Working In Active Burst Mode - Infineon CoolSET-F3R ICE3BRXX65JZ series Design Manual

V
DS
V
CC
20ms
V
FB
V
BA
Figure 8 Entering Burst Mode
6.3.2

Working in Active Burst Mode

In the active burst mode, the IC is constantly monitoring the output voltage by feedback pin, V
controls burst duty cycle and burst frequency. The burst "ON" starts when V
when V is dropped to 3.0V. During burst "ON", the primary current limit is set to 34% of maximum peak
FB
current (V =0.34V) to reduce the conduction losses and to avoid audible noise. The FB voltage is changing
CS
like a saw tooth between 3.0V and 3.5V. The corresponding secondary output ripple (peak to peak) is
controlled to be small. It can be calculated by equation (5).
R
=
V
out _ ripple _ pp
⋅
R G
FB opto
where, R : series resistor with opto-coupler at secondary side (e.g. Rc6 in Figure 5), R
opto
up resistor connected to FB pin (R
transfer gain of the loop compensation network (e.g. Rc1, Rc2, Rc3, Rc4, Cc1, Cc2 in Figure 5),
feedback voltage change (0.5V)
Usually there is a noise coupling capacitor at the FB pin to filter the switching noise and spike (e.g. C8 in
Figure 5). However, if this capacitor is too large (>10nF), it would affect the normal operation of the
controller. This capacitor should be as small as possible (without the capacitor is the best). In the mean time,
it is found that this filter capacitor will also affect the output ripple voltage during burst mode; larger
capacitance will get larger ripple voltage and smaller capacitance get lower ripple voltage.
Figure 9 is the output ripple waveform of the 12W demo board. The burst ripple voltage is about 50mV.
V
o
Figure 9 Output ripple during Active Burst Mode at light load
Application Note
opto
∆ ⋅
V
FB
⋅
G
TL 431
=15.4KΩ), G
FB
50mV
Channel 1; C1 : Drain-Source Voltage (V
Channel 2; C2 : Supply Voltage (V
Channel 3; C3 : FB voltage (V
Channel 4; C4 : BA Voltage (V
Entering Active Burst mode with preset
time 20ms when load changes from full to
light @ Vin=85Vac
: current transfer gain of opto-coupler, G
opto
Channel 2; C2 : Output ripple Voltage (V
Output ripple voltage at light load @ Vin=85Vac
11
ICE3BRXX65J(Z)(G)
)
DS
)
CC
)
FB
)
BA
, which
FB
reaches 3.5V and it stops
FB
: IC internal pull
FB
: voltage
TL431
)
o
2010-06-20
(5)
∆
V :
FB