UCC1973 datasheet(8/26 Pages) TI2

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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:

()

watts

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 0.16

W. 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:

RES

PRIMARY

RES

820

600

(6)

Buck inductor current is calculated in the next section.

Secondary current is simply the lamp current, the second-

ary winding has 176

W 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 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 1k

W resistor was se-

lected in this application. Losses scale with buck voltage

as:

BLOSS

BUCK

() =

(7)

APPLICATION INFORMATION (cont.)

Manufacturer

Capacitance Type

Series

Dissipation Factor

(1kHz)

Ballast Capacitor

Cera-Mite (414) 377-3500

High Voltage Disk Capacitor (3kV)

564C

NOVA-CAP (805) 295-5920

SMT 1808 (3kV)

COG

Murata Electronics

SMT 1808 (3kV)

GHM

Resonant Capacitor

Wima (914)347-2474

Polypropylene foil film FKP02

FKP02

0.0003

Metalized Polypropylene

MKP2

0.0005

SMT Metalized polyphenylene-sulfide

MKI

0.0015

Paccom (800)426-6254

SMT Metalized polyphenylene-sulfide

CHE

0.0006

NOVA-CAP

SMT Ceramic

COG

0.001

Table 2. Capacitor selection

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