a capacitor
with low dissipation factor should be selected. Power
loss in the resonant tank capacitor will be:
67·
0.1
Buck inductor current is calculated in the next section.
Secondary current is simply the lamp current, the second-
ary winding has 176W of resistance.
Core losses are a function of core material, cross sectional
area of the core, operating frequency, and transformer
voltage. For ferrite material, the hysteresis core losses in-
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.
CRES _LOSS watts =
(5)
(
)
V
(
2 ·2p·FRESONANT ·CRES ·Dissipation Factor
)
TANK
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.
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 VCESAT 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
(hFE) of 200 and a forward drop (VCESAT) 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 RB is sized to provide full VCE saturation for
all operating conditions assuming a worst case hFE. 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:
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
V 2
(7)
BUCK
RB(LOSS )
=
RB
Table 2. Capacitor selection
Manufacturer
Ballast Capacitor
Capacitance Type
Series
Dissipation Factor
(1kHz)
Cera-Mite (414) 377-3500
NOVA-CAP (805) 295-5920
Murata Electronics
High Voltage Disk Capacitor (3kV)
SMT 1808 (3kV)
SMT 1808 (3kV)
564C
COG
GHM
Resonant Capacitor
Wima (914)347-2474
Polypropylene foil film FKP02
Metalized Polypropylene
SMT Metalized polyphenylene-sulfide
SMT Metalized polyphenylene-sulfide
SMT Ceramic
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 ]
