NCP1028 ON Semiconductor, NCP1028 Datasheet - Page 25

• Manufacturer Part Number: NCP1028
• Description: High?voltage Switcher For Medium Power Offline Smps
• Manufacturer: ON Semiconductor

•
an inductance of:
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…
DI L +
CV
I peak +
From Equation 16, a K factor of 0.8 (40% ripple), gives
The peak current can be evaluated to be:
In Figure 43,
Large K: approaching BCM where the rms losses are
the worse, but smaller inductance, leading to a better
leakage inductance.
CC
5. Based on the above numbers, we can now
I d, rms + I 1 d
HV
LF SW
1 ) 1
evaluate the conduction losses:
I 1 + I peak -
V in d
+
I avg
d
3
L +
+ 120
2
)
I
3.8m
1
0.258
60k
1
2
3
4
DI L
2
can also be calculated:
0.318
(120
DI L
2
+ I peak + 156m
1 ) 1
a.
0.8
0.49
+ 0.447- 0.129 + 318 mA
60k
2 + 228 mA rms
3
0.49) 2
+ 258 mA peak * to * peak
DI L
2I 1
8
7
6
5
18.75
Figure 44. Different Options to Clamp the Leakage Spike
2 + 0.318
0.49
+ 3.8 mH
C
)
CV
DI L
2
CC
HV
+ 447 mA
0.7
+
http://onsemi.com
R
clamp
NCP1028
1
2
3
4
25
temperature, i.e. 11 W, then conduction losses worse case
are:
= 723 mW.
Power Switch Circuit Protection
excursion to a safe value, e.g. below the power switch
circuit BVdss which is 700 V. Figures 44a, b, c present
possible implementations:
b.
Figure 44b: The most standard circuitry called the RCD
network. You calculate
following formulae:
P off +
If we take the maximum R
The theoretical total power is then 0.571 + 0.13 + 0.022
As in any Flyback design, it is important to limit the drain
C
clamp
6. Off-time and on-time switching losses can be
7. The ramp compensation will be calculated as
R clamp +
8
7
6
5
D
estimated based on the following calculations:
suggested by Equation 13 giving a resistor of
78 kW or 82 kW for the normalized value.
I peak V ds t off
P cond + I 2 d, rms R ds(on) + 571 mW
P on +
6T SW
C clamp +
+ 0.447
2V clamp (V clamp - (V out ) V f ) N)
CV
I peak N(V out ) V f )t on
CC
HV
+ 0.447
6
+
L peak I 2 peak F SW
V ripple F SW R clamp
6T SW
R
114
15u
clamp
DS(on)
6
V clamp
1
2
3
4
40n
650
and
15u
for a 120°C junction
Dz
+ 22 mW
c.
C
40n
clamp
+ 130 mW
8
7
6
5
D
using the
(eq. 18)
(eq. 19)
(eq. 20)
(eq. 21)