SNVS856B – JUNE 2012 – REVISED MAY 2013
V
SW
Low Freq state
Steady state
Startup
state
t
V
CC
26V
t
V
ZCD
V
ZCD-OVP
V
ZCD-ARM
t
Figure 15. Start up Bias Waveforms
Zero Crossing Detection
To minimized the switching loss of the power MOSFET, a zero crossing detection circuit is embedded in the
TPS92314/14A. V
LAUX
is AC voltage coupled from V
SW
by means of the transformer, with the lower part of the
waveform clipped by D
ZCD
. V
LAUX
is fed back to the ZCD pin to detect a zero crossing point through a resistor
divider network which consists of R
AUX1
and R
AUX2
. The next turn on time of Q
1
is selected V
SW
is the minimum,
an instant corresponding to a small delay after the zero crossing occurs. (Figure
The actual delay time
depends on the drain capacitance of the Q
1
and the primary inductance of the transformer (L
P
). Such delay time
is set by a single external resistor as described in Delay Setting section.
During the off-period at steady state, V
ZCD
reaches its maximum V
ZCD-PEAK
which is scalable by the
turn ratio of the transformer and the resistor divider network R
AUX1
and R
AUX2
. It is recommended that V
ZCD-PEAK
is set to 3V during normal operation.
n
u
V
LED
+V
IN
n
u
V
LED
V
SW
t
DLY
Figure 16. Switching Node Waveforms
10
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