Design Idea DI-35

TOPSwitch-GX

16 W Audio Amplifier Power Supply

Design Highlights

• 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

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,

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.

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

www.powerint.com

Figure 1. Audio Amplifier Power Supply.

DI-35

®

July 2004

Application

Device

Power Output

Input Voltage

Output Voltage

Topology

Audio Amplifier

TOP245P

16 W (35 W Peak)

85-265 VAC

16 V

Flyback

PI-3731-070704

85-265

VAC

16 V, 1 A /

2.2 A

RTN

L

N

CX1

220 nF

250 VAC

F1

3.15 A

250 V

C1

100

µF

400 V

R1

1 M

Ω

R3

200 k

Ω

1/2 W

R5

3.3

Ω

R7

10

Ω

1

9,10

T1

6,7

4

5

3

R6

33

Ω

1/4 W

R8

150

Ω

R10

1 k

R4

33

Ω

1/4 W

R2

1 M

Ω

C2

100 nF

50 V

C3

100

µF

16 V

C10

47

µF

25 V

U2

PC817A

VR1

IN5244B

5%

C9

100

µF

25 V

C8

470

µF

25 V

C7

470

µF

25 V

C5

1

µF

50 V

C4

1 nF

1 kV

CY1

2.2 nF

(Safety)

C6

470 pF

100 V

D6

BAV20

D5

1N4007G

D7

SB5100

L1

6.8 mH

0.8 A

L2

3.3

µH

U1

TOP245P

D1-D4

1N4007

TOPSwitch-GX

D

S

C

M

CONTROL