TOP232-234
35 W Multiple Output Power Supply
pin instead of the SOURCE pin in video noise sensitive
applications to allow for heavier snubbing without significant
impact on efficiency.
Figure 25 shows a five output, 35 W, secondary regulated
powersupplyutilizingaTOP233formultipleoutputapplications
such as set-top box, VCR, DVD, etc. The circuit shown is
designedfora230VACinputbutcanbeusedovertheuniversal
range at a derated output power of 25 W. Alternatively, a
doubler input stage can be used at 100 or 115 VAC for the full
power rating of 35 W. TOPSwitch-FX provides several
advantages in the above mentioned applications.
This design achieves 5% load regulation on 3.3 V and 5 V
outputsusingdualsensedoptocouplerfeedbackthroughresistors
R9,R10andR11.Otheroutputvoltagesaresetbythetransformer
turnsratio. Outputvoltageonthelowpower-5Voutputisshunt
regulated by resistor R12 and Zener diode VR2. Dummy load
resistor R13 is required to maintain regulation of the 30 V
outputunderlightloadconditions. CompensationoftheTL431
(U3) is achieved with resistor R8 and capacitor C7. Primary
side compensation and auto-restart frequency are determined
byresistorR5andcapacitorC5. SecondstageLCpost-filtering
is used on the 3.3 V, 5 V and 18 V high power outputs (L2, L3,
L4 and C13, C15, C17) for low ripple. Full load operating
efficiency exceeds 75% across the AC input range. Primary
clamp components VR1 and D1 limit peak drain voltage to a
safe value.
A single line sense resistor R1 (2 MΩ) implements an under-
voltagedetect(at100V), over-voltageshutdown(at450V)and
line feed forward with DCMAX reduction features. Under-
voltage detect ensures that the outputs are glitch free on power
down. The over-voltage shutdown turns off the TOPSwitch-FX
MOSFET above 450 V on the DC input rail, eliminating
reflected voltage and leakage inductance spikes, and hence,
extending the surge withstand to the 700 VDC rating of the
MOSFET. This feature prevents field failures in countries
where prolonged line voltage surges are common.
The frequency jittering in TOPSwitch-FX helps reduce EMI,
maintainingemissionsbelowCISPR22(FCCB)levelsthrough
properchoiceofY1capacitor(CY1)andinputfilteringelements
(CX1, L1). To minimize coupling of common mode transients
totheTOP233, Y1capacitoristiedtothepositiveinputDCrail.
Lightning strike immunity to 3 kV is attained with the addition
of a 275 V MOV (RV1).
Thisdesignalsotakesadvantageofsoft-startandhigheroperating
frequency to reduce transformer size. A snubber circuit (R4,
C4) is used to slowdown dv/dt of the switching waveform
minimizing radiated video noise that could interfere with TV
reception. The half frequency option of the TOPSwitch-FXcan
beusedbyconnectingtheFREQUENCYpintotheCONTROL
30 V
@ 100 mA
C11
D8
MUR120
C10
L2
3.3 µH
1 µF
100 µF
18 V
@ 550 mA
50 V
50 V
C12
220 µF
25 V
D9
UF5402
C13
100 µF
25 V
R13
24 kΩ
CY1
2.2 nF
L3
3.3 µH
5 V
@ 2.5 A
D10
MBR1045
C14
C15
100 µF
10 V
1000 µF
L4
3.3 µH
3.3 V
@ 3 A
25 V
VR1
P6KE200
D11
BYW29-
100
C16
C17
100 µF
10 V
1000 µF
25 V
D1
UF4007
BR1
400 V
RTN
C18
C19
330 µF
VR2
1N5231
D12
1N5819
R12
5 Ω
-5 V
100 µF
10 V
@ 100 mA
10 V
C1
L1
20 mH
33 µF
400 V
D2
1N4148
R6
51 Ω
R10
15.0 kΩ
R1
2 MΩ
U2
LTV817
T1
R7
510 Ω
1/2 W
CX1
0.1 µF
250 VAC
C4
C6
100 nF
R9
9.53 kΩ
47 pF
TOPSwitch-FX
D
S
M
R4
2 kΩ
CONTROL
C7
R8
F1
3.15 A
C
10 Ω 0.1 µF
TOP233Y
U1
R5
6.8 Ω
F
J1
U3
TL431CLP
C5
47 µF
C8
22 µF
L
RV1
275 V
R11
10.0 kΩ
N
PI-2536-040401
Figure 25. 35 W Set-Top Box Power Supply.
B
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PAM [ POWER ANALOG MICOELECTRONICS ]
