NCP1219PRINTGEVB
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5
Auxiliary Supply Regulator
The HV pin of the NCP1219 can be tied directly to the
bulk storage capacitor and used to supply the IC in the
absence of an auxiliary winding, for instance, during the
startup of the adapter. The startup current is controlled
internally and supplied to the V
CC
capacitor through the
V
CC
pin. While V
CC
is less than the Inhibit threshold
voltage, the V
CC
capacitor is charged with a current source
of 200
m
A (typical). Once the inhibit threshold is exceeded,
the startup current (typically 13.5 mA) is supplied to the
V
CC
capacitor. When V
CC(on)
is exceeded, the internal
current source is disabled, and the V
CC
capacitor is
discharged until V
CC
decreases to less than V
CC(MIN)
, at
which time the startup current source is enabled, starting the
DSS cycle over again.
The evaluation board contains several options for the HV
pin connection and the biasing of V
CC
. An auxiliary winding
is used to supply V
CC
at high line conditions in order to
satisfy the low standby power requirement of 1 W.
Option 1 – Bulk Connection with Forward Auxiliary
Winding
Connecting the HV pin to the bulk voltage and using a
forward auxiliary winding provides an IC bias dependant on
input voltage, but independent of the output voltage. This is
required in this design due to the dual output voltage design.
Otherwise the converter would require additional circuitry
to prevent the converter from entering DSS mode during the
standby conditions. Figure 3 shows this configuration. The
voltage is supplied by the auxiliary winding through a series
diode.
Figure 3. V
CC
Connection Using a Forward Auxiliary
Winding with DSS at Low
−
Line
Skip/
latch
FB
CS
GND
VCC
HV
NCP1219
D12
V
out
DRV
The voltage on the V
CC
pin can not exceed 20 V.
Therefore the ratio between the number of turns on the
auxiliary winding, NA, and the primary winding, N
A
/N
P
, is
chosen to maintain V
CC
below 20 V at the maximum input
voltage. N
A
/N
P
is calculated using Equation 19.
N
A
ń
N
P
+
ǒ
V
CC
)
V
f
Ǔ
V
bulk
(eq. 19)
This implies that, as the input voltage drops, the auxiliary
winding can not supply the IC. When V
CC
reduces to
V
CC(MIN)
, the startup circuit is enabled and the IC bias is
supplied to the V
CC
capacitor by the internal current source.
Alternately, an auxiliary voltage greater than 20 V can be
used by clamping V
CC
using a zener diode, minimizing the
input voltage at which the controller enters DSS mode. This
is shown in Figure 4.
Figure 4. V
CC
Connection using a Forward Auxilliary
Winding with Added Zener
Skip/
latch
FB
CS
GND
VCC
HV
NCP1219
D12
V
out
DRV
Option 2 – Full
−
time DSS Mode (No Auxiliary Winding)
The auxiliary winding is not necessary with DSS mode, so
the connection to the auxiliary winding can be removed
altogether, as shown in Figure 5.
Figure 5. V
CC
Connection with Full
−
Time DSS Mode
(No Auxiliary Winding)
Skip/
latch
FB
CS
DRV
GND
VCC
HV
NCP1219
D12
V
out
If standby power dissipation is not an issue, this option
eliminates the extra components used with the auxiliary
