UCC29421 Datasheet, PDF (29/35 Pages) Texas Instruments – Multimode High Frequency PWM Controller

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UCC29421, UCC29422, UCC39421, UCC39422
MULTIMODE HIGHĆFREQUENCY PWM CONTROLLER
SLUS246C − OCTOBER 1999 − REVISED FEBRUARY 2005
APPLICATION INFORMATION
selecting the inductor
The inductor must be chosen based on the desired operating frequency and the maximum load current. Higher
frequencies allow the use of lower inductor values, reducing component size. Higher load currents require larger
inductors with higher current ratings and less winding resistance to minimize losses. The inductor must be rated
for operation at the highest anticipated peak current. Refer to equation (8) and equation (12) to calculate the
peak inductor current for a boost or flyback design, based on V_IN, V_OUT, maximum load, frequency, and inductor
value. Some manufacturers rate their parts for maximum energy storage in microjoules (µJ). This is expressed
by:
E + 0.5 L IPEAK2
(21)
where E is the required energy rating in microjoules. L is the inductor value in microhenries (µH) (with current
applied), and I_PEAK is the peak current in amps that the inductor sees in the application. Another way in which
inductor ratings are sometimes specified is the maximum volt-seconds applied. This is given simply by:
E
T
+
VIN
f
D
(22)
where ET is the required rating in V-µs, D is the duty cycle for a given V_IN and V_OUT, and f is the switching
frequency in MHz. Refer to equations (7) and (11) to calculate the duty cycle for a CCM boost or flyback
converter.
In any case, the inductor must use a low loss core designed for high-frequency operation. High-frequency ferrite
cores are recommended. Some manufacturers of off-the-shelf surface-mount designs are listed in Table 3. For
flyback and SEPIC topologies, use a two-winding coupled inductor. SEPIC designs can also use two discrete
inductors.
Table 3. MT COMMERCIAL INDUCTOR MANUFACTURERS
Coilcraft Inc. ⋅ (800) 322−2645.1102 Silver Lake RD, Cary, IL 60013
Coiltronics Inc. ⋅ (407) 241−7876 6000 Park of Commerce Blvd, Boca Raton, FL 33487
Dale Electronics, Inc. ⋅ (605) 665−9301East Highway 50, Yankton, SD 57078
Pulse Engineering Ltd. ⋅ (204) 633−4321300 Keewatin Street, Winnipeg, MB R2X 2R9
Sumida ⋅ Voice (65) 296−3388 ⋅ Fax (65) 293−3390 Block 996, Bendemeer Rd., #04−05/06 Singapore 33944
BH Electronics ⋅ (612) 894−9590 12219 Wood Lake Drive, Burnsville, MN 55337
Tokin America Inc. ⋅ (408) 432−8020155 Nicholson Lane, San Jose CA 95134
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