Datasheet

8
UCC1972/3
UCC2972/3
UCC3972/3
the operating frequencies of a particular design are
within the synchronizable frequencies of the controller.
Component Selection for the Resonant Tank and Out
-
put Circuit
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:
()
()
C watts
V F C Dissipatio
RES LOSS
TANK RESONANT RES
_
=
·· · ·
2
2p n Factor
(5)
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
half cycles (i.e. only ½ of the primary winding sees the
buck current depending upon which transistor is on). Maxi
-
mum resonant current is equal to:
I
V
L
C
mA
RES
PRIMARY
PRIMARY
RES
==
·
=
820
67
44
01
600
.
(6)
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.
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
V
R
BLOSS
BUCK
B
()
=
2
(7)
APPLICATION INFORMATION (cont.)
Manufacturer Capacitance Type Series Dissipation Factor
(1kHz)
Ballast Capacitor
Cera-Mite [phone] High Voltage Disk Capacitor (3kV) 564C
NOVA-CAP [phone] SMT 1808 (3kV) COG
Murata Electronics SMT 1808 (3kV) GHM
Resonant Capacitor
Wima [phone] Polypropylene foil film FKP02 FKP02 0.0003
Metalized Polypropylene MKP2 0.0005
SMT Metalized polyphenylene-sulfide MKI 0.0015
Paccom [phone] SMT Metalized polyphenylene-sulfide CHE 0.0006
NOVA-CAP SMT Ceramic COG 0.001
Table 2. Capacitor selection