參數(shù)資料
型號: L6566ATR
廠商: 意法半導(dǎo)體
元件分類: 基準(zhǔn)電壓源/電流源
英文描述: Multi-mode controller for SMPS with PFC front-end
中文描述: 多模式開關(guān)電源控制器,PFC前端
文件頁數(shù): 28/51頁
文件大?。?/td> 410K
代理商: L6566ATR
Application information
L6566A
28/51
Figure 16. Typical power capability change vs input voltage in QR flyback converters
It acts on the overcurrent setpoint V
csx
, 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 V
csx
(with the error amplifier saturated high in the attempt of keeping output voltage
regulation):
Equation 5
Note:
If the voltage on the pin exceeds 3V 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
of
Figure 16
.
The optimum value of k, k
opt
, 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 k
opt
can be found:
Equation 6
Experience shows that this value is typically lower than the real one. Once the maximum
peak primary current, I
PKpmax
, occurring at minimum input voltage Vinmin has been found,
the value of Rs can be determined from (2):
V
in
V
inmin
P
i
i
P
i
i
1
1.5
2
2.5
3
3.5
4
0.5
1
1.5
2
2.5
system optimally
compensated
system not
compensated
k = 0
k = k
opt
k
Vin
3
k
1
3
V
1
V
VFF
csx
=
=
(
)
R
max
in
min
in
max
in
min
in
R
opt
V
V
V
V
V
V
3
k
+
+
=
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