IX9908
INTEGRATED
C
IRCUITS
D
IVISION
2.2 Soft-Start
suppression circuit is implemented. The suppression
time has two values that depend on the voltage at
ZCV. If VZCV is greater than 0.7V, then the time is
2.5S nominal. If VZCV is less than 0.7V, then the time
is 42S nominal. Turn-on of Q1 can not occur during
the suppression time, but does occur after a
zero-crossing is detected. In the case of a missed
zero-crossing, a maximum off-time is implemented.
After Q1 has been off for 42S nominal (toffMax), it is
turned back on.
Once VCC reaches VVCC_on (typically 18V), the device
will initiate a soft-start sequence. This is intended to
minimize the electrical stresses on Q1, DOUT, DVCC
and the transformer. The soft-start operates as shown
in Figure 3. The duration of this soft-start is 12mS
nominal and steps VCS, the current sense voltage, to
four values, as shown.
,
Maximum Current (Sense Voltage)
During Soft-Start
1.1
2.3.2 Switch-Off Determination
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
In the application circuit the primary current is sensed
by RCS. The voltage across this resistor, VCS , is
applied to the CS input of the device. It is processed
internally (VCSINT = VCS(3) + 0.7V), and compared to
the voltage at the VR pin, which is a scaled version of
the rectified line voltage. When VCSINT > VR, the
power switch Q1 is turned off.
0
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15
Time (ms)
Leading-edge blanking is used to prevent a false
trigger caused by the voltage spike across RCS at the
moment of Q1 turn-on. This blanking time, tBLKCS , is
nominally 330nS. To prevent transformer saturation, a
maximum on-time circuit is implemented. Max on-time
for Q1 (GD=H) is 30S nominal.
Figure 3. Soft-Start
2.3 Normal Operation
Because the IX9908 employs quasi-resonant
operation, its PWM switch-on is set by the zero
crossing of the auxiliary winding voltage, and the
switch-off is set by the current sense voltage.
2.3.3 Foldback Point Correction
When the AC line voltage increases, the Q1 on-time
decreases, which increases the operating frequency.
As a result, with a constant primary current limit, the
output power increases. To provide output power
regulation with respect to line voltage, the internal
foldback point correction circuit varies the VCS limit.
The VCS limit is decreased in response to an increase
in AC line voltage. The relationship between VCSMax
and VIN is shown in Figure 4.
2.3.1 Zero Crossing & Switch-On Determination
As the application schematic on Page 1 shows, the
voltage from the auxiliary winding is connected to the
zero crossing pin, ZCV, through an RC network. This
network provides a delay so that switch-on can occur
at voltage valley thus enhancing efficiency. The
required time delay, t, should be approximately
one-fourth of the oscillation period (determined by
transformer primary inductor and drain-source
capacitance of Q1) minus the propagation delay from
Variation of VCS Limit Voltage
According to the IZC Current
1.1
zero-cross detect to Q1 switch-on, tdelay
.
1.0
0.9
0.8
0.7
tOSC
t =
- tdelay
4
This time delay, t, should be matched by adjusting
the RC network.
tRC = CZCV (RZCV1 // RZCV2
)
0.6
50 100 150 200 250 300 350 400 450
VIN (V)
After Q1 is turned off, its VDS will show some
oscillation. This will also show on the ZCV input. To
avoid a mis-triggered Q1 turn-on, a ringing
Figure 4. VCSMax vs. VIN
R01
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