參數(shù)資料
型號(hào)： NCP1381
廠商： ON SEMICONDUCTOR
英文描述： Low−Standby High Performance PWM Controller
文件頁(yè)數(shù)： 15/26頁(yè)
文件大?。?/td> 559K
代理商： NCP1381
NCP1381
http://onsemi.com
15
As one can observe, the output power runs out of the initial
100 W specification when we enter the high line region. To
cope with this problem, we need to compensate the
controller in such a way that its peak capability gets reduced
at higher input voltages. How much do we need to
compensate the peak excursion We can find the answer by
calculating I
P
= I
PLL
I
PHL
, with V
inLL
= 200 V and V
inHL
= 400 V. With our previous numbers, I
P
= 588 mA. We
therefore need to instruct the controller to reduce its peak
excursion by 588 mA at high line. Otherwise speaking, if we
think in voltages, the CS pin excursion shall drop from 0.8 V
(at low line, the maximum peak is 0.8 / R
S
) to (3.2 0.588).
0.25 = 653 mV at high line. Figure 19 shows the situation at
both line levels. A possible solution lies in offsetting the
current floor by the necessary value, which is, in our case,
0.8 0.653 = 147 mV. The traditional way of doing this goes
through the wiring of a high value resistor to the bulk
capacitor. This unfortunately dissipates heat. The NCP1381
offers a more elegant option since it transforms the voltage
available from the Brownout pin into a fixed current,
routed to the CS pin. That way, we can calculate a resistor
value which, once inserted in series with current sense
voltage image, will create our necessary offset. Figure 20
shows this internal connection:
Figure 20. A Transconductance Amplifiers
Transforms the BO Voltage into a Current
Figure 21. The Compensated Converter Output
Power Response to Input Variations
105
100
95
90
200
250
300
350
400
V
in
, VOLTAGE (V)
P
O
+
V
bulk
G1
80 S
To BO
Comp.
BO
CS
R
sense
To CS Comp.
R
offset
We can now calculate our R
offset
resistor to generate the
necessary static voltage. Suppose that the BO network
divides the bulk voltage by 400 (V
BO
= . V
in
= 0.0025 x
V
in
). Therefore, in presence of a 400 V input voltage, we will
have 1 V on the BO pin. due to the transconductance
amplifier of a 80 S gm, it will turn into a 80 A offset
current. To get our 147 mV, we just divide it by 80 A:
R
offset
= V
offset
/ V
inHL
x . g
m
= 1.8 k .
We can now update Equation 4 with Equation 5, where
the peak current is affected by the variable offset:
PO(Vin) :
0.8
RS
Vin
LP
tP
Vin
gm
Roffset
2
(
(Vin(Vout
VF)))
Vout
VF
Vin
N
(eq. 5)
If we now plot the compensated curve, we obtain
Figure 21 graph. The output power is slightly above what we
were originally shooting for and the total power excursion
is now kept within 15 W.
Overvoltage Protection
The NCP1381 features an overvoltage protection made by
sensing the plateau voltage at the switch turnoff. However,
a sampling delay is introduced to avoid considering the
leakage inductance. When the demagnetization pin goes
above V
demlatch
, the comparator goes high. If this condition
is maintained when the sampling pulse arrives, then a fault
is latched. Figure 22 shows the arrangement and Figure 23
portrays a typical waveform. Once latched, the controller
stops all driving pulses and V
CC
is clamped to 6 V. Reset
occurs when the user unplugs the converter from the mains
and V
CC
reduces below 4 V.
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