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参数资料
| 型号： | TPS43000PWRG4 |
| 厂商： | TEXAS INSTRUMENTS INC |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, 2000 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封装： | GREEN, TSSOP-16 |
| 文件页数： | 16/31页 |
| 文件大小： | 615K |
| 代理商： | TPS43000PWRG4 |
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TPS43000
SLUS489 OCTOBER 2001
23
www.ti.com
APPLICATION INFORMATION
The SEPIC topology offers the same advantage of the flyback in that it can generate an output voltage that is
greater or less than the input voltage. However, it also offers improved efficiency. Although it requires an
additional capacitor in the power stage, it greatly reduces ripple current in the input capacitor and improves
efficiency by transferring the energy in the leakage inductance of the coupled inductor to the output. This also
provides snubbing for the primary and secondary windings, eliminating the need for RC snubbers. Note that
the capacitor must have low ESR, with sufficient ripple current rating for the application. Another advantage of
the SEPIC is that the inductors do not have to be on the same core.
theory of operation
When the energizing MOSFET (Q1) is on, VIN is applied across L1A with the dotted end negative (see Figure 8).
At the same time, the voltage across the SEPIC capacitor C9 (equal to VIN) is applied across L1B with its dotted
end also negative. Since L1A = L1B and the voltages across the two inductors are equal (VIN), the inductor
currents ramp up at the same rate (di/dt=VIN/L). The energizing MOSFET current is the sum of the two inductor
currents. When the energizing MOSFET turns off, the inductor current wants to continue to flow causing the
inductor voltages to reverse until the output rectifer begins to conduct. The voltage across L1B is clamped to
VOUT (plus the rectifier drop) and with its dotted end positive. The voltage across the SEPIC capacitor (VIN)
cancels with VIN and the voltage across L1A is also VOUT with its dotted end positive. Again, since L1A=L1B
and the voltage across the two inductors are equal (VOUT), the inductor current ramps down at the same rate
(di/dt=VOUT/L). The SEPIC capacitor is charged by L1A when the energizing MOSFET is off and is discharged
by L1B when the energizing MOSFET is on. Since the voltages across the inductors are identical at all times
throughout the switcing cycle, the inductors can be coupled on a single magnetic core with an equal number
of turns. This improves the SEPIC application’s dynamic performance and also allows reduction of the input
filtering requirements through ripple steering.
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 equations 17, 19, and 21 to calculate the peak
inductor current for a buck, boost, flyback, or SEPIC design, based on VIN, VOUT, duty cycle, maximum load,
frequency, and inductor value. Some manufacturers rate their parts for maximum energy storage in microjoules
(
J). This is expressed by:
E
+ 0.5
L
I
pk
2
where E is the required energy rating in microjoules, L is the inductor value in microhenries (
H) (with current
applied), and Ipk 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:
ET
+
V
IN
D
f
SW
where ET is the required rating in V-
s, D is the duty cycle for a given VIN and VOUT, and fSW is the switching
frequency in MHz. Refer to equations 18, 20, and 22 to calculate the duty cycle for a buck, boost, flyback, or
SEPIC 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 1. For
flyback and SEPIC topologies, use a two-winding coupled inductor. SEPIC designs can also use two discrete
inductors.
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