NCP1219PRINTGEVB
http://onsemi.com
7
The average HV pin voltage, V
AVG(half
−
wave)
, is
calculated using Equation 26.
V
AVG(half
*
wave)
+
V
Peak
p
(eq. 26)
In comparison, using the example from option 2
(full
−
time DSS mode with the HV pin connected to V
bullk
),
power dissipation, P
DSS
, of 270 mW, and a junction
temperature of 107
°
C is achieved.
The techniques mentioned above can be explored in
different combinations to optimize standby power and
thermal performance of the NCP1219.
Feedback Network
The negative feedback loop that controls the output
voltage senses the output voltage using a voltage divider and
compares it to the internal reference voltage of a TL431
precision reference. The output current of the TL431 is then
a function of the bias that is required to force the internal
reference of the TL431 and the output voltage to be equal.
The TL431 output drives the cathode of an optocoupler,
providing isolation between the primary and secondary side
of the converter. The collector of the optocoupler is
connected to the FB pin of the NCP1219, closing the
feedback loop, as shown in Figure 8.
Figure 8. Feedback Network
V
FB
is compared to V
CS
to determine the on time. If there
is an increase in load current, V
1
begins to decrease with
V
out
. This causes I
1
to decrease. The optocoupler collector
current, I
2
, also decreases causing V
FB
to increase,
increasing on time for the next switching cycle. The timing
diagram describing the feedback loop is shown in Figure 9.
Figure 9. Feedback Loop Timing Diagram
time
I
out
V
out
,V
1
I
1
,I
2
V
FB
I
L,pri
Standby Reconfiguration Control
The evaluation board has a dual output voltage mode. In
normal operation, the converter provides a 24 V regulated
output. During standby mode, the output supplies 7.25 V
with a standby current of 70 mA. The output voltage level
is selected by actively altering the voltage divider supplying
the feedback loop. An additional resistor is connected in
series with R32. A small signal MOSFET (Q6) is placed in
parallel with the added resistance, as shown in Figure 10.
