FSDH321 Datasheet, PDF (11/20 Pages) Fairchild Semiconductor – Green Mode Fairchild Power Switch (FPS TM)

FSDH321, FSDL321
5. Soft Start : The FPS has an internal soft start circuit that
slowly increases the feedback voltage together with the
Sense FET current after it starts up. The typical soft start
time is 15msec, as shown in Figure 8, where progressive
increments of the Sense FET current are allowed during the
start-up phase. The pulse width to the power switching
device is progressively increased to establish the correct
working conditions for transformers, inductors, and capaci-
tors. The voltage on the output capacitors is progressively
increased with the intention of smoothly establishing the
required output voltage. It also helps to prevent transformer
saturation and reduce the stress on the secondary diode.
Drain current
0.7A
1ms
15steps
Current limit
0.4A
t
Figure 8. Soft Start Function
V_FB
V_BURH
V_BURL
Current
Waveform
Burst
Operation
Burst
Operation
Normal
Operation
Switching
OFF
Switching
OFF
Vfb
3
+
VBURH -
VBURL/VBURH
Vcc Vcc
I_DELAY
I_FB
Vcc
I_BUR(pk)
Normal
2.5R
R
Burst
PWM
MOSFET
Current
Figure 9. Burst Operation Function
6. Burst Operation : In order to minimize power dissipation
in standby mode, the FPS enters burst mode operation. As
the load decreases, the feedback voltage decreases. As
shown in Figure 9, the device automatically enters burst
mode when the feedback voltage drops below
VBURH(500mV). Switching still continues but the current
limit is set to a fixed limit internally to minimize flux density
in the transformer. The fixed current limit is larger than that
defined by VFB = VBURH and therefore, VFB is driven
down further. Switching continues until the feedback voltage
drops below VBURL(350mV). At this point switching stops
and the output voltages start to drop at a rate dependent on
the standby current load. This causes the feedback voltage to
rise. Once it passes VBURH(500mV), switching resumes.
The feedback voltage then falls and the process repeats.
Burst mode operation alternately enables and disables
switching of the power Sense FET thereby reducing switch-
ing loss in Standby mode.
7. Frequency Modulation : Modulating the switching fre-
quency of a switched power supply can reduce EMI. Fre-
quency modulation can reduce EMI by spreading the energy
over a wider frequency range than the bandwidth measured
by the EMI test equipment. The amount of EMI reduction is
directly related to the depth of the reference frequency. As
can be seen in Figure 10, the frequency changes from 97KHz
to 103KHz in 4ms for the FSDH321 (48.5KHz to 51.5KHz
for FSDL321). Frequency modulation allows the use of a
cost effective inductor instead of an AC input mode choke to
satisfy the requirements of world wide EMI limits.
f_s=1/t_s
103kHz
100kHz
97kHz
Drain
Current
t_s
4ms
t
Figure 10. Frequency Modulation Waveform
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