Design Idea DI-35
TOPSwitch-GX
16 W Audio Amplifier Power Supply
DI-35
Application
Audio Amplifier
TOP245P
16 V
Power Output
16 W (35 W Peak)
Input Voltage
85-265 VAC
Output Voltage
Topology
Flyback
Device
Figure 1. Audio Amplifier Power Supply.
Design Highlights
No heatsinks required!
77% efficiency (min), @ 85 VAC input/35 W output
Low no-load input power: < 0.7 W @ 230 VAC input
Low component count: 37, excluding I/O connectors
Stable operation with or without 10,000
μ
F 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
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.
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.
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.
OV and UV thresholds are determined by: V
=
225
μ
A
x
(R1+R2) and V
UV
= 50
μ
A
x
(R1+R2).
PI-3731-012604
16 V, 1 A / 2.2 A
RTN
TOU1
C
U2
U2
IN5%
2CX1
250 V
VAC
N
L
C1
100
μ
F
400 V
C2
50 V
C3
100
μ
F
16 V
PU2
1N4007
1N4007
(CY1
0.8 A
9, 10
1
3
6, 7
4
T1
5
R1
2R3
R5
3.3
C4
R4
33
IND5
R2
R7
10
C5
μ
F
1
BD6
1 k
R8
150
R6
33
100 V
3.L2
C9
100
μ
F
25 V
SD7
C10
47
μ
F
25 V
C7
470
μ
F
25 V
C8
470
μ
F
25 V
TOPSwitch-GX
D
S
CONTROL
M
www.powerint.com
March 2004