Datasheet

(ovp) ZCD1
ZCD2
B
OUT(pk) (ovp)
S
ZCD R
R =
N
V × -ZCD
N
´
æ ö
ç ÷
è ø
OUT F
B
ZCD1
S
V + V
N
R =
100 A N
´
m
R
Z CD 1
R
ZC D 2
ZCD
N
B
N
P
N
S
Zero
Current
Detect
Output
Voltage
Sense
5V
PWM
Flip-Flop
Fault Timing
and Control
OV
Fault
2
C
Z C D
UCC28610
www.ti.com SLUS888F –JANUARY 2009–REVISED SEPTEMBER 2012
Zero crossing is detected using a resistive divider across the bias winding, as shown in Figure 29. The bias
winding operates in phase with the output winding. The ZCD function detects transformer demagnetization when
the ZCD voltage has a high to low crossing of the 20-mV ZCD threshold, ZCD
TH
. The voltage at the ZCD pin is
internally clamped to contain negative excursions at -160mV (ZCD
CLAMP
). A small delay, 50 ns to 200 ns, can be
added with C
ZCD
to align the turn-on of the primary switch with the resonant valley of the primary winding
waveform.
Figure 29. Zero Crossing Detection.
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