NCP1351 ON Semiconductor, NCP1351 Datasheet - Page 20

• Manufacturer Part Number: NCP1351
• Description: Variable Off Time Controller Featuring Short-circuit Protection
• Manufacturer: ON Semiconductor

Let us round it to 0.25 or 1/N = 4
50%. The design should thus be free of subharmonic
oscillations in steady-state conditions. If necessary,
negative ramp compensation is however feasible by the
auxiliary winding.
where K = DI
in CCM (see Figure 26).
•
•
d max +
L +
In this equation, the CCM duty-cycle does not exceed
Small K: deep CCM, implying a large primary
inductance, a low bandwidth and a large leakage
inductance.
Large K: approaching BCM where the RMS losses are
the worse, but smaller inductance, leading to a better
leakage inductance.
Figure 26. Primary Inductance Current Evolution
2. Calculate the maximum operating duty-cycle for
3. To obtain the primary inductance, we can use the
( V in_min d max ) 2
this flyback converter operated in CCM:
following equation which expresses the inductance
in relationship to a coefficient k. This coefficient
actually dictates the depth of the CCM operation.
If it goes to 2, then we are in DCM.
DT
F SW KP in
SW
V out N ) V in_min
L
/I
I
V out N
and defines the amount of ripple we want
T
SW
in CCM
I
1
+
19
19
4 ) 100
I
I
I
peak
valley
valley
I
4
avg
+ 0.43
(eq. 21)
(eq. 22)
DI
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L
t
NCP1351
20
From Equation 17, a K factor of 0.8 (40% ripple) ensures a
good operation over universal mains. It leads to an
inductance of:
The peak current can be evaluated to be:
On Figure 26,
The valley current is also found to be:
To generate 1 V, the offset resistor will be 3.7 kW, as already
explained. Using Equation 29, the power dissipated in the
sense element reaches:
Figure 27 portrays a possible application schematic
implementing what we discussed in the above lines.
L +
DI L +
I in_avg +
I peak +
I valley + I peak * DI L + 2.33 * 1.34 + 1.0 A
I d_rms + I I d
R sense +
P sense + R sense I d_rms 2 + 0.4
I I + I peak *
4. Based on the above numbers, we can now evaluate
5. The current peaks to 2.33 A. Selecting a 1 V drop
6. To switch at 65 kHz, the
7. As the load changes, the operating frequency will
+ 1.34 A peak-to-peak
65 k
the RMS current circulating in the MOSFET and
the sense resistor:
across the sense resistor, we can compute its value:
pin 2 will be selected to 180 pF.
automatically adjust to satisfy either equation 5
(high power, CCM) or equation 6 in lighter load
conditions (DCM).
( 100
V in_min d max
+ 1.65
+ 1.1 A
I avg
d
LF SW
h V in_min
I peak
0.8
I
P out
1
1
)
43 ) 2
DI L
can also be calculated:
2
DI L
1 )
+
2
0.65
72
+ 2.33 *
2.5
+
+
1
+
+ 493 mH
1
3
0.8
0.712
+ 0.4 W
0.43
493 u
DI L
2I 1
19
100
1 )
2
)
1.34
100
C
3
2
0.43
1.34
t
1
3
65 k
capacitor connected to
2
+ 712 mA
+ 1.65 A
2
1.1 2 + 484 mW
+ 2.33 A
1.34
1.65
2
(eq. 23)
(eq. 24)
(eq. 25)
(eq. 26)
(eq. 30)
(eq. 31)
(eq. 27)
(eq. 28)
(eq. 29)