NCP1606
http://onsemi.com
13
out the bulk voltage ripple, then this on time is truly
constant over the ac line cycle.
Note that the maximum on time of the controller occurs
when V
CONTROL
is at its maximum. Therefore, the Ct
capacitor must be sized to ensure that the required on time
can be delivered at full power and the lowest input voltage
condition. The maximum on time is given by:
t
ON(max)
+
Ct @ V
CTMAX
I
CHARGE
(eq. 4)
Combining this equation with equation 1, gives:
Ct w
2 @ P
OUT
@ L @ I
CHARGE
h @ Vac
RMS
2
@ V
CTMAX
(eq. 5)
where V
CTMAX
= 2.9 V (min)
I
CHARGE
= 297 mA (max)
OFF TIME SEQUENCE
While the on time is constant across the ac cycle, the off
time in CRM operation varies with the instantaneous input
voltage. The NCP1606 determines the correct off time by
sensing the inductor voltage. When the inductor current
drops to zero, the drain voltage ( Vd in Figure 23) is
essentially floating and naturally begins to drop. If the
switch is turned on at this moment, then CRM operation
will be achieved. To measure this high voltage directly on
the inductor is generally not economical or practical.
Rather, a smaller winding is taken off of the boost inductor.
This winding, called the zero current detector (ZCD)
winding, gives a scaled version of the inductor output and
is more useful to the controller.
Figure 28. Voltage Waveforms for Zero Current
Detection
DRIVE
Winding
Pin
Drain
0.6 V
V
OUT
5.7 V
2.1 V
1.6 V
ZCD
Figure 28 gives typical operating waveforms with the
ZCD winding. When the drive is on, a negative voltage
appears on the ZCD winding. And when the drive is off, a
positive voltage appears. When the inductor current drops
to zero, then the ZCD voltage falls and starts to ring around
zero volts. The NCP1606 detects this falling edge and starts
the next driver on time. To ensure that a ZCD event has
truly occurred, the NCP1606s logic (Figure 29) waits for
the ZCD pin voltage to rise above V
ZCDH
(2.1 V typical)
and then fall below V
ZCDL
(1.6 V typical). In this way,
CRM operation is easily achieved.
Figure 29. Implementation of the ZCD Winding
ZCD
+
200 mV
+
2.1 v
VCL(POS)
Clamp
Shutdown
Demag
VCL(NEG)
Active
Clamp
+
1.6 V
Reset
Dominant
Latch
R
Q
S
DRIVE
R
SENSE
R
ZCD
V
DD
V
in
N
ZCD
Q
N
B
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