TOP232-234
achieved by turning the power supply off when the input
voltagegoesbelowalevelneededtomaintainoutputregulation
and keeping it off until the input voltage goes above the
under-voltage threshold (VUV), when the AC is turned on again.
The under voltage threshold is set at 200VDC, slightly below
the required lowest operating DC input voltage, for start-up at
170VAC. This feature saves several components needed to
implement the glitch free turn off with discrete or
TOPSwitch-II based designs.
17 W PC Standby Power Supply
Figure 26 shows a 17 W PC standby application with 3.3 V and
5 V secondary outputs and a 15 V primary output. The supply
uses the TOP232 operating from 230 VAC or 100/115 VAC
with doubler input. This design takes advantage of the soft-
start, line under-voltage detect, tighter current limit variation
andhigherswitchingfrequencyfeaturesofTOPSwitch-FX. For
example, the higher switching frequency with tighter current
limit variation allows use of an EE19 transformer core.
Furthermore, thespacingbetweenhighvoltageDRAINpinand
low voltage pins of the TOPSwitch-FX packages provides large
creepage distance which is a significant advantage in high
pollution environments such as fan cooled PC power supplies.
The bias winding is rectified and filtered by D2 and C6 to create
a bias voltage for the TOP232 and to provide a 15V primary
bias output voltage for the main power supply primary control
circuitry. Both 3.3V and 5V output voltages are sensed by R9,
R10 and R11 using a TL431 (U3) circuit shown. Resistor R6
limits current through optocoupler U2 and sets overall AC
control loop gain. Resistor R7 assures that there is sufficient
biascurrentfortheTL431whentheoptocouplerisataminimum
current. CapacitorC8providesasoft-finishfunctiontoeliminate
turn-on overshoot. The no load regulation (cycle-skipping) of
TOPSwitch-FX permits the circuit to meet the low standby
power requirement of the Blue Angel specification for PCs.
Capacitor C1 provides high frequency decoupling of the high
voltage DC supply, and is necessary only if there is a long trace
length from the source of the DC supply to the inputs of this
standby circuit. The line sense resistor R1 senses the DC input
voltage for line under-voltage. When AC is turned off, the
under-voltage detect feature of the TOPSwitch-FX prevents
auto-restart glitches at the output caused by the slow discharge
of large storage capacitance in the main converter. This is
CY1
1 nF
L1
3.3 µH
5 V
C10
1000 µF
10 V
C11
100 µF
10 V
@ 2 A
D3
L2
3.3 µH
+
SB540
VR1
BZY97C-200
3.3 V
@ 2 A
C12
1000 µF
10 V
C13
100 µF
10 V
D4
SB540
R1
200-375
VDC
3.9 MΩ
RTN
D1
UF4005
15 V
D2
BAV21
@ 30 mA
R6
301 Ω
(Primary
Referenced)
C1
R7
0.01 µF
C6
35 V
510 Ω
T1
1 kV
R9
16.2 kΩ
(optional)
U2
SFH615-2
D
M
TOPSwitch-FX
U1
TOP232Y
R10
12.1 kΩ
CONTROL
C
C7
R5
6.8 Ω
0.1 µF
S
F
C8
10 µF
35 V
U3
TL431CLP
R11
10 kΩ
C5
47 µF
-
PI-2537-040401
Figure 26. 17 W PC Standby Supply.
B
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PAM [ POWER ANALOG MICOELECTRONICS ]
