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參數(shù)資料
| 型號： | NCP1927DR2G |
| 廠商： | ON Semiconductor |
| 文件頁數(shù)： | 12/24頁 |
| 文件大小： | 531K |
| 描述： | IC CTLR PFC/FLYBACK 16-SOIC |
| 標(biāo)準(zhǔn)包裝： | 2,500 |
| 系列： | * |
NCP1927
http://onsemi.com
12
Figure 6. V
CC
Behavior During Shutdown Mode
THERMAL SHUTDOWN
When the junction temperature exceeds T
TSHDN
(140癈
minimum), a temperature sensing circuit disables the gate
drives and a V
CC
hiccup occurs (see Figure 5). When V
CC
reaches V
CC(on)
, the cycle repeats unless the junction
temperature drops below T
TSHDN
.
CLAMPED DRIVERS
The NCP1927 includes two powerful MOSFET drivers
capable of sourcing 800 mA and sinking 1200 mA each.
Since V
CC
is rated at 30 V (maximum), each driver output
is internally clamped to 16 V (maximum) to allow the use of
20 V MOSFETs.
FLYBACK CONTROLLER
The NCP1927 flyback stage implements a standard
current mode architecture where the switchoff event is
dictated by the peak current setpoint.
Oscillator with Maximum Duty Ratio and Frequency
Jittering
The NCP1927 flyback controller includes an oscillator
that sets the switching frequency with an accuracy of
$7.7%. The maximum duty ratio of the FDRV pin is 80%
(typical).
In order to improve the EMI signature, the switching
frequency jitters at f
MOD
($6% typical) around its nominal
value, with a trianglewave shape and at a frequency of f
jitter
(125 Hz typical). The frequency jittering is fully disabled
during softstart and frequency foldback. Figure 7 depicts
the jittering operation.
Figure 7. Frequency Jittering
Time
f
OSC
+ f
MOD
8 ms
(125 Hz)
Nominal f
OSC
f
OSC
- f
MOD
f
OSC
Current Sensing
NCP1927 is a currentmode controller, which means that
the feedback voltage sets the peak current flowing in the
transformer and the MOSFET. This is done through the
PWM comparator. The switch current is sensed across a
resistor and the resulting voltage is applied to the FCS pin.
It is then applied to one input of the PWM comparator
through a 250 ns leading edge blanking (LEB) block. On the
other input, the feedback voltage divided by K
FFB
(typically
5) sets the current limit threshold. When the current reaches
this threshold, the output driver is turned off. A dedicated
comparator monitors the current sense voltage, and if it
reaches the maximum value, V
ILIM
(typically 0.7 V), the
output driver is turned off immediately. This occurs even if
the limit imposed by the feedback voltage is higher than
V
ILIM
. Figure 8 shows the schematic of the current sense
circuit.
Figure 8. Current Sense Block Schematic
FFB
V
FFB(open)
R
FFB
V
ILIM
?/DIV>
S
Q
R
V
CC
FDRV
f
(OSC)
FCS
t
LEB
K
FFB
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