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NCP1028
L +
+ 3.8 mH
+ 120
0.49 + 258 mA peak * to * peak
D IL +
+ 0.447
40n + 130 mW
Poff +
+ Ipeak + 156m )
Ipeak +
Iavg D IL D IL
) + 447 mA
I1 + Ipeak-
D IL
+ 0.447-0.129 + 318 mA
Pon +
+ 0.447
40n + 22 mW
? Large K: approaching BCM where the rms losses are
the worse, but smaller inductance, leading to a better
leakage inductance.
From Equation 16, a K factor of 0.8 (40% ripple), gives
an inductance of:
(120 0.49)2
60k 0.8 18.75
Vind
LFSW 3.8m 60k
The peak current can be evaluated to be:
d 2 0.49 2
In Figure 43, I 1 can also be calculated:
2
5. Based on the above numbers, we can now
evaluate the conduction losses:
If we take the maximum R DS(on) for a 120 ° C junction
temperature, i.e. 11 W , then conduction losses worse case
are:
Pcond + I2d, rms Rds(on) + 571 mW
6. Off-time and on-time switching losses can be
estimated based on the following calculations:
IpeakVdstoff 650
6TSW 6 15u
(eq. 18)
IpeakN(Vout ) Vf)ton
6TSW (eq. 19)
114
6 15u
The theoretical total power is then 0.571 + 0.13 + 0.022
= 723 mW.
7. The ramp compensation will be calculated as
suggested by Equation 13 giving a resistor of
78 k W or 82 k W for the normalized value.
1 ) 1
2 + 228 mA rms
HV
3 2 0.318
0.258
Id, rms + I1 d 1 ) 1 D IL 2 + 0.318
3 2I1
0.7
HV
Power Switch Circuit Protection
As in any Flyback design, it is important to limit the drain
excursion to a safe value, e.g. below the power switch
circuit BVdss which is 700 V. Figures 44a, b, c present
possible implementations:
HV
R clamp
C clamp
D
Dz
D
1
2
3
8
7
6
1
2
3
8
7
6
1
2
3
8
7
6
CV CC +
4
5
CV CC +
4
5
CV CC +
4
5
C
a.
b.
Figure 44. Different Options to Clamp the Leakage Spike
c.
Figure 44a: The simple capacitor limits the voltage
according to Equation 14. This option is only valid for low
power applications, e.g. below 5.0 W, otherwise chances
Figure 44b: The most standard circuitry called the RCD
network. You calculate R clamp and C clamp using the
following formulae:
exist to destroy the MOSFET. After evaluating the leakage
inductance, you can compute C with Equation 15. Typical
values are between 100 pF and up to 470 pF. Large
Rclamp +
2Vclamp(Vclamp-(Vout ) Vf) N)
LpeakI2peak FSW
(eq. 20)
capacitors increase capacitive losses …
Cclamp +
Vclamp
VrippleFSWRclamp
(eq. 21)
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