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When output voltage decreases, secondary diode
conduction time is increased and the linear frequency
control lengthens the switching period, which retains
DCM operation in the wide output voltage range, as
shown in Figure 18. The frequency control lowers the
primary rms current with better power efficiency in the
full-load condition.
nV O
L m
Figure 20 shows operational waveforms in short-LED
condition. Output voltage is quickly lowered to 0V right
after a short-LED event. Then the reflected auxiliary
voltage is also 0 V, making V S less than 0.4 V. 0.2 V
OCP level limits primary-side current and V DD hiccups
up and down between UVLO hysteresis.
V O
t DIS
n
3
4
L m
t
V
4
3
t DIS
n
4
3
3
5
L m
t
O
5
3
t DIS
5
3
t
Figure 20.
Waveforms in Short-LED Condition
Figure 18.
Primary and Secondary Current
Open-LED Protection
BCM Control
The end of secondary diode conduction time is possibly
over a switching period set by linear frequency control.
In this case, FL7732 doesn’t allow CCM and the
operation mode changes from DCM to BCM. Therefore,
magnetizing inductance can be largely designed to add
BCM for better efficiency if PF and THD meet
specification with enough margin.
Short-LED Protection
In case of a short-LED condition, the switching
MOSFET and secondary diode are stressed by the high
powering current. However, FL7732 changes the OCP
level in a short-LED condition. When V S voltage is lower
than 0.4 V, OCP level becomes 0.2 V from 0.7 V, as
shown in Figure 19, so powering is limited and external
components current stress is reduced.
FL7732 protects external components, such as diode
and capacitor, at secondary side in open-LED condition.
During switch-off, the V DD capacitor is charged up to the
auxiliary winding voltage, which is applied as the
reflected output voltage. Because the V DD voltage has
output voltage information, the internal voltage
comparator on the VDD pin can trigger output Over-
Voltage Protection (OVP), as shown in Figure 21. When
at least one LED is open-circuited, output load
impedance becomes very high and output capacitor is
quickly charged up to V OVP x N S / N A Then switching is
shut down and the V DD block goes into Hiccup Mode
until the open-LED condition is removed, as shown
in Figure 22.
Figure 19.
Internal OCP Block
? 2011 Fairchild Semiconductor Corporation
FL7732 ? Rev. 1.0.6
10
www.fairchildsemi.com
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