ON Semiconductor NCP1351B, Примечание к применению

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8
The voltage at which the LLC stops pulsing is set at 295 V.
Using this voltage in Equation 8 yields:
V
BO_bulk2
+
47.465
@
10 3
2.74
@
10
6
)
47.465
@
10
3
@
295
*
0.55
(eq. 10)
V
BO_bulk2
^
4.47 V
Using Equation 6, the value of the lower resistor is obtained:
R
BO_lower
+
1
@
5.84
*
4.47
18.2
@
10
*
6 (4.47
*
1)
(eq. 11)
R
BO_lower
^
21.69 k
W
Using Equation 7, the value of the upper resistor is obtained:
R
BO_upper
+
21.69
@
10
3
@
5.84
*
1
1
(eq. 12)
R
BO_upper
^
75.27 k
W
For R
BO_lower
, a parallel combination of resistors 24 k
W
and 220 k
W
is used and their total value is 21.69 k
W
. For
R
BO_upper
, a parallel combination of resistors 82 k
W
and
910 k
W
is used which results in a resistance of 75.27 k
W
.
As mentioned above, the BO pin has two functions.
The first is V
bulk
monitoring, if the voltage on this pin drops
below 1 V, the NCP1392 stops pulsing after 20
m
s. The other
function is enable, if the voltage on this pin exceeds 2 V,
the IC stops switching after 0.5
m
s and restarts without any
delay if the voltage drops back below 1.9 V (please see the
NCP1392 data sheet for additional information on the
functionality of this pin). This feature is used for skip and
hiccup mode. The voltage to capacitor C46 is connected and
stabilized by Zener diode D22 through resistor R60. This
circuit is used because the BO pin does not accept V
cc
voltage. The function of the skip and hiccup mode will be
explained hereinafter.
The R
t
pin is the only pin used for setting the operating
frequency of this IC. The soft start of this LLC is set by R83,
which dictates the frequency at which soft start begins.
Capacitor C51 dictates the duration of the soft start.
A minimum operating frequency is set by resistor R82 when
no FB signal is available. This frequency is set for cases
when the LLC is not able to keep the output voltage at
a nominal level. It is important to set this resistance at such
a value as to avoid the fall of LLC into the zero current
switching (ZCS) operating area. A maximum operating
frequency is set by R93 and the resistance of OK1.
Skip Mode:
Figure 9. The Skip Mode Schematic
Skip mode improves efficiency by skipping switching
cycles during light loading in order to minimize switching
losses. Skip mode is implemented via the BO pin. If the
output power diminishes, the current flowing through
optocoupler OK1 increases. The increased current creates
a voltage drop on R93. Once this drop reaches the V
BE
of
Q11, Q11 is turned on. Q6 is turned on through diode D25
and R86, the BO pin exceeds 2 V, and NCP1392 stops pulses
immediately. When the output voltage diminishes,
the voltage drop on R93 is lower, Q11 and Q6 are turned off,
the BO pin drops below 1.9 V, and NCP1392 restores output
pulses. The output voltage drop and primary current during
a skip period are depicted in Figure 32 and Figure 33.
NOTE: The skip cycle is an option. If a minimum load for LLC is
defined, it is not necessary to implement this function.