NCP1252
www.onsemi.com
16
LEB
CS
Rsense
S
R
Q
FB
2R
R
Buffered
Ramp
Rramp
Rcomp
Clock
Vdd
+
−
DRV
path
Ccs
Figure 39. Ramp Compensation Setup
Q
In the NCP1252, the internal ramp swings with a slope of:
S
int
+
V
ramp
DC
max
F
SW
(eq. 6)
In a forward application the secondary−side downslope
viewed on a primary side requires a projection over the sense
resistor R
sense
. Thus:
S
sense
+
(V
out
) V
f
)
L
out
N
s
N
p
R
sense
(eq. 7)
where:
• V
out
is output voltage level
• V
f
the freewheel diode forward drop
• L
out
, the secondary inductor value
• N
s
/N
p
the transformer turn ratio
• R
sense
: the sense resistor on the primary side
Assuming the selected amount of ramp compensation to
be applied is δ
comp
, then we must calculate the division ratio
to scale down S
int
accordingly:
Ratio +
R
sense
d
comp
S
int
(eq. 8)
A few line of algebra determined Rcomp:
R
comp
+ R
ramp
Ratio
1 * Ratio
(eq. 9)
The previous ramp compensation calculation does not
take into account the natural primary ramp created by the
transformer magnetizing inductance. In some case
illustrated here after the power supply does not need
additional ramp compensation due to the high level of the
natural primary ramp.
The natural primary ramp is extracted from the following
formula:
S
natural
+
V
bulk
L
mag
R
sense
(eq. 10)
Then the natural ramp compensation will be:
d
natural_comp
+
S
natural
S
sense
(eq. 11)
If the natural ramp compensation (δ
natural_comp
) is higher
than the ramp compensation needed (δ
comp
), the power
supply does not need additional ramp compensation. If not,
only the difference (δ
comp
−δ
natural_comp
) should be used to
calculate the accurate compensation value.
Thus the new division ratio is:
(eq. 12)
if d
natural_comp
t d
comp
å Ratio +
S
sense
(d
comp
* d
natural_comp
)
S
int
Then R
comp
can be calculated with the same equation used
when the natural ramp is neglected (Equation 9).
Ramp Compensation Design Example:
2 switch−Forward Power supply specification:
• Regulated output: 12 V
• L
out
= 27 mH
• V
f
= 0.7 V (drop voltage on the regulated output)
• Current sense resistor : 0.75 W
• Switching frequency : 125 kHz
• V
bulk
= 350 V, minimum input voltage at which the
power supply works.
• Duty cycle max: DC
max
= 84%
• V
ramp
= 3.5 V, Internal ramp level.
• R
ramp
= 26.5 kW, Internal pull−up resistance
• Targeted ramp compensation level: 100%
• Transformer specification:
− L
mag
= 13 mH
− N
s
/N
p
= 0.085
