NCP1072STBT3G(2012) 查看數據表（PDF） - ON Semiconductor
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NCP1072STBT3G Datasheet PDF : 26 Pages
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NCP1072, NCP1075
installed from the drain to the bulk voltage. We will
see how to calculate it later on.
4. Calculate the maximum operating duty−cycle for
this flyback converter operated in CCM:
dmax
+
NǒVout ) VfǓ
NǒVout ) VfǓ ) Vin,min
+
1
)
1
Vin,min
NǒVout)VfǓ
+ 0.44
(eq. 5)
5. To obtain the primary inductance, we have the
choice between two equations:
where
L + ǒVindǓ2
fswKPin
(eq. 6)
K + DIL
ILavg
and defines the amount of ripple we want in CCM
(see Figure 36).
♦ 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 worse, but smaller inductance,
leading to a better leakage inductance.
From Equation 6, a K factor of 1 (50% ripple), gives
an inductance of:
(127 0.44)2
L+
+ 3.8 mH
65k 1 12.75
DIL
+
Vin,min @ dmax
LFSW
+
127
3.8
0.44
65k
+ 223 mA peak−to−peak
The peak current can be evaluated to be:
Ipeak
+
Iavg
d
)
DIL
2
+
Ipeak
+
98m
0.44
)
DIL
2
+ 335 mA
On IL, ILavg can also be calculated:
ILavg
+
Ipeak
*
DIL
2
+
0.34
*
0.112
+
223
mA
6. Based on the above numbers, we can now evaluate
the conduction losses:
Ǹ ǒ Id,rms +
d
Ipeak
2
*
IpeakDIL
)
DIL
3
2
Ǹ ǒ + 0.44 0.3352 * 0.335 @ 0.223 ) 0.2232
3
If we take the maximum Rds(on) for a 125°C junction
temperature, i.e. 24 W, then conduction losses worse
case are:
Pcond + Id,rms 2RDS(on) + 570 mW
7. Off−time and on−time switching losses can be
estimated based on the following calculations:
ǒ Ǔ Ipeak Vbulk ) Vclamp toff
Poff +
2Tsw
0.335 (127 ) 120 @ 2) 10n
+
2 15.4m
(eq. 7)
+ 36 mW
Where, assume the Vclamp is equal to two times of
reflected voltage.
ǒ Ivalley Vbulk ) NǒVout ) VfǓǓton
Pon +
6Tsw
0.111 (127 ) 100) 20n
+
6 15.4m
(eq. 8)
+ 5.5 mW
It is noted that the overlap of voltage and current seen
on MOSFET during turning on and off duration is
dependent on the snubber and parasitic capacitance
seen from drain pin. Therefore the toff and ton in
Equations 7 and 8 have to be modified after
measuring on the bench.
8. The theoretical total power is then 0.570 + 0.036 +
0.0055 = 611 mW
9. If the NCP107X operates at DSS mode, then the
losses caused by DSS mode should be counted as
losses of this device on the following calculation:
PDSS + ICC1 @ Vin,max + 1m @ 375 + 375 mW
(eq. 9)
MOSFET protection
As in any Flyback design, it is important to limit the drain
excursion to a safe value, e.g. below the MOSFET BVdss
which is 700 V. Figure 37a, b, c present possible
implementations:
+ 154 mA
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