ice2qr4765z Infineon Technologies Corporation, ice2qr4765z Datasheet - Page 7

• Manufacturer Part Number: ice2qr4765z
• Description: Off-line Smps Quasi-resonant Pwm Controller With Integrated 650v Coolmos And Startup Cell In Dip- 7
• Manufacturer: Infineon Technologies Corporation

3
3.1
In ICE2QR4765Z, a startup cell is integrated into the
CoolMOS
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
IC begins to operate.
Once the mains input voltage is applied, a rectified voltage
shows across the capacitor C
provides a current to charge the VCC capacitor C
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
shown in the time phase I in
increase near linearly and the charging speed is independent
of the mains voltage level.
Figure 3
The time taking for the VCC pre-charging can then be
approximately calculated as:
where I
which is 1.05mA, typically.
When the VCC voltage exceeds the VCC turned-on
threshold V
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 t
constant value depending on output load.
Version 2.0
t 1
V
V
=
V
VCCon
VCCoff
VCCcharge2
VCC
V
----------------------------------------- -
I VCCch
®
VCCon
VCCon
. As shown in Figure 2, the start cell consists of a
Functional Description
VCC Pre-Charging and Typical
VCC Voltage During Start-up
VCC voltage at start up
at time t
is the charging current from the startup cell
arg

C
2
vcc
e2
onward. The VCC will then reach a
I
1
, the startup cell is switched off and
bus
t1
Figure
. The high voltage device
II
t2
3, the VCC voltage
III
VCCon
vcc
VCCon
. Before
and the
t
. As
[1]
7
3.2
As shown in
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 ICE2QR4765Z 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.
Figure 4
3.3
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
the current sensing signal V
off determination. Details about the full operation of the
PWM controller in normal operation are illustrated in the
following paragraphs.
3.3.1
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
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
Vcs_sst
(V)
1.00
0.83
0.66
0.49
0.32
t
on
Soft-start
Normal Operation
Digital Frequency Reduction
Up/down counter
Figure
Maximum current sense voltage during
3
softstart
4, at the time t
CS
Functional Description
6
are necessary for the switch-
CoolSET
ICE2QR4765Z
9
on
, the IC begins to
May 25, 2011
12
®
Time(ms)
- Q1
FB
and