IX9907
INTEGRATED
C
IRCUITS
D
IVISION
2.2 Normal Operation
is 42s nominal. Turn-on of the power switch cannot
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 the power switch has been off for
42s nominal (toffMax), it is turned back on.
Because the IX9907 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.
2.2.1 Zero Crossing & Switch-On Determination
2.2.2 Switch-Off Determination
The application example in Figure 1 shows that 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 a voltage valley, thus enhancing efficiency. The
required time delay, t, should be approximately
one-fourth of the oscillation period (determined by the
inductance of the transformer’s primary and the
drain-source capacitance of the built-in power switch
MOSFET) minus the propagation delay from
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 is turned off.
Leading-edge blanking is used to prevent a false
trigger caused by the voltage spike across RCS at the
moment of power switch turn-on. This blanking time,
zero-cross detect to power switch-on, tdelay
.
t
BLKCS, is nominally 450nS. To prevent transformer
tOSC
t = ---------- – tdelay
saturation, a maximum on-time circuit is implemented.
Max on-time for the power switch is 30s nominal.
4
This time delay, t, should be matched by adjusting
the RC network.
2.2.3 Foldback Point Correction
RZCV1
When the AC line voltage increases, the power switch
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
---------------
tRC = CZCV
RZCV2
After the power switch is turned off, its VDS will show
some oscillation. This will also show on the ZCV input.
To avoid a mis-triggered switch turn-on, a ringing
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
VCSMax and VIN is shown in Figure 3.
Figure 3 VCSMax vs. VIN
Variation of VCS Limit Voltage
With Respect to VIN
1.1
1.0
0.9
0.8
0.7
0.6
50 100 150 200 250 300 350 400 450
VIN (V)
R02
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