Datasheet

IN
PRE BIAS
RAMP SR(ramp)
PRE BIAS
V
V
2 n
V V
2 V
-
-
-
´
= ´
´
( )
COMP( final) MAX
V SR _ D 0.5 3 V 2= - ´ ´
( )
COMP( final)
RAMP
MAX
V
V
1 SR _ D 2
=
- ´
IN(max)
CS
RAMP CS sw
V 2
R
V C f
´
=
´ ´
UCC28250
www.ti.com SLUSA29C –APRIL 2010– REVISED JULY 2011
The input voltage feedforward also helps on pre-biased start up. When doing primary-side control to pre-biased
start up, three conditions need to be considered:
Condition 1
At initial start up the primary side needs to provide enough energy to prevent output voltage dip;
Condition 2
At the end of soft start, it is required to keep the SR duty cycle change to be as small as possible. With input
voltage feedforward, the COMP pin voltage is virtually fixed for different input voltages. Therefore, before the end
of soft start, the duty cycle is the same for different input voltages. Choose the R
CS
and C
CS
following the
procedure below.
Considering initial start up, the RAMP peak voltage should be:
(23)
In this equation, VIN is the input voltage because of the feedforward any input voltage should be fine; V
PRE-BIAS
is
the highest pre-bias start-up voltage required by the system; n is the tranformer primary to secondary turns ratio
and V
SR(ramp)
is the internal SR ramp peak voltage 3 V.
Another consideration is at the end of soft start, the SR duty cycle changes from controlled by the soft start, to
complimentary to the primary-side duty cycle. The design should keep the transition as smooth as possible.
Considering this, based on the output voltage and input voltage range, as well as the transformer turns ratio,
calculate the SR duty cycle at different line voltages.
Next, based on the maximum duty cycle on the SR_D
MAX
, and the internal fixed ramp amplitude 3 V, the COMP
voltage at regulation can be chosen as:
(24)
Condition 3
Use the calculated COMP pin voltage to derive the external ramp amplitude
(25)
According to the calculated ramp voltage from Equation 23 and Equation 25 some trade off is required to pick up
the appropriate ramp voltage. Based on the selected ramp capacitor C
CS
value, choose the ramp resistor R
CS
value:
(26)
In this equation, V
IN(max)
is the maximum input voltage, f
SW
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.
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