鈥?/div>
The EMI filter has been significantly simplified to three
components (L1, CX1 and CY1), thanks to the built-in
frequency jitter function of the
TOPSwitch-GX
family.
The RCD network of C4, R3, R4, and D5 limits (clamps) the
peak voltage that the U1-DRAIN sees. A glass-passivated
normal recovery diode (D5) recycles stored leakage energy,
increasing overall efficiency. DRAIN ringing is dampened
by R4 (which is necessary when a normal recovery diode is
used).
The bias winding provides operating power and feedback
current to the CONTROL pin of U1. The U2-LED, R8 and
VR1 comprise the feedback circuit that senses the output
voltage. The current through the U2-LED regulates the
output voltage, by modulating the feedback current that
flows into the CONTROL pin of U1.
Operation
Many of the built-in
TOPSwitch-GX
features have been used to
reduce the component count, transformer size, and overall cost
of this universal input flyback power supply. Designed around
a TOP245P (DIP-8 package), the device uses PCB copper for
its heatsink, eliminating the costs of both a heatsink and the
installation labor. Output diode D7 is also cooled the same way.
Resistors R1 and R2 set the nominal Under-Voltage (UV)
lockout and Overvoltage (OV) shutdown limits to 84 V and
378 V, respectively. UV lockout protects the supply from
overheating at low line and eliminates power-up and power-
down glitches. OV shutdown protects the power supply from
line surges.
Key Design Points
鈥?Ensure that the control loop has enough phase margin
at the extremes of the rated input voltage and output
load, both with and without the output buffer capacitor.
鈥?A 5% VR1 should be acceptable for an audio amplifier.
Greater accuracy will require a 2% VR1.
鈥?OV and UV thresholds are determined by: V
OV
=
225
碌A(chǔ)
x
(R1+R2) and V
UV
= 50
碌A(chǔ)
x
(R1+R2).
CY1
2.2 nF
(Safety)
1
9, 10
D7
SB5100
R7
10
鈩?/div>
C7
C8
470
碌F
470
碌F
25 V 25 V R8
C5
150
鈩?/div>
1
碌F
50 V
U2
PC817A
U2
U2
R10
1k
RTN
R6
C6
33
鈩?/div>
470 pF
1/4 W 100 V
L2
3.3 uH
R3
200 k
1/2 W
D1 & D2
1N4007
C1
100
碌F
400 V
D3 & D4
1N4007
L1
6.8 mH
0.8 A
CX1
220 nF
250 VAC
F1
3.15 A
250 V
85-265
VAC
L
N
S
C4
1 nF
1 KV
C9
100
碌F
25 V
16 V, 1 A / 2.2 A
R1
1M
3
6, 7
4
R2
1M
R4
33
鈩?/div>
1/4 W
D5
IN4007G
M
5
T1
D6
BAV20
D
CONTROL
TOPSwitch-GX
U1
TOP245P
C
C2
100 nF
50 V
R5
3.3
鈩?/div>
C3
100
碌F
16 V
VR1
IN5244B
5%
C10
47
碌F
25 V
Figure 1. Audio Amplifier Power Supply.
DI-35
PI-3731-012604
www.powerint.com
March 2004
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