Converter design using the quasi-resonant
PWM controller ICE2QS03G
AN-PS0045
Application Note 8 7 April 2010
After the secondary side current falls to zero, the drains-source voltage of the power switch shows another
oscillation (oscillation 2 in Figure 3, this is also mentioned as the main oscillation in this document). This
oscillation happens in the circuit consisting of the equivalent main inductance of the transformer Lpand the
capacitor across the drain-source (or drain-ground) terminal CDS. The frequency of this oscillation is
calculated as:
DSP
OSC CL2π
1
f
⋅
=(3)
The amplitude of this oscillation begins with a value of vRefl and decreases exponentially with the elapsing
time, which is determined by the losses factor of the resonant circuit. The first minimum of the drain voltage
appears at the half of the oscillation period after the time t4and can be apporximated as:
ReflbusdsMin V-VV =(4)
In the quasi-resonant control, the power switch is switched on at the minimum of the drain-source voltage.
From this kind of operation, the switching-on losses are minimized, and switching noise due to dvds/dt is
reduced compared to a normal hard-switching flyback converter.
4 Functions and Application Overview
4.1 VCC Pre-Charging and Typical VCC Voltage During Start-up
In the controller ICE2QS03G, a power cell is integrated and it consists of a 500V high voltage device and a
controller, whereby the high voltage device is controlled by the controller. The power cell provides a pre-
charging of the VCC capacitor till VCC voltage reaches the VCC turned-on threshold VVCCon and the IC
begins to operate, while it may keep the VCC voltage at a constant value during burst mode operation when
the output voltage is pulled down or the power from the auxiliary winding is not enough, or when the IC is
latched off in certain protection mode.
Once the mains input voltage is applied, a rectified voltage shows across the capacitor Cbus. The high voltage
device provides a current to charge the VCC capacitor Cvcc. 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 VVCCon. As shown as the time phase I in Figure 4, the VCC voltage
increase nearly linearly.
Figure 4 VCC voltage at start up
The time taken for the charging VCC to turn-on threshold can then be approximately calculated as:
VCCcharge2
VCCVCCon
1I
CV
t⋅
=[5]
where IVCCcharge2 is the charging current from the power cell which is 1.05mA, typically.
When the VCC voltage exceeds the turned-on threshold VVCCon of at time t1, the power cell is switched off,
and the IC begins to operate with a soft-start. Because the energy from the auxiliary winding is not enough to
supply the IC operation when output voltage is low, the VCC voltage drops (Phase II). Once the output
voltage is high enough, the VCC capacitor receives energy from the auxiliary winding from the time point t2 on.
The VCC voltage will then reach a constant value depending on output load.
Since there is a VCC undervoltage protection, the capacitance of the VCC capacitor should be selected to be
high enough to ensure that enough energy is stored in the VCC capacitor so that the VCC voltage will never
touch the VCC under voltage protection threshold VVCCUVP before the output voltage is built up. Therefore, the
capacitance should fulfill the following requirement:
