ICL8001G Datasheet, PDF (7/16 Pages) Infineon Technologies AG – Single-Stage Flyback And PFC Controller For LED Lighting Applications

Single-Stage Flyback and PFC Controller
ICL8001G
Functional Description
3
Functional Description ^{then will reach a constant value depending on output}
load.
3.1
VCC Pre-Charging and Typical
VCC Voltage During Start-up
In ICL8001G, a high voltage startup cell is integrated.
As shown in Figure 2, the start cell consists of a high
voltage device and a controller, whereby the high
voltage device is controlled by the controller. The
startup cell provides a pre-charging of the VCC
capacitor till VCC voltage reaches the VCC turned-on
threshold V_VCCon and the IC begins to operate.
Once the mains input voltage is applied, a rectified
voltage shows across the capacitor C_bus. The high
voltage device provides a current to charge the VCC
capacitor C_vcc. Before the VCC voltage reaches a
certain value, the amplitude of the current through the
high voltage device is only determined by its channel
resistance and can be as high as several mA. After the
VCC voltage is high enough, the controller controls the
high voltage device so that a constant current around
1mA is provided to charge the VCC capacitor further,
until the VCC voltage exceeds the turned-on threshold
V_VCCon. As shown as the time phase I in Figure 3, the
VCC voltage increase near linearly and the charging
speed is independent of the mains voltage level.
3.2
Soft-start
At the time t_on, the IC begins to operate with a soft-start.
By this soft-start the switching stresses for the switch,
diode and transformer are minimised. The soft-start
implemented in ICL8001G is a digital time-based
function. The preset soft-start time is 12ms with 4
steps. If not limited by other functions, the peak voltage
on CS pin will increase step by step from 0.32V to 1V
finally.
Vcs_sst
(V)
1.00
0.83
0.66
0.49
0.32
ton
3
6
9
12 _Time(ms)
Figure 4
Maximum current sense voltage during
softstart
V_VCC
V_VCCon
V_VCCoff
i
ii
iii
t1 t2
t
Figure 3 VCC voltage at start up
The time taking for the VCC pre-charging can then be
approximately calculated as:
t1
=
V-----V-----C----C-----o---n-----⋅_{---C----v---c----c-}
IVCCch _{arg e2}
[1]
where I_VCCcharge2 is the charging current from the
startup cell which is 1.05mA, typically.
Exceeds the VCC voltage the turned-on threshold
V_VCCon of at time t₁, the startup cell is switched off, and
the IC begins to operate with a soft-start. Due to power
consumption of the IC and the fact that still no energy
from the auxiliary winding to charge the VCC capacitor
before the output voltage is built up, the VCC voltage
drops (Phase II). Once the output voltage is high
enough, the VCC capacitor receives then energy from
the auxiliary winding from the time point t₂ on. The VCC
3.3
Normal Operation
The PWM controller during normal operation consists
of a digital signal processing circuit including a
comparator, and an analog circuit including a current
measurement unit and a comparator. The switch-on
and -off time points are each determined by the digital
circuit and the analog circuit, respectively. As input
information for the switch-on determination, the zero-
crossing input signal is needed, while the voltages
sense signal at pin VR and the current sensing signal
V_CS are necessary for the switch-off determination.
Details about the full operation of the PWM controller in
normal operation are illustrated in the following
paragraphs.
3.3.1
Zero crossing
In the system, the voltage from the auxiliary winding is
applied to the zero-crossing pin through a RC network,
which provides a time delay to the voltage from the
auxiliary winding. Internally, this pin is connected to a
clamping network, a zero-crossing detector, an output
overvoltage detector and a ringing suppression time
controller.
During on-state of the power switch a negative voltage
applies to the ZCV pin. Through the internal clamping
network, the voltage at the pin is clamped to certain
level.
Version 1.0
7
May 6, 2010