NCP1252APG Ver la hoja de datos (PDF) - ON Semiconductor
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NCP1252APG Datasheet PDF : 18 Pages
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NCP1252
Internal ramp compensation level
Sint
+
Vramp
DCmax
Fsw
å
Sint
+
3.5
0.84
125
kHz
+
520
mV
ń
ms
Secondary−side downslope projected over the sense resistor is:
Ssense
+
(Vout )
Lout
Vf)
Ns
Np
Rsense
å
Ssense
+
(12
27
) 0.7)
@ 10−6
0.085
0.75 + 29.99 mV ń ms
Natural primary ramp:
Snatural
+
Vbulk
Lmag
Rsense
å
Snatural
+
350
13 @ 10−3
0.75
+
20.19
mV
ń
ms
Thus the natural ramp compensation is:
dnatural_comp
+
Snatural
Ssense
å
dnatural_comp
+
20.19
29.99
+
67.3%
Here the natural ramp compensation is lower than the desired ramp compensation, so an external compensation should be
added to prevent sub−harmonics oscillation.
Ratio
+
Ssense(dcomp * dnatural_comp)
Sint
å
Ratio
+
29.99
@
(1.00
520
*
0.67)
+
0.019
We can know calculate external resistor (Rcomp) to reach the correct compensation level.
Rcomp
+
Rramp
1
Ratio
* Ratio
å
Rcomp
+
26.5
@
103
1
0.019
* 0.019
+
509
W
Thus with Rcomp = 510 W, 100% compensation ramp is applied on the CS pin.
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
• Lout = 27 mH
• Vf = 0.7 V (drop voltage on the regulated output)
• Current sense resistor: 0.75 W
• 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%
• Switching frequency: 125 kHz
• Transformer specification:
• Vbulk = 350 V, minimum input voltage at which the
power supply works.
− Lmag = 7 mH
− Ns/Np = 0.085
Secondary−side downslope projected over the sense resistor is:
Ssense
+
(Vout ) Vf)
Lout
Ns
Np
Rsense
å
Ssense
+
(12 ) 0.7)
27 @ 10−6 0.085
0.75 + 29.99 mV ń ms
The natural primary ramp is:
Snatural
+
Vbulk
Lmag
Rsense
å
Snatural
+
7
350
@ 10−3
0.75
+
37.5
mV
ń
ms
And the natural ramp compensation will be:
dnatural_comp
+
Snatural
Ssense
å
dnatural_comp
+
37.5
29.99
+
125%
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%.
Table 4. ORDERING INFORMATION
Device
Version
Marking
Shipping†
NCP1252ADR2G
A version
1252A
2500/Tape & Reel
NCP1252BDR2G
B version
1252B
2500/Tape & Reel
NCP1252CDR2G
C version
1252C
2500/Tape & Reel
NCP1252APG
A version
1252AP
50 Units / Rail
†For information on tape and reel specifications, including part orientation and tape sizes, please refer to our Tape and Reel Packaging
Specification Brochure, BRD8011/D.
http://onsemi.com
16
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