Design Idea DI-35
16 W Audio Amplifier Power Supply
Power Output Input Voltage
16 W (35 W Peak) 85-265 VAC
• 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 µF output buffer
• 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
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
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
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: VOV =
D1 & D2
D3 & D4
200 k 1 nF
1/2 W 1 KV
1 9, 10
33 Ω 470 pF
1/4 W 100 V
4 10 Ω
470 µF 470 µF
25 V R8
S C2 3.3 Ω
Figure 1. Audio Amplifier Power Supply.
25 V 16 V, 1 A / 2.2 A