NCP1252
Vdd
FB
2R
R
Rcomp CS
Rsense Ccs
Rramp
Buffered
Ramp
LEB
Clock
S
Q
Q
R
DRV
path
Figure 36. Ramp Compensation Setup
In the NCP1252, the internal ramp swings with a slope of:
Sint
+
Vramp
DCmax
FSW
(eq. 6)
In a forward application the secondary−side downslope
viewed on a primary side requires a projection over the sense
resistor Rsense. Thus:
Ssense
+
(Vout )
Lout
Vf)
Ns
Np
Rsense
(eq. 7)
where:
• Vout is output voltage level
• Vf the freewheel diode forward drop
• Lout, the secondary inductor value
• Ns/Np the transformer turn ratio
• Rsense: the sense resistor on the primary side
Assuming the selected amount of ramp compensation to
be applied is δcomp, then we must calculate the division ratio
to scale down Sint accordingly:
Ratio
+
Rsensedcomp
Sint
(eq. 8)
A few line of algebra determined Rcomp:
Rcomp
+
Rramp
1
Ratio
* 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:
Snatural
+
Vbulk
Lmag
Rsense
(eq. 10)
Then the natural ramp compensation will be:
dnatural_comp
+
Snatural
Ssense
(eq. 11)
If the natural ramp compensation (δnatural_comp) is higher
than the ramp compensation needed (δcomp), the power
supply does not need additional ramp compensation. If not,
only the difference (δcomp−δnatural_comp) should be used to
calculate the accurate compensation value.
Thus the new division ratio is:
if
dnatural_comp
t
dcomp
å
Ratio
+
Ssense(dcomp * dnatural_comp)
Sint
(eq. 12)
Then Rcomp can be calculated with the same equation used
when the natural ramp is neglected (Equation 9).
• Vbulk = 350 V, minimum input voltage at which the
power supply works.
Ramp Compensation Design Example:
2 switch−Forward Power supply specification:
• Regulated output: 12 V
• Lout = 27 mH
• Vf = 0.7 V (drop voltage on the regulated output)
• Current sense resistor : 0.75 W
• Switching frequency : 125 kHz
• Duty cycle max: DCmax = 84%
• Vramp = 3.5 V, Internal ramp level.
• Rramp = 26.5 kW, Internal pull−up resistance
• Targeted ramp compensation level: 100%
• Transformer specification:
− Lmag = 13 mH
− Ns/Np = 0.085
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