NCL37733
Line Range Detection
As sketched in Figure 6, this circuit detects the low−line
soon as the V pin voltage exceeds V (2.0 V typical).
These levels roughly correspond to 152 V rms and
160 V rms line voltages if the external resistors divider
S
HL
range if the V pin remains below the V threshold (1.9 V
S
LL
typical) for more than the 25 ms blanking time. The
high−line range is detected (“HL” of Figure 6 is high) as
applied to the V pin is designed to provide a 1 V peak value
S
at 80 V rms.
Input Voltage
Rail
V
S
reset
+
HL
25 ms
blanking time
LL
−
V
HL
V
LL
if HL is low
if HL is high
Figure 6. Line Range Detection Circuitry
In the low−line range, conduction losses are generally
dominant. Adding a dead−time would further increase these
losses by forcing increased switching current. In high−line
conditions, switching losses generally are the most critical.
It is thus efficient to skip one valley to lower the switching
frequency. Hence, under normal operation, the NCL37733
optimizes the efficiency over the line range by turning on the
MOSFET at the first valley in low−line conditions and at the
second valley in the high−line case. This is illustrated by
Figure 7 that sketches the MOSFET Drain−source voltage
in both cases.
Figure 7. Quasi−resonant Mode in Low Line (left), Turn on at Valley 2 when in High Line (right)
In addition, the gain of the current control block is divided
by two when the high−line range is detected. This allows for
an optimal resolution of the output current over the line
range.
current. Now, the MOSFET cannot turn off at the very
moment when the current−sense voltage exceeds V
There actually exists a propagation delay for which the
primary current keeps rising. As a result, the primary current
.
control
does not exactly peak to the expected ( V
value but to a higher level. The NCL37733 features the line
feedforward function to compensate for this effect.
/ R
)
control
SENSE
Line Feedforward
The NCL37733 computes the current setpoint (V
for power factor correction and proper regulation of the LED
)
control
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