Quasi-resonant CoolSET design guide
ICE2QRxx65/80x
Design Guide
15
8 August 2011
V
ZC
is lower than 0.7V. During the ring suppression time, CoolSET can not be turned on again. Therefore, the
ring suppression time can also be thought as a minimum off time.
4.8
Protections
The ICE2QRxx65/80x CoolSET provides full protection functions. The following table summarizes these
protection functions.
VCC Overvoltage
Auto Restart Mode
VCC Undervoltage
Auto Restart Mode
Overload/Open Loop
Auto Restart Mode
Over Temperature
Auto Restart Mode
Output Overvoltage
Latched Off Mode
Short Winding
Latched Off Mode
During Operation, the VCC over voltage is continuously monitored. In case of an under- or an over-voltage,
the CoolSET is reset and the main power switch is then kept off. After the VCC voltage falls below the
threshold V
VCCoff
, the startup cell is activated. The VCC capacitor is then charged up. Once the voltage
exceeds the threshold V
VCCon
, the CoolSET begins to operate with a new soft-start.
In case of open control loop or output over load, the feedback voltage will be pulled up. After a blanking time
of 24ms, the CoolSET enters auto-restart mode. The blanking time here enables the converter to provide a
high power in case the increase in V
FB
is due to a sudden load increase.
During off-time of the power switch, the voltage at the zero-crossing pin is monitored for output over-voltage
detection. If the voltage is higher than the preset threshold V
ZCOVP
, the CoolSET is latched off after the preset
blanking time.
If the junction temperature of CoolSET exceeds 140
o
C, the CoolSET enters into auto-restart mode.
If the voltage at the current sensing pin is higher than the preset threshold V
CSSW
during on-time of the power
switch, the CoolSET is latched off. This is short-winding protection.
During latch-off protection mode, when the VCC voltage drops to 10.5V, the startup cell is activated and the
VCC voltage is charged to 18V then the startup cell is hut down again and repeats the previous procedure.
The latch-off mode can only be reset if the VCC voltage < 6.23V.
4.9
Others
For quasi-resonant flyback converters, it is possible that the operation frequency goes too low, which
normally resulted in audible noise. To prevent it, in ICE2QRxx65/80x, a maximum on time and maximum
switching period is provided.
The maximum on time in ICE2QRxx65/80x is 30 μs typically. If the gate is maintained ON for 30 μs, CoolSET
will turn off the gate regardless of the current sense voltage.
When the MOSFET is off and CoolSET can not detect enough number of ZC to turn on the MOSFET,
CoolSET will turn on the MOSFET when the maximum switching period, 50 μs typically, is reached. Please
note that even a non-zero ZC pin voltage can not prevent CoolSET from turning on the MOSFET. Therefore,
during soft start, a CCM operation of the converter is expected.
5
Typical application circuit
A 12W evaluation board with ICE2QR4780z is shown below as an example. The detailed information can be
found in [5]. The application circuit is shown in Figure 9.
