Quasi-resonant CoolSET design guide
ICE2QRxx65/80x
Design Guide
8
8 August 2011
Figure 3 Key waveforms of a quasi-resonant flyback converter
4
Functional description and component design
4.1
VCC Pre-Charging and Typical VCC Voltage During Start-up
In the CoolSET ICE2QRxx65/80xx, a startup cell is integrated to the CoolMOS. The startup cell provides a
pre-charging of the VCC capacitor till VCC voltage reaches the VCC turned-on threshold
V
VCCon
and the
CoolSET 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 rises to certain level, the CoolSET controls the startup
cell so that a constant current around 1mA is provided to charge the VCC capacitor. It stops until the VCC
voltage exceeds the turned-on threshold V
VCCon
. As shown in the time phase I of Figure 4, the VCC voltage
increase almost linearly.
The time taken for the charging VCC to turn-on threshold can then be approximately calculated as:
VCCcharge2
VCC
VCCon
1
I
C
V
t
[5]
where
I
VCCcharge2
is the charging current from the startup cell which is 1.1mA, typically.
