ICE2QR0665G Datasheet, PDF (7/21 Pages) Infineon Technologies AG – Off-Line SMPS Quasi-Resonant PWM Controller with integrated 650V CoolMOS® and startup cell in DSO-16/12

CoolSET^® - Q1
ICE2QR0665G
Functional Description
3
Functional Description
3.1
VCC Pre-Charging and Typical
VCC Voltage During Start-up
In ICE2QR0665G, 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 in 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
As shown in Figure 4, 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 ICE2QR0665G 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
VVCC
VVCCon
VVCCoff
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--I--VV-----CC----C-C----oc---nh----a-^--r-C-g---ve---c-2---c-
[1]
where I_VCCcharge2 is the charging current from the startup cell
which is 1.05mA, typically.
When the VCC voltage exceeds the VCC turned-on
threshold V_VCCon at time t₁, the startup cell is switched off and
the IC begins to operate with soft-start. Due to power
consumption of the IC and the fact that there is 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 the energy from the auxiliary winding
from the time point t₂ onward. The VCC will then 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 determined by the digital circuit and the
analog circuit respectively. The zero-crossing input signal
and the value of the up/down counter are needed for the
switch-on determination 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
period or output short circuit. Functionality of these parts is
described in the following.
3.3.1.1 Up/down counter
The up/down counter stores the number of the zero crossing
where the main power switch is switched on after
demagnetisation of the transformer. This value is fixed
Version 2.0
7
July 4, 2011