UCC3973PWTRG4 Datasheet, PDF (8/24 Pages) Texas Instruments – BiCMOS Cold Cathode Fluorescent Lamp Driver Controller

UCC1972/3
UCC2972/3
APPLICATION INFORMATION (cont.)
UCC3972/3
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-
Component Selection for the Resonant Tank and O- ut ^{mum resonant current is equal to:}
put Circuit
Since high efficiency is a primary goal of the backlight
I _RES =
V_PRIMARY
L_PRIMARY
= ⁸²⁰ = 600mA
67· ⁴⁴
(6)
converter design, the selection of each component
C _RES
0.1
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
with low dissipation factor should be selected. Power
loss in the resonant tank capacitor will be:
C_{RES _LOSS (}watts₎ =
(5)
(^V_TANK )^{2 · 2p · F}_RESONANT ^·C_RES ^{· Dissipation Factor}
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-
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.
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
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
have the highest possible value. A 1kW resistor was se-
lected in this application. Losses scale with buck voltage
as:
current of the primary winding includes the sinusoidal
resonant current and the DC buck current on alternate
R_{B(LOSS )}
=
V
2
BUCK
R_B
(7)
Manufacturer
Ballast Capacitor
Cera-Mite [phone]
NOVA-CAP [phone]
Murata Electronics
Resonant Capacitor
Wima [phone]
Paccom [phone]
NOVA-CAP
Table 2. Capacitor selection
Capacitance Type
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
Series
564C
COG
GHM
FKP02
MKP2
MKI
CHE
COG
Dissipation Factor
(1kHz)
0.0003
0.0005
0.0015
0.0006
0.001
8