ICE2QR4765_12 Datasheet, PDF (7/22 Pages) Infineon Technologies AG – Off-Line SMPS Quasi-Resonant PWM Controller with integrated 650V Startup Cell/Depletion CoolMOS In DIP-8

CoolSET^® - Q1
ICE2QR4765
Functional Description
3 Functional Description
3.1
VCC Pre-Charging and Typical
VCC Voltage During Start-up
In ICE2QR4765, a startup cell is integrated into the
CoolMOS^®. 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 ICE2QR4765 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-----^â‹---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
then will reach a constant value depending on output
load.
3.3
Normal Operation
The PWM controller during normal operation consists
of a digital signal processing circuit including an up/
down counter, a zero-crossing counter (ZC counter)
and 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 and the value of the up/down
counter are needed, while the feedback signal V_FB 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
Digital Frequency Reduction
As mentioned above, the digital signal processing
circuit consists of an up/down counter, a ZC counter
and a comparator. These three parts are key to
implement digital frequency reduction with decreasing
load. In addition, a ringing suppression time controller
is implemented to avoid mistriggering by the high
frequency oscillation, when the output voltage is very
low under conditions such as soft start or output short
circuit . Functionality of these parts is described as in
the following.
Version 2.3
7
November 14, 2012