AP3703 データシートの表示（PDF） - BCD Semiconductor
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AP3703 Datasheet PDF : 11 Pages
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Preliminary Datasheet
LOW-POWER OFF-LINE PRIMARY SIDE REGULATION CONTROLLER
Operation Description
AP3703
Bridge
Vg
VIN
+
C1
VS D1
LM NP
NS IS
VO
+CO
IO
Q1
OUT
AP3703
IP
FB
GND
CS
VAUX
NAUX
RCS
Figure 7. Simplified Flyback Converter Controlled by AP3703
Figure 7 illustrates a simplified flyback converter
controlled by AP3703.
The energy stored in the magnetizing inductance LM
each cycle is therefore:
Constant Primary Peak Current
The primary current ip(t) is sensed by a current sense
resistor RCS as shown in Figure 7.
The current rises up linearly at a rate of:
Eg
=
1
2
×
LM
⋅
Ipk2
......(3)
So the power transferring from the input to the output
is given by:
dip(t) = vg(t)
dt
LM
......(1)
P
=
1
2
×
LM
×
Ipk2
×
f
SW
......(4)
See equation 2
Where the fsw is the switching frequency. When the
peak current Ipk is constant, the output power depends
on the switching frequency fsw.
Ip
0A
Figure 8. Primary Current Waveform
As illustrated in Figure 8, when the current ip(t) rises
up to Ipk, the switch Q1 turns off. The constant peak
current is given by:
Ipk = Vcs
Rcs
......(2)
Constant Voltage Operation
The AP3703 captures the auxiliary winding feedback
voltage at FB pin and operates in constant-voltage
(CV) mode to regulate the output voltage. Assuming
the secondary winding is master, the auxiliary winding
is slave during the D1 on-time and the auxiliary
voltage is given by:
( ) VAUX
=
N AUX
NS
×
Vo + Vd
......(5)
Feb. 2009 Rev. 1.0
BCD Semiconductor Manufacturing Limited
7
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