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UCC3972PWG4 Datasheet(PDF) 8 Page - Texas Instruments |
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UCC3972PWG4 Datasheet(HTML) 8 Page - Texas Instruments |
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8 / 24 page  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 0.16 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: 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 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: R V R BLOSS BUCK B () = 2 (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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