Quasi-resonant CoolSET design guide
ICE2QRxx65/80x
Design Guide
6
8 August 2011
2.3
Pin functions
2.3.1
ZC (Zero Crossing)
Three functions are incorporated at the ZC pin. First, during MOSFET off time, the de-magnetization of the
transformer is detected when the ZC voltage falls below V
ZCCT
(100mv). Second, after the MOSFET is turned
off, an output overvoltage fault will be assumed if V
ZC
is higher than V
ZCOVP
(3.7V). Finally, during the
MOSFET on time, a current depending on the bus voltage flows out of this pin. Information on this current is
then used to adjust the maximum current limit. More details on this function are provided in Section 4.
2.3.2
FB (Feedback)
Usually, an external capacitor is connected to this pin to smooth the feedback voltage. Internally, this pin is
connected to the PWM signal generator for switch-off determination (together with the current sensing signal),
and to the digital signal processing for the frequency reduction with decreasing load during normal operation.
Additionally, the openloop/overload protection is implemented by monitoring the voltage at this pin.
2.3.3
CS (Current Sensing)
This pin is connected to the shunt resistor for the primary current sensing externally and it is also used to
determine the PWM signal generator for switch-off (together with the feedback voltage) internally. Moreover,
short-winding protection is realised by monitoring the
V
cs
voltage during on-time of the main power switch.
2.3.4
Drain
This pin is connected to the drain of the 650V/800V CoolMOS
®
.
2.3.5
VCC (Power supply)
The VCC pin is the positive supply of the CoolSET and should be connected to auxiliary winding of the main
transformer.
2.3.6
GND (Ground)
This is the common ground of the CoolSET. Note that the current sense resistor ground should be connected
to bulk capacitor ground in order to avoid strong noise interruption.
3
Overview of quasi-resonant flyback converter
Figure 2 shows a typical application of ICE2QRxx65/80x in quasi-resonant flyback converter. In this
converter, the mains input voltage is rectified by the diode bridge and then smoothed by the capacitor C
bus
where the bus voltage V
bus
is available. The transformer has one primary winding W
p
, one or more secondary
windings (here one secondary winding W
s
), and one auxiliary winding W
a
. When quasi-resonant control is
used for the flyback converter, the typical waveforms are shown in Figure 3. The voltage from the auxiliary
winding provides information about demagnetization of the power transformer, the information of input
voltage and output voltage.
As shown in Figure 3, after switch-on of the power switch the voltage across the shunt resistor V
CS
shows a
spike caused by the discharging of the drain-source capacitor. After the spike, the voltage V
CS
shows
information about the real current through the main inductance of the transformer L
p
. Once the measured
current signal V
CS
exceeds the maximum value determined by the feedback voltage V
FB
, the power switch is
turned off. During this on-time, a negative voltage proportional to the input bus voltage is generated across
the auxiliary winding.