NCP1340 Datasheet, PDF (13/32 Pages) ON Semiconductor – High-Voltage, Quasi-Resonant, Controller Featuring Valley Lock-Out Switching

NCP1340
−
⎛
⎢
⎝
N_AUX
N_P
. _VBULK
Figure 9. Auxiliary Winding Voltage
Since the auxiliary winding voltage during the power
switch on time is a reflection of the input voltage scaled by
the primary to auxiliary winding turns ratio, N_P:AUX (see
Figure 9), OPP is achieved by scaling down reflected
voltage during the on−time and applying it to the ZCD pin
as a negative voltage, V_OPP. The voltage is scaled down by
a resistor divider comprised of R_OPPU and R_OPPL. The
maximum internal current setpoint (V_CS(OPP)) is simply the
sum of V_OPP and the peak current sense threshold, V_ILIM1.
Figure 8 shows the schematic for the OPP circuit.
The adjusted peak current limit is calculated using
Equation 2. For example, a V_OPP of −150 mV results in a
peak current limit of 650 mV in NCP1340.
V_CS(OPP) + V_OPP ) V_ILIM1
(eq. 2)
To ensure optimal zero−crossing detection, a diode is
needed to bypass R_OPPU during the off−time. Equation 3 is
used to calculate R_OPPU and R_OPPL.
R_ZCD ) R_OPPU
R_OPPL
+
*
N_P:AUX
@ V_bulk
V_OPP
*
V_OPP
(eq. 3)
R_OPPU is selected once a value is chosen for R_OPPL.
R_OPPL is selected large enough such that enough voltage is
available for the zero−crossing detection during the
off−time. It is recommended to have at least 8 V applied on
the ZCD pin for good detection. The maximum voltage is
internally clamped to V_CC. The off−time voltage on the ZCD
Pin is given by Equation 4.
V_ZCD
+
R_OPPL
R_ZCD ) R_OPPL
@
ǒ_VAUX
*
V_F^Ǔ
(eq. 4)
Where V_AUX is the voltage across the auxiliary winding
and V_F is the D_OPP forward voltage drop.
The ratio between R_ZCD and R_OPPL is given by
Equation 5. It is obtained by combining Equations 3 and 4.
R_ZCD
R_OPPL
+
V_AUX
* V_F *
V_ZCD
V_ZCD
A design example is shown below:
System Parameters:
(eq. 5)
V_AUX + 18 V
V_F + 0.6 V
N_P:AUX + 0.18
The ratio between R_ZCD and R_OPPL is calculated using
Equation 5 for a minimum V_ZCD of 8 V.
R_ZCD
R_OPPL
+
18
V
*
0.6 V
8V
*
8
V
+
1.2
kW
R_ZCD is arbitrarily set to 1 kW. R_OPPL is also set to 1 kW
because the ratio between the resistors is close to 1.
The NCP1340 maximum overpower compensation or
peak current setpoint reduction is 31.25% for a V_OPP of
−250 mV. We will use this value for the following example:
Substituting values in Equation 3 and solving for R_OPPU
we obtain:
R_ZCD ) R_OPPU
R_OPPL
+
0.18
@
370 V * (−0.25
−0.25 V
V)
+
271
R_OPPU + 271 @ R_OPPL * R_ZCD
R_OPPU + 271 @ 1 kW * 1 kW + 270 kW
For optimum performance over temperature, it is
recommended to keep R_OPPL below 3 kW.
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