NCP1252
Internal ramp compensation level
Vramp
3.5
0.84
Sint
+
Fsw å Sint
+
125 kHz + 520 mV ń ms
DCmax
Secondary−side downslope projected over the sense resistor is:
(Vout ) V )
Ns
Np
(12 ) 0.7)
27 @ 10−6
f
Ssense
+
Rsense å Ssense
+
0.085 0.75 + 29.99 mV ń ms
0.75 + 20.19 mV ń ms
Lout
Natural primary ramp:
Vbulk
Lmag
350
Snatural
+
Rsense å Snatural +
13 @ 10−3
Thus the natural ramp compensation is:
S
20.19
dnatural_comp
+
natural å dnatural_comp
+
+ 67.3%
29.99
Ssense
Here the natural ramp compensation is lower than the desired ramp compensation, so an external compensation should be
added to prevent sub−harmonics oscillation.
S
sense(dcomp * dnatural_comp)
29.99 @ (1.00 * 0.67)
Ratio +
å Ratio +
+ 0.019
520
Sint
We can know calculate external resistor (R
) to reach the correct compensation level.
comp
Ratio
0.019
1 * 0.019
Rcomp + R
å Rcomp + 26.5 @ 103
+ 509 W
ramp 1 * Ratio
Thus with R
= 510 W, 100% compensation ramp is applied on the CS pin.
comp
The following example illustrates a power supply where the natural ramp offers enough ramp compensation to avoid external
ramp compensation.
2 switch−Forward Power supply specification:
• Regulated output: 12 V
• Duty cycle max: DC
= 84%
max
• L = 27 mH
• V
• R
= 3.5 V, Internal ramp level.
out
ramp
• V = 0.7 V (drop voltage on the regulated output)
= 26.5 kW, Internal pull−up resistance
f
ramp
• Current sense resistor: 0.75 W
• Switching frequency: 125 kHz
• Targeted ramp compensation level: 100%
• Transformer specification:
− L
= 7 mH
• V
= 350 V, minimum input voltage at which the
power supply works.
mag
bulk
− N /N = 0.085
s
p
Secondary−side downslope projected over the sense resistor is:
(Vout ) Vf)
Ns
Np
(12 ) 0.7)
27 @ 10−6
Ssense
+
Rsense å Ssense
+
0.085 0.75 + 29.99 mV ń ms
0.75 + 37.5 mV ń ms
Lout
The natural primary ramp is:
Vbulk
Lmag
350
Snatural
+
Rsense å Snatural
+
7 @ 10−3
And the natural ramp compensation will be:
S
natural å dnatural_comp
+
+ 125%
29.99
37.5
dnatural_comp
+
Ssense
So in that case the natural ramp compensation due to the magnetizing inductance of the transformer will be enough to prevent
any sub−harmonics oscillation in case of duty cycle above 50%.
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