ON Semiconductor NCP1351B, Application Note

Page: 7 / 28

AND8344/D
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7
NOTE: The main LLC SMPS includes sub-circuits for various
protection features. These can be removed if the given
function is not required.
The voltage used for the main LLC is taken from bulk
capacitors C12 and C13 which are charged by the PFC stage.
A power loop of the LLC is closed through Q1 and Q3, main
transformer TR1, and resonant capacitor(s) C21 (and C2).
The gates of the MOSFETs are protected by R10 and R21.
The NCP1392 features a 600 V high-side drive, which
allows connection of both transistors directly to the
controller. Resistors R33 and R34 damp the gate charging,
suppress overshoots on the gates, and control EMI noise.
The energy required for controlling the high side MOSFET
is taken from bootstrap capacitor C38, whose voltage floats
on the bridge voltage. If Q3 is turned on, the HB pin is
grounded and bootstrap capacitor C38 is charged through
resistor R67 and high-voltage diode D14. The NCP1392
always turns on M
lower
first after any restart of the IC to
charge this bootstrap capacitor. For situations when the
standby is not loaded and the main LLC must be operated at
full load, it is important to self-power the LLC.
Self-powering is ensured by winding W6 of transformer
TR1. The current from W6 is limited by resistor R4, rectified
by diode D1, and connected to C56.
Design of the Brown-Out Divider:
The NCP1392 features a BO pin, which continuously
senses bulk voltage to ensure sufficient voltage is available
on the bulk capacitor for normal operation. To sense BO
voltage it is necessary to use a resistor divider connected to
V
bulk
. If V
bulk
range between 295 V and 375 V is required,
the recommended total resistance of the BO divider is
approximately 4.4 M
W
. If the SMPS is powered from 265 V
ac, the power dissipation on this divider is almost 32 mW.
This power loss contributes to increased losses in standby
mode. Because there is a feedback divider for the PFC stage,
it is possible to save the 32 mW by using an emitter follower
based on Q4.
Because the LLC controller sinks 18.2
m
A from the BO
pin when the V
bulk
is lower than the set level, it is not
possible to connect the BO pin directly to the PFC feedback
divider. As soon as this is connected, the current sunk from
the divider diminishes the voltage on the divider, so the
circuit cannot regulate at the correct voltage level.
The solution is to set the voltage that will be connected to the
base of Q4. The level of this voltage is best kept higher
because of the thermal dependence of the transistor’s V
BE
.
In this application, 6.5 V is chosen as a nominal V
bulk
. It is
necessary as well to keep the emitter voltage above the
voltage of D22 to prevent too high of negative voltage on the
base-emitter junction of Q4:
V
B
+
V
bulk
R
lower
R
lower
)
R
upper
*
I
E
h
FE
@
ǒ
R
lower
@
R
upper
R
lower
)
R
upper
Ǔ
(eq. 1)
Where:
V
B
= Voltage on Base Q4
h
FE
= dc Current Gain of Q4
V
bulk
= Nominal Bulk Voltage
R
lower
= serial-parallel combination R39, R47, R48 and R51
and is calculated by:
R
lower
+
1
1
R47
@
R48
R47
)
R48
)
R39
)
1
R51
(eq. 2)
R
upper
= serial combination of resistors R7, R11, R15 and
R28, which is calculated by:
R
upper
+
R7
)
R11
)
R15
)
R28
(eq. 3)
I
E
= current from the emitter of Q4, which is calculated by:
I
E
+
V
BO
@
(R87
)
R88)
R87
@
R88
)
(V
E
*
V
BO
)
@
(R53
)
R54)
R53
@
R54
(eq. 4)
Where:
V
BO
= BO voltage of the NCP1392B, which is 1.0 V
V
E
= voltage on the emitter of Q4, which is:
V
E
+
V
B
*
V
BE
(eq. 5)
Because the right side of Equation 1 can reach only a very
low value and h
FE
depends on transistor Q4 (which
according to the datasheet ranges from 250 to 600), this
value can be set approximately to 0.4
m
A for 385 V
bulk
and
linearly decreases with the voltage on the base to zero. This
value has been measured in this application. Once the V
B
value is known, we can determine the resistors necessary to
set V
bulk_ON
and V
bulk_OFF
.
According to the datasheet, the equation for determining
R
lower
of the BO pin is as follows:
R
BO_lower
+
Vref
BO
@
V
BO_bulk1
*
V
BO_bulk2
I
BO
@
(V
BO_bulk2
*
Vref
BO
)
(eq. 6)
And for R
upper
:
R
BO_upper
+
R
BO_lower
@
V
BO_bulk2
*
Vref
BO
Vref
BO
(eq. 7)
Where:
Vref
BO
= 1.0 V (see Datasheet)
I
BO
= 18.2
m
A (see Datasheet)
V
BO_bulk
+
R
lower
R
upper
)
R
lower
@
V
bulk
*
V
BE
(eq. 8)
V
bulk
is the voltage at which the LLC can start. This LLC
should start at 375 V.
If the values are put into Equation 8 then:
このを
8
に すると、のようになります。
V
BO_bulk1
+
47.465
@
10 3
2.74
@
10 6
)
47.465
@
10 3
@
375
*
0.55
(eq. 9)
V
BO_bulk1
^
5.84 V