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参数资料
| 型号： | NCP1252BDR2G |
| 厂商： | ON Semiconductor |
| 文件页数： | 16/18页 |
| 文件大小： | 0K |
| 描述： | IC REG CTRLR FLYBK ISO PWM 8SOIC |
| 标准包装： | 2,500 |
| PWM 型： | 电流模式 |
| 输出数： | 1 |
| 频率 - 最大： | 500kHz |
| 占空比： | 84% |
| 电源电压： | 9 V ~ 28 V |
| 降压： | 无 |
| 升压： | 无 |
| 回扫： | 是 |
| 反相： | 无 |
| 倍增器： | 无 |
| 除法器： | 无 |
| Cuk： | 无 |
| 隔离： | 是 |
| 工作温度： | -25°C ~ 125°C |
| 封装/外壳： | 8-SOIC（0.154"，3.90mm 宽） |
| 包装： | 带卷 (TR) |
NCP1252
Internal ramp compensation level
V ramp
DC max sw
S int +
F
3.5
? S int + 0.84 125 kHz + 520 mV
m s
Secondary ? side downslope projected over the sense resistor is:
(V out ) V f ) N s
N p sense
L out
S sense +
R
? S sense +
(12 ) 0.7)
27 @ 10 ? 6
0.085
0.75 + 29.99 mV
m s
Natural primary ramp:
V bulk
L mag sense
S natural +
R
? S natural +
350
13 @ 10 ? 3
0.75 + 20.19 mV
m s
Thus the natural ramp compensation is:
d natural_comp +
S natural
S sense
? d natural_comp + 20.19 + 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 +
S sense ( d comp * d natural_comp )
S int
? Ratio +
29.99 @ (1.00 * 0.67)
520
+ 0.019
? R comp + 26.5 @ 10 3
R comp + R ramp
+ 509 W
We can know calculate external resistor (R comp ) to reach the correct compensation level.
Ratio 0.019
1 * Ratio 1 * 0.019
Thus with R comp = 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
? L out = 27 m H
? 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 k W , Internal pull ? up resistance
? Targeted ramp compensation level: 100%
? Transformer specification:
? L mag = 7 mH
? N s /N p = 0.085
Secondary ? side downslope projected over the sense resistor is:
(V out ) V f ) N s
N p sense
L out
S sense +
R
? S sense +
(12 ) 0.7)
27 @ 10 ? 6
0.085
0.75 + 29.99 mV
m s
The natural primary ramp is:
V bulk
L mag sense
S natural +
R
? S natural +
350
7 @ 10 ? 3
0.75 + 37.5 mV
m s
And the natural ramp compensation will be:
d natural_comp +
S natural
S sense
37.5
? d natural_comp + 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
NCP1252ADR2G
NCP1252BDR2G
NCP1252CDR2G
NCP1252APG
Device
Version
A version
B version
C version
A version
Marking
1252A
1252B
1252C
1252AP
Shipping ?
2500/Tape & Reel
2500/Tape & Reel
2500/Tape & Reel
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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