ncp1239f ON Semiconductor, ncp1239f Datasheet - Page 32

• Manufacturer Part Number: ncp1239f
• Description: Lowstandby High Performance Pwm Controller
• Manufacturer: ON Semiconductor

Suppose we select a 65 kHz version. Over a 65 kHz
frequency, it corresponds to a 130 mV/ms ramp. In our
FLYBACK design, let’s assume that our primary inductance
Lp is 350 mH, and the SMPS delivers 12 V with a Np:Ns
ratio of 1:0.1. The OFF time slope of the primary current is:
projected over a 0.1 W Rsense for instance. If we select 75%
of the down−slope as the required amount of ramp
compensation, then we shall inject 27 mV/ms. Our internal
compensation being of 208 mV/ms, the divider ratio
(divratio) between R
of
(Vout ) Vf) @
R ramp +
In the NCP1239, the ramp features a swing of 3.2 V.
Lp
algebra
19 k @ divratio
(1 * divratio)
Np
Ns
that is, 371 mA/ms or 37 mV/ms, once
ramp
The V
to
and the 32 kW is 0.178. A few lines
+ 6.92 kW
pin6
ramp is used to introduce frequency jittering on the oscillator saw−tooth
determine
62.4kHz
.
Internal
ramp
65kHz
10ms
http://onsemi.com
R
ramp
Figure 53.
NCP1239
:
32
low). This inhibition avoids that the ramp compensation
modifies the setpoint above which the NCP1239 enables
PFC.
Frequency Jittering
signature by spreading the energy in the vicinity of the main
switching component. NCP1239 offers a +3.5% deviation of
the nominal switching frequency. The sweep saw−tooth is
internally generated and modulates the clock up and down
with a period depending on the Pin 6 capacitor (10 ms
typically with 390 nF, 10 mS * Cpin6 / 390 nF in general).
Again, if one selects a 65 kHz version, the frequency will
equal 65 kHz in the middle of the ripple and will increase as
V
portrays the behavior we have adopted:
67.6kHz
pin6
The ramp is disabled during standby (i.e., when pfcON is
Frequency jittering is a method used to soften the EMI
rises or decrease as V
Internal
sawtooth
pin6
ramps down. Figure 53