参数资料
型号: L6566A
厂商: STMICROELECTRONICS
元件分类: 稳压器
英文描述: 0.8 A POWER FACTOR CONTROLLER WITH POST REGULATOR, 300 kHz SWITCHING FREQ-MAX, PDSO16
封装: LEAD FREE, SOP-16
文件页数: 21/51页
文件大小: 646K
代理商: L6566A
Application information
L6566A
28/51
Figure 16.
Typical power capability change vs input voltage in QR flyback
converters
It acts on the overcurrent setpoint Vcsx, so that it is a function of the converter’s input voltage
Vin (output of the PFC pre-regulator) sensed through a dedicated pin (15, VFF): the higher
the input voltage, the lower the setpoint. This is illustrated in the diagram on the left-hand
side of Figure 17 on page 29: it shows the relationship between the voltage on the pin VFF
and Vcsx (with the error amplifier saturated high in the attempt of keeping output voltage
regulation):
Equation 5
Note:
If the voltage on the pin exceeds 3 V switching ceases but the soft-start capacitor is not
discharged. The schematic in Figure 17 on page 29 shows also how the function is included
in the control loop.
With a proper selection of the external divider R1-R2, i.e. of the ratio k = R2 / (R1+R2), it is
possible to achieve the optimum compensation described by the lower curve in the diagram
The optimum value of k, kopt, which minimizes the power capability variation over the input
voltage range, is the one that provides equal power capability at the extremes of the range.
The exact calculation is complex, and non-idealities shift the real-world optimum value from
the theoretical one. It is therefore more practical to provide a first cut value, simple to be
calculated, and then to fine tune experimentally.
Assuming that the system operates exactly at the boundary between DCM and CCM, and
neglecting propagation delays, the following expression for kopt can be found:
Equation 6
Vin
Vinmin
P
in
lim
@
V
in
P
in
lim
@
V
in
mi
n
11.5
22.5
3
3.5
4
0.5
1
1.5
2
2.5
system optimally
compensated
system not
compensated
k = 0
k = kopt
k
Vin
3
k
1
3
V
1
V
VFF
csx
=
=
()
R
max
in
min
in
max
in
min
in
R
opt
V
3
k
+
=
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