ICE2QR0665_10 Datasheet, PDF (10/21 Pages) Infineon Technologies AG – Off-Line SMPS Quasi-Resonant PWM Controller

CoolSET^® - Q1
ICE2QR0665
Functional Description
versus various input bus voltage can be calculated, which is about Active Burst Mode operation are explained in the
shown in Figure 6.
following paragraphs.
1
0.9
0.8
0.7
0.6
80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400
Vin(V)
Figure 6 Variation of the VCS limit voltage according to
the I_ZC current
According to the typical application circuit, when MOSFET
is turned on, a negative voltage proportional to bus voltage
will be coupled to auxiliary winding. Inside CoolSET^® - Q1,
an internal circuit will clamp the voltage on ZC pin to nearly
0V. As a result, the current flowing out from ZC pin can be
calculated as
IZC
=
-V----B-----U----S----N-----a-
RZC1NP
[5]
When this current is higher than I_{ZC_1}, the amount of current
exceeding this threshold is used to generate an offset to
decrease the maximum limit on V_CS. Since the ideal curve
shown in Figure 6 is a nonlinear one, a digital block in
CoolSET^® - Q1 is implemented to get a better control of
maximum output power. Additional advantage to use digital
circuit is the production tolerance is smaller compared to
analog solutions. The typical maximum limit on V_CS versus
the ZC current is shown in Figure 7.
1
0.9
0.8
0.7
3.5.1
Entering Active Burst Mode Operation
For determination of entering Active Burst Mode
operation, three conditions apply:
• the feedback voltage is lower than the threshold of
V_FBEB(1.25V). Accordingly, the peak current sense
voltage across the shunt resistor is 0.17;
• the up/down counter is 7; and
• a certain blanking time (t_BEB).
Once all of these conditions are fulfilled, the Active
Burst Mode flip-flop is set and the controller enters
Active Burst Mode operation. This multi-condition
determination for entering Active Burst Mode operation
prevents mistriggering of entering Active Burst Mode
operation, so that the controller enters Active Burst
Mode operation only when the output power is really
low during the preset blanking time.
3.5.2
During Active Burst Mode Operation
After entering the Active Burst Mode the feedback
voltage rises as V_OUT starts to decrease due to the
inactive PWM section. One comparator observes the
feedback signal if the voltage level V_BH (3.6V) is
exceeded. In that case the internal circuit is again
activated by the internal bias to start with swtiching.
Turn-on of the power MOSFET is triggered by the
timer. The PWM generator for Active Burst Mode
operation composes of a timer with a fixed frequency of
50kHz, typically, and an analog comparator. Turn-off is
resulted by comparison of the voltage signal v₁ with an
internal threshold, by which the voltage across the
shunt resistor V_csB is 0.34V, accordingly. A turn-off can
also be triggered by the maximal duty ratio controller
which sets the maximal duty ratio to 50%. In operation,
the output flip-flop will be reset by one of these signals
which come first.
If the output load is still low, the feedback signal
decreases as the PWM section is operating. When
feedback signal reaches the low threshold V_BL(3.0V),
the internal bias is reset again and the PWM section is
disabled until next time regultaion siganl increases
beyond the V_BH threshold. If working in Active Burst
Mode the feedback signal is changing like a saw tooth
between 3.0V and 3.6V shown in Figure 8.
0.6
300
500
700
900 1100 1300 1500 1700 1900 2100
Iz c(uA)
Figure 7
V_CS-max versus I_ZC
3.5
Active Burst Mode Operation
At light load condition, the IC enters Active Burst Mode
operation to minimize the power consumption. Details
3.5.3
Leaving Active Burst Mode Operation
The feedback voltage immediately increases if there is a high
load jump. This is observed by one comparator. As the
current limit is 34% during Active Burst Mode a certain load
is needed so that feedback voltage can exceed V_LB (4.5V).
After leaving active busrt mode, maximum current can now
be provided to stabilize V_O. In addition, the up/down counter
will be set to 1 immediately after leaving Active Burst
Version 2.3
10
April 27, 2010