Datasheet

UCC28700, UCC28701
UCC28702, UCC28703
www.ti.com SLUSB41 – JULY 2012
Transformer Parameter Verification
The transformer turns ratio selected affects the MOSFET V
DS
and secondary rectifier reverse voltage so these
should be reviewed. The UCC28700 controller requires a minimum on time of the MOSFET (T
ON
) and minimum
D
MAG
time (T
DMAG
) of the secondary rectifier in the high line, minimum load condition. The selection of F
MAX
, L
P
and R
CS
affects the minimum T
ON
and T
DMAG
.
The secondary rectifier and MOSFET voltage stress can be determined by the equations below.
(19)
For the MOSFET V
DS
voltage stress, an estimated leakage inductance voltage spike (V
LK
) needs to be included.
(20)
The following equations are used to determine if the minimum T
ON
target of 300 ns and minimum T
DMAG
target of
1.1 µs is achieved.
(21)
(22)
Output Capacitance
The output capacitance value is typically determined by the transient response requirement from no-load. For
example, in some USB charger applications there is a requirement to maintain a minimum V
O
of 4.1 V with a
load-step transient of 0 mA to 500 mA . The equation below assumes that the switching frequency can be at the
UCC28700 minimum of f
SW(min)
.
(23)
Another consideration of the output capacitor(s) is the ripple voltage requirement which is reviewed based on
secondary peak current and ESR. A margin of 20% is added to the capacitor ESR requirement in the equation
below.
(24)
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