NCP1028 ON Semiconductor, NCP1028 Datasheet - Page 22

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

connected from pin 2 to ground, to impose a current
the transistor collector.
following:
where Vp, the total voltage swing, equals 2.75 V.
Application example:
Vout = 5.0 V
Vf = 1.0 V
The principle consists in selecting the RR resistor,
The equation to get the right compensation level is the
Suppose we have the following flyback specifications:
Figure 38. Maximum Peak Current Setpoint
Variations versus Ramp Compensation
RR +
output voltage
secondary diode forward drop
@ Iout nominal
S a @T SW
V p 2.75 k
Figure 39. 1.0 ms Soft-Start Sequence
http://onsemi.com
(eq. 9)
I
RR
NCP1028
in
22
Np:Ns = 1:N = 1:0.052 transformer turn ratio
Lp = 3.8 mH
to evaluate
primary inductance. The slope is projected over a complete
switching period. Here, we use a 65 kHz part.
will become a voltage slope having a value of:
compensation ramp will present a slope of:
We then have:
purposely been omitted to avoid bringing in another
variable. In case no ramp compensation is required, pin 2
must be tied to V
Soft-Start
reduces the power-on stress, but also contributes to lower
the output overshoot. Figure 39 shows a typical operating
waveform. The NCP1028 features a novel patented
structure which offers a better soft-start ramp, almost
ignoring the startup pedestal inherent to traditional
current-mode supplies.
S off +
We can calculate the off slope, the one actually needed
Due to the internal sense arrangement, this current slope
If we chose 50% of this downslope, then the final
In the above calculations, the internal ESD resistor has
The NCP1028 features a 1.0 ms soft-start, which
RR +
V out ) V f
S off + 455m
NL p
S
S a @T SW
a
V p 2.75 k
, by reflecting the output voltage over the
S a + 170m
CC
T SW +
, the adjacent pin.
2
+ 2.75
primary inductance
0.375 + 170 mV 15 ms
0.052
+ 85 mV 15 ms
6
85m
2.75k
15u
3.8m
+ 89 kW
+ 455 mA 15 ms
(eq. 10)
(eq. 12)
(eq. 11)
(eq. 13)