ICE2QS02G Datasheet, PDF (10/17 Pages) Infineon Technologies AG – Quasi-Resonant PWM Controller

Quasi-Resonant PWM Controller
ICE2QS02G
Funtional Description
3.5
Protection Functions
ICE2QS02G provides various protection functions. The
following table summarizes these protection functions.
Table 2 Protection features
VCC Undervoltage
Overload/Openloop
Protection
Main undervoltage
Protection
Output Overvoltage
Short Winding
Latch off
Auto restart
Block Gate
recover with soft start
Latch off
Latch off
During normal operation, the VCC voltage is
continuously monitored. In case of a VCC
undervoltage, the IC is reset and the main power switch
is kept off.
The Overload and Open Loop Protection contains an
adjustable blanking time and variable restart time.
Such an adjustable buffer time is indeed, useful, for
applications that usually work in low output power, but
require a short duration of high output
poweroccasionally. Here, when the regulation voltage,
V_FB exceeds the threshold voltage of V_FBOLP, an
internal current source of I_BL starts charging the
external capacitor C_BL. This current source turns off
only when the capacitor voltage, V_BL reaches V_BLH or
when V_FB decreases below V_FBH. Once V_BL exceeds
V_BLH, the overload/openloop protection is triggered by
turning off the GATE signal, and pulling high the
feedback voltage. From this time, C_BL slowly
discharges through the external resistance R_BL. When
V_BL drops below V_BLL, the IC restarts its operation
beginning with soft-start. The charging time and the
discharging time of the capacitor C_BL, fix respectively
the openloop/overload protection blanking time and the
restart time of the IC. One example about how this
protection works is shown in Figure 7.
VGATE
The blanking time for Over Load Protection can be
calculated using equation [5].
T_OLP = –R_BLC_BL ln _1 – _I---B^V---_L-^B--R--^L--_B-^H--_L-_
[5]
The restart time for Over Load Protection can be
calculated using equation [6].
T_RESTART
=
–R_B
L
C_B
L
ln


V-V----_BB^---_LL^---_HL^{--}
[6]
During the switch off time, the voltage at the zero-
crossing pin, ZC, is monitored for output overvoltage
detection. If this voltage is higher than the preset
threshold V_ZCOVP, the IC enters latch-off mode.
If the voltage at the current sensing pin is higher than
the preset threshold V_CSSW of 1.68V during the on-time
of the power switch, the IC is latched off. This
consitutes a short winding protection.
Finally, this IC has an adjustable main undervoltage
detection system. Given the resistances R_VINS1 and
R_VINS2 connected to the VINS pin, the main turn off
voltage is given by equation [7].
V_BUSOFF
=
V_VINSTH
⋅
R-----_{V---I--N---S---1----+-----R----V---I--N---S---2-}
R_VINS2
[7]
For system stability, a hysteresis is implemented in the
main undervoltage protection using an internal current
source I_VINS, so that the main turn on voltage is given
by equation [8].
V_BUSON = V_BUSOFF + I_VINSHys ⋅ R_VINS1
[8]
Everytime IC recovers from a mains undervoltage
protection, IC will begin with a soft start.
VBL
t
3.9V
0.5V
VFB
t
4.5V
tOLP
trestart
tss _tOLP
trestart
tss
t
Figure 7 Over Load Protection and timers
Version 2.0
10
13 June 2008