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

Output Voltage

Topology

Device

85-265 VAC

Flyback

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

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

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

µA x (R1+R2) and V

PI-3731-012604

16 V, 1 A / 2.2 A

RTN

U1

TOP245P

U2

U2

VR1

IN5244B

5%

CX1

220 nF

250 VAC

F1

3.15 A

250 V

85-265

VAC

N

L

C1

100 µF

400 V

C2

100 nF

50 V

C3

100 µF

16 V

U2

PC817A

D1 & D2

1N4007

D3 & D4

1N4007

CY1

2.2 nF

(Safety)

L1

6.8 mH

0.8 A

9, 10

1

3

6, 7

4

T1

5

R1

1 M

R3

200 k

1/2 W

R5

3.3 Ω

C4

1 nF

1 KV

R4

33 Ω

1/4 W

D5

IN4007G

R2

1 M

R7

10 Ω

C5

1 µF

50 V

D6

BAV20

R10

1 k

R8

150 Ω

R6

33 Ω

1/4 W

C6

470 pF

100 V

L2

3.3 uH

C9

100 µF

25 V

D7

SB5100

C10

47 µF

25 V

C7

470 µF

25 V

C8

470 µF

25 V

TOPSwitch-GX

D

S

C

CONTROL

M

www.powerint.com

March 2004