UCC28951
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ZHCSIQ7A –AUGUST 2018 –REVISED DECEMBER 2021
The standard resistor value selected for RT is 61.9 kΩ.
The UCC28951 provides slope compensation. The amount of slope compensation is set by the resistor RSUM
.
As suggested earlier, set the slope compensation ramp to be half the inductor current ramp downslope (inductor
current ramp during the off time), reflected through the main transformer and current sensing networks as
explained earlier in 节7.3.11.
Calculate required slope compensation ramp:
VOUT ìRCS
LOUT ì a1ì CTRAT
12 ì 47
mV
me = 0.5ì
= 0.5 ì
= 67
2 ì10-6 ì 21ì100
ms
(143)
The magnetizing current of the power transformer provides part of the slope compensation ramp. The slope of
this current is calculated using 方程式 144 where VINHU is the minimum voltage for VOUT holdup purposes. It is
the voltage at which the converter is operating at the maximum dudy cycle (DMAX) while maintaining VOUT
:
VINHU ´ RCS
260 ´ 47
mV
mMAG
=
=
» 44
2.76x10-3 ´100
LMAG ´ CTRAT
ms
(144)
Calculate the required compensating ramp:
mV
mV
mSUM = me - mMAG = (67 - 44)
= 23
ms
ms
(145)
The value for the resistor, RSUM, may be found from the graph in 图7-10, calculated from rearranged versions of
方程式 13, or calculated by 方程式 13, depending on whether the controller is operating in current mode or
voltage control mode. This design uses current mode control and 方程式146 is rearranged and evaluated:
2.5
2.5
RSUM
=
=
» 200kW
0.5 ´ 23x10-3
0.5 ´ mSUM
(146)
Confirm that the 300 mV allowed for the slope compensation ramp is sufficient when choosing RCS in 方程式
100.
mV
23
´ 0.7
mSUM ´ DMAX
2 ´ FSW
ms
DVSLOPE -COMP
=
=
= 80mV
2 ´100kHz
(147)
To increase efficiency at lighter loads the UCC28951 is programmed (Pin 12, DCM) under light-load conditions to
disable the synchronous FETs on the secondary side of the converter (QE and QF). This threshold is
programmed with resistor divider formed by RDCMHI and RDCM. This DCM threshold needs to be set at a level
before the inductor current goes discontinuous. 方程式 148 sets the level at which the synchronous rectifiers are
disabled at roughly 15% load current.
æ
ç
è
ö
÷
ø
POUT ´0.15 DILOUT
+
´R
CS
VOUT
2
VRCS
=
= 0.29V
a1´CTRAT
(148)
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67
Product Folder Links: UCC28951
English Data Sheet: SLUSDB2
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