Datasheet

NCP1028
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25
• 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:
L +
(120 0.49)
2
60k 0.8 18.75
+ 3.8mH
DI
L
+
V
in
d
LF
SW
+
120 0.49
3.8m 60k
+ 258mApeak * to * peak
The peak current can be evaluated to be:
I
peak
+
I
avg
d
)
DI
L
2
+ I
peak
+
156m
0.49
)
DI
L
2
+ 447mA
In Figure 43, I
1
can also be calculated:
I
1
+ I
peak
-
DI
L
2
+ 0.447-0.129 + 318mA
5. Based on the above numbers, we can now
evaluate the conduction losses:
I
d, rms
+ I
1
d
Ǹ
1 )
1
3
ǒ
DI
L
2I
1
Ǔ
2
Ǹ
+ 0.318 0.7
1 )
1
3
ǒ
0.258
2 0.318
Ǔ
2
Ǹ
+ 228mArms
If we take the maximum R
DS(on)
for a 120°C junction
temperature, i.e. 11 W, then conduction losses worse case
are:
P
cond
+ I
2
d, rms
R
ds(on)
+ 571mW
6. Off-time and on-time switching losses can be
estimated based on the following calculations:
P
off
+
I
peak
V
ds
t
off
6T
SW
+
0.447 650 40n
6 15u
+ 130mW
(eq. 18)
P
on
+
I
peak
N(V
out
) V
f
)t
on
6T
SW
(eq. 19)
+
0.447 114 40n
6 15u
+ 22mW
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 kW or 82 kW for the normalized value.
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:
+
1
2
3
4
5
7
8
Figure 44. Different Options to Clamp the Leakage Spike
6
CV
CC
C
HV
+
1
2
3
4
5
7
8
6
CV
CC
HV
R
clamp
C
clamp
D
+
1
2
3
4
5
7
8
6
CV
CC
HV
D
Dz
a. b. 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
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
capacitors increase capacitive losses…
Figure 44b: The most standard circuitry called the RCD
network. You calculate R
clamp
and C
clamp
using the
following formulae:
R
clamp
+
2V
clamp
(V
clamp
-(V
out
) V
f
)N)
L
peak
I
2
peak
F
SW
(eq. 20)
C
clamp
+
V
clamp
V
ripple
F
SW
R
clamp
(eq. 21)