ICE2QS03G_14 Datasheet, PDF (9/24 Pages) Infineon Technologies AG – Active burst mode for low standby power

Quasi-Resonant PWM controller
ICE2QS03G
Functional Description
3
Functional Description
3.1
VCC Pre-Charging and Typical VCC Voltage During Start-up
In ICE2QS03G, a high voltage startup cell is integrated. As shown in Figure 3, 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 4, the VCC voltage
increase near linearly and the charging speed is independent of the mains voltage level.
Figure 4 VCC voltage at start up

The time taking for the VCC pre-charging can then be approximately calculated as:
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 then will reach a constant value depending on
output load.
3.2
Soft-start
As shown in Figure 5, 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 minimized. The soft-start implemented in ICE2QS03G is a
digital time-based function. The preset soft-start time is t_SS (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.
Data Sheet
9
V2.3, 2014-03-06