Design Idea DI-123

TinySwitch-III

9.65 W, Dual Output Power Supply for Residential

Heating Control

Figure 1. TinySwitch-III 9.65 W Residential Heating Controller Power Supply.

DI-123

September 2006

Application

Device Power Output Input Voltage

Output Voltage

Topology

Residential Heating Control TNY275P

9.65 W

185-265 VAC 5V / 0.25 A, 24 V / 0.35 A Flyback

Design Highlights

• Simple, low cost design only requires 38 parts

• <150 mW no-load consumption at 230 VAC input

• Meets CEC / ENERGY STAR active-mode efficiency

and no-load input power requirements

• Both outputs exhibit good cross regulation

• Meets CISPR-22 Class B/EN55022 B without input

X or Y capacitors or an input common mode choke

Operation

The flyback converter shown in Figure 1 uses a TNY275

(U1) to provide two output voltages: 5 V at 250 mA and

24 V at 350 mA. Typical applications are residential heating

controllers (furnace) or any application where 2 outputs are

required. The 9.65 W of output power is delivered by using

the increased current limit of U1, which is selected by the

value of C11.

The MOSFET integrated within U1 switches the primary of

transformer T1. Each time it turns on, the primary current

ramps until it reaches an internal current limit and the

MOSFETisturnedoff. ThentheenergystoredinT1transfers

to the secondary where it is rectified and filtered by D1 and

C2 and D4 and C5. The primary side RCD clamp (D8, C4

R1 and R8) limits the peak DRAIN voltage spike caused by

transformer leakage inductance.

Power Integrations E-Shield™ transformer construction

techniques, the RCD clamp, a snubber (R11 and C14), a

simple π filter (C7, C8, L1 and L5) and the frequency jitter

function of the TinySwitch-III family, provide good EMI

margin, even with the output return connected to safety earth

ground (see Figure 3).

Key Design Points

• The turns ratio of the two secondary winding was

optimized for output voltage centering.

• The use of a fast instead of an ultrafast diode for

D8 improves efficiency by recovering some leakage

inductance energy.

®

U2

PC817A

C12

47

µF

35 V

C2

220

µF

35 V

C6

100

µF

16 V

C10

100 nF

C5

330

µF

10 V

C13

470 pF

C4

1 nF

1 kV

C14

33 pF

1 kV

C11

10

µF

50 V

C15

0.1

µF

50 V

C8

10

µF

400 V

C7

10

µF

400 V

24 V, 350 mA

L

N

185-265

VAC

5 V, 250 mA

RTN

D8

FR106

D1 UF4003

D4 1N5819

U3

LM431

2%

L3, L4 and L5 are

3.5 mm × 10 mm

Ferrite beads.

F1

1 A

D2

1N4937

D3

1N4007

D5

1N4937

D7

1N4007

L2

330

µH

PI-4495-090706

D

S

EN/UV

BP/M

U1

TNY275P

TinySwitch-III

L5

FB

L4

FB

1

10

6

8, 9

3

L3

FB

R11

100

Ω

R12

30

Ω

R1

100

Ω

R8

75 k

Ω

R2

47

Ω

R3

15 k

Ω

1%

R9

261 k

Ω

1%

R4

1 k

Ω

R5

3.3 k

Ω

R10

10

Ω

1/2 W

T1

EF20

R6

10 k

Ω

1%