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UCC28251 Datasheet(PDF) 33 Page - Texas Instruments

Part No. UCC28251
Description  UCC28251 Advanced PWM Controller With Prebias Operation
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Maker  TI [Texas Instruments]
Homepage  http://www.ti.com
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UCC28251 Datasheet(HTML) 33 Page - Texas Instruments

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RAMP/CS
VREF
UCC28251
CS
RS
CT
1:n
Transformer Primary Side
Current
BLANK
OUTA
OUTB
IN(max)
CS
RAMP
CS
sw
V
2
R
V
C
f
´
=
´
´
(
)
COMP( final)
RAMP
MAX
V
V
1 SR _ D
2
=
-
´
UCC28251
www.ti.com
SLUSBD8E – FEBRUARY 2013 – REVISED DECEMBER 2014
Application Information (continued)
9.1.3.3 Condition 3
Use the calculated COMP pin voltage to derive the external ramp amplitude
(24)
According to the calculated ramp voltage from Equation 22 and Equation 24 some trade off is required to pick up
the appropriate ramp voltage. Based on the selected ramp capacitor CCS value, choose the ramp resistor RCS
value:
(25)
In this equation, VIN(max) is the maximum input voltage, fSW is the switching frequency.
Because these calculations ignore the dead time and the non-linearity of the ramp, slight modification is expected
to achieve the optimal design. When the input voltage feed forward is not used, refer to the RAMP pin discussion
for RC calculation.
9.1.4 Peak Current Mode Control
For peak current mode control, RAMP/CS pin is connected directly with the current signal generated from a
current transformer. The current signal must be compatible with the input range of the COMP pin. External slope
compensation is required to prevent sub-harmonic oscillation and to maintain flux-balance. The slope
compensation can be implemented by using OUTA and OUTB to charge external capacitors and use the voltage
follower to add into the sensed the current signal, as shown in Figure 42. Follow the peak current mode control
theory to select compensation slope or refer to Modeling, Analysis and Compensation of the Current-Mode
Converter (SLUA101).
Figure 42. UCC28251 Set Up for Peak Current Mode Control
Copyright © 2013–2014, Texas Instruments Incorporated
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