Design Idea DI-35
®
TOPSwitch-GX
16 W Audio Amplifier Power Supply
Application
Device
Power Output
Input Voltage
Output Voltage
Topology
Audio Amplifier
TOP245P
16 W (35 W Peak)
85-265 VAC
16 V
Flyback
Design Highlights
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.
• No heatsinks required!
• 77% efficiency (min), at 85 VAC input/35 W output
• Low no-load input power: < 0.7 W at 230 VAC input
• Low component count: 37, excluding I/O connectors
• Stable operation with or without 10,000 µF output buffer
capacitor
• Robust transformer design enables 35 W peak power
• Meets CISPR22B EMI with >10 dB of margin
• Ideal for both class AB and class D audio amplifiers,
6 channel Dolby Digital with 6 ohm speakers
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,
increasingoverallefficiency.DRAINringingisdampenedby
R4 (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. ThecurrentthroughtheU2-LEDregulatestheoutput
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.
Key Design Points
• 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.
• A 5% VR1 should be acceptable for an audio amplifier.
Greater accuracy will require a 2% VR1.
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.
• OV and UV thresholds are determined by:
VOV = 225 µA • (R1+R2) and VUV = 50 µA • (R1+R2).
CY1
R6
C6
2.2 nF
(Safety)
33 Ω 470 pF
1/4 W 100 V
R3
C4
C9
100 µF
25 V
L2
3.3 µH
16 V, 1 A /
2.2 A
200 kΩ 1 nF
1/2 W 1 kV
1
3
9,10
6,7
D1-D4
1N4007
D7
SB5100
R1
C1
1 MΩ
100 µF
400 V
R7
10 Ω
C7
C8
RTN
470 µF 470 µF
4
5
R2
1 MΩ
25 V
25 V
R4
33 Ω
C5
1 µF
50 V
R8
150 Ω
1/4 W
D5
L1
6.8 mH
0.8 A
T1
1N4007G
D6
BAV20
TOPSwitch-GX
R10
1 k
D
S
M
U1
TOP245P
C
CX1
CONTROL
220 nF
250 VAC
U2
PC817A
R5
3.3 Ω
F1
C2
100 nF
50 V
3.15 A
VR1
IN5244B
5%
C10
47 µF
25 V
250 V
C3
100 µF
16 V
85-265
VAC
L
N
PI-3731-070704
Figure 1. Audio Amplifier Power Supply.
July 2004
DI-35
www.powerint.com