NCP1230A Datasheet, PDF (13/21 Pages) ON Semiconductor – Low--Standby Power Soft Skip Mode Controller

NCP1230A
Feedback
The feedback pin has been designed to be connected
directly to the open--collector output of an optocoupler. The
pin is pulled--up through a 20 kΩ resistor to the internal Vdd
supply (6.5 volts nominal). The feedback input signal is
divided down, by a factor of three, and connected to the
negative (--) input of the PWM comparator. The positive (+)
input to the PWM comparator is the current sense signal
(Figure 30).
The NCP1230A is a peak current mode controller, where
the feedback signal is proportional to the output power. At
the beginning of the cycle, the power switch is turns--on and
the current begins to increase in the primary of the
transformer, when the peak current crosses the feedback
voltage level, the PWM comparators switches from a logic
level low, to a logic level high, resetting the PWM latching
Flip--Flop, turning off the power switch until the next
oscillator clock cycle begins.
Vdd
20k
FB
55k
2
-- PWM
10 V
25k
+
2.3 Vpp
Ramp
18k
3
LEB
Figure 30.
The feedback pin input is clamped to a nominal 10 volt for
ESD protection.
Soft Skip Mode
The feedback input is connected in parallel with the skip
cycle logic (Figure 32). When the feedback voltage drops
below 25% of the maximum peak current (1.0 V/Rsense) the
IC prevents the current from decreasing any further and
starts to blank the output pulses. This is called the soft skip
mode. While the controller is in the soft skip mode the power
transfer now depends upon the duty cycle of the pulse burst
width which reduces the average input power demand.
When the controller determines that more energy is
required to meet the output power demands, the NCP1230A
goes through a soft--start cycle ramping up the current to the
output. (Refer to Figure 31)
Figure 31.
When the feedback signal determines that the desired
output power has been reached, the NCP1230A
immediately turns off the skip mode (it does not ramp down
the output current). This Soft Skip Mode is ideal for
applications where a low standby power is required and a
minimization of any audible noise that may be associated
with magnetics and snubber components is needed.
Vc = Ipk ⋅ Rs ⋅ 3
where:
V_c = control voltage (Feedback pin input),
I_pk = Peak primary current,
R_s = Current sense resistor,
3 = Feedback divider ratio.
SkipLevel = 3V ⋅ 25% = 0.75V
Ipk
=
0.75
Rs ⋅ 3
where:
Ipk ⋅ Rs = 1V
 Ipk =
2 ⋅ Pin
Lp ⋅ f
where:
P_in = is the power level where the NCP1230A will go into
the skip mode
L_p = Primary inductance
f = NCP1230A controller frequency
Pin
=
Lp
⋅
f⋅
2
Ipk2
Pin
=
Pout
Eff
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