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Design Idea DI-42

(Buck/Boost) Converter

September 2002

DI-42

®

Application

Appliances

TNY254P

12 V

Power Output

1.2 W

Input Voltage

Output Voltage

Topology

Device

85-265 VAC

Buck/Boost

Figure 1. TinySwitch – TNY254P, 1.2 W, 12 V 100 mA.

.

Design Highlights

• Lowest cost non-isolated flyback converter

• Inherently short circuit & thermal overload protected

• Excellent line and load regulation characteristics

• Operates at 85 ˚C, without forced cooling

•Meets EN61000-4-5 (2 kV line surge – simple

modification provides a 6 kV line surge withstand)

• Input AC line is fed directly to the output

• Available for prototyping in DAK-7

Circuit Features

This non-isolated Flyback (Buck-Boost) converter has been

implemented using a TinySwitch TNY254P, which minimizes

cost, PCB space and design time.

The most typical application for the circuit of Figure 1 is

powering a TRIAC drive and associated circuitry, in a consumer

or industrial appliance, having TRIAC controlled motors.

The combined voltage drops of Zener diode (VR1) and

optocoupler diode (U2) set the output voltage. Due to the digital

nature of the TinySwitch output voltage regulation scheme, the

current transfer ratio (CTR) of the optocoupler is not critical and

there is no analog control loop to compensate. The low

switching frequency under light and no-load conditions

provides very low power consumption

under these

conditions – see Figures 2 and 3.

Providing power for driving TRIACs normally requires the

output of the supply to be referenced to the input line as

shown in Figure 1. Therefore, only half-wave rectification

is used and the switch must be in the return or neutral side of

the primary switching circuit. In these applications, EMI

filtering is normally done at the system level and is therefore

not shown here.

The circuitry shown allows this converter to meet 2 kV surge

requirements according to IEC standards. To extend surge

withstand to 6 kV, an MOV must be added between the Line

input and the node where D3 connects to R2. Additionally,

D3 must be replaced with 100

Ω, 5 W surge-rated resistor

(See DAK-7 for details).

The high efficiency of this converter (see Figure 4) allows

the circuit to operate to very high ambient temperatures of up

to 85 ˚C, as sometimes specified in household appliances and

industrial applications.

®

+

SD

BP EN

R2

180 Ω, 3 W

Fusible

U2

85 - 265

VAC

L

N

+12 VDC

PI-3280-090502

TNY254P

TinySwitch

D3

1N4007

D1

1N4007

D2

UF4005

C1

6.8 uF

400 V

L1

1.5 mH

C3

220 uF

25 V

C2

0.1 uF

50 V

U2

SFH615-2

VR1

1N5342B