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NCP1351
Let us round it to 0.25 or 1/N = 4
From Equation 17, a K factor of 0.8 (40% ripple) ensures a
good operation over universal mains. It leads to an
inductance of:
I peak
L +
(100
65 k
43)2
0.8 72
+ 493 m H
(eq. 23)
I 1
D I L
D IL +
Vin_mind max
LFSW
+
100
493 u
0.43
65 k
(eq. 24)
+ 1.34 A peak-to-peak
I valley
The peak current can be evaluated to be:
Iin_avg +
Pout
h Vin_min
+
19
0.8
3
100
+ 712 mA
(eq. 25)
I avg
Ipeak +
Iavg
d
)
D IL
2
+
0.712
0.43
)
1.34
2
+ 2.33 A
(eq. 26)
t
On Figure 26, I 1 can also be calculated:
DT SW
T SW
II + Ipeak *
D IL
2
+ 2.33 *
1.34
2
+ 1.65 A
(eq. 27)
Figure 26. Primary Inductance Current Evolution
The valley current is also found to be:
in CCM
Ivalley + Ipeak * D IL + 2.33 * 1.34 + 1.0 A
(eq. 28)
2. Calculate the maximum operating duty-cycle for
this flyback converter operated in CCM:
4. Based on the above numbers, we can now evaluate
the RMS current circulating in the MOSFET and
the sense resistor:
d max +
Vout N
Vout N ) Vin_min
+
19
19 4
4 ) 100
+ 0.43
(eq. 21)
Id_rms + II d
1 )
1 D IL 2
3 2I1
In this equation, the CCM duty-cycle does not exceed
50%. The design should thus be free of subharmonic
+ 1.65
0.65
1 )
1
3 2
1.34
1.65
2
(eq. 29)
oscillations in steady-state conditions. If necessary,
negative ramp compensation is however feasible by the
auxiliary winding.
3. To obtain the primary inductance, we can use the
+ 1.1 A
5. The current peaks to 2.33 A. Selecting a 1 V drop
across the sense resistor, we can compute its value:
following equation which expresses the inductance
in relationship to a coefficient k. This coefficient
Rsense +
1
Ipeak
+
1
2.5
+ 0.4 W
(eq. 30)
L +
actually dictates the depth of the CCM operation.
If it goes to 2, then we are in DCM.
(Vin_mind max)2
(eq. 22)
FSWKPin
To generate 1 V, the offset resistor will be 3.7 k W , as already
explained. Using Equation 29, the power dissipated in the
sense element reaches:
Psense + Rsense Id_rms2 + 0.4 1.12 + 484 mW
where K = D I L /I I and defines the amount of ripple we want
in CCM (see Figure 26).
? 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.
(eq. 31)
6. To switch at 65 kHz, the C t capacitor connected to
pin 2 will be selected to 180 pF.
7. As the load changes, the operating frequency will
automatically adjust to satisfy either equation 5
(high power, CCM) or equation 6 in lighter load
conditions (DCM).
Figure 27 portrays a possible application schematic
implementing what we discussed in the above lines.
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