Rev. B 03/10

17

TOP264-271

www.powerint.com

Very Low No-load, High Efficiency, 30 W, Universal Input,

Open Frame, Power Supply

The circuit shown in Figure 25 below shows an 85 VAC to

265 VAC input, 12 V, 2.5 A output power supply. The goals of

the design were highest full load efficiency, average efficiency

(average of 25%, 50%, 75% and 100% load points), very low no-

load consumption. Additional requirements included latching

output overvoltage shutdown and compliance to safety agency

limited power source (LPS) limits. Actual efficiency and no-load

performance is summarized in the table shown in the schematic

which easily exceed current energy efficiency requirements.

In order to meet these design goals the following key design

decisions were made.

PI part selection

•

Ambient of 40 °C allowed one device size smaller than

indicated by the power table

The device selected for this design was based on the 85-265 VAC,

Open Frame, PCB heat sinking column of power table (Table 1).

One device size smaller was selected (TOP266V vs TOP267V)

due to the ambient specification of 40 °C (vs the 50°C assumed

in the power table) and the optimum PCB area and layout for

the device heatsink. The subsequent thermal and efficiency

data confirmed this choice. The maximum device temperature

was 107°C at full load, 40 °C, 85 VAC, 47 Hz (worst case

conditions) and average efficiency exceeded 83% ENERGY

STAR and EuP Tier 2 requirements.

Transformer Core Selection

•

132 kHz switching frequency allowed the selection of smaller

core for lower cost

The size of the magnetic core is a function of the switching

frequency. The choice of the higher switching frequency of

132 kHz allowed for the use of a smaller core size. The higher

switching frequency does not negatively impact the efficiency in

TOPSwitch-JX designs due its small drain to source capacitance

(C

Line Sense Resistor Values

•

Increasing line sensing resistance from 4 M

W to 10.2 MW to

reduce no-load input power dissipation by 16 mW

Line sensing is provided by resistors R1 and R2 and sets the

line undervoltage and overvoltage thresholds. The combined

value of these resistors was increased from the standard 4 M

W

to 10.2 M

W. This reduced the resistor, and therefore contribution

to no-load input power, from ~26 mW to ~10 mW. To compensate

the resultant change in the UV threshold resistor R12 was

added between the CONTROL and VOLTAGE-MONITOR pins.

This adds a DC current equal to ~16

mA into the V pin, requiring

only 9

mA to be provided via R1 and R2 to reach the V pin UV

threshold current of 25

mA and setting the UV threshold to

approximately 95 VDC.

This technique does effectively disable the line OV feature as

the resultant OV threshold is raised from ~450 VDC to ~980

VDC. However in this design there was no impact as the value

of input capacitance (C3) was sufficient to allow the design to

withstand differential line surges greater than 1 kV without the

peak drain voltage reaching the BV

Specific guidelines and detailed calculations for the value of R12

may be found in the TOPSwitch-JX Application Note.

Figure 25. Schematic of High Efficiency 12 V, 30 W, Universal Input Flyback Supply With Very Low No-load.

PI-5775-030810

R17

22

Ω

R23

10 k

Ω

1%

R21

86.6 k

Ω

1%

D10

LL4148

R19

470

Ω

R18

110

Ω

U3

LMV431A

1%

D5

FR107

D6

BAV19WS

VR3

ZMM5245B-7

R16

6.8

Ω

1/8 W

R9

10

Ω

R12

191 k

Ω

1%

R5

10 k

Ω

1/2 W

U2B

LTV817D

U2A

LTV817D

L1

14 mH

U1

TOP266VG

C10

47

µF

25 V

C18

47 nF

50 V

C7

47

µF

25 V

D7

BAV21WS-

7-F

D8,9

SB560

C9

100 nF

50 V

D1

1N4007

D2

1N4007

D3

1N4007

D4

1N4007

C14

680

µF

25 V

C15

680

µF

25 V

L2

3.3

µH

C16

100

µF

25 V

C12

1 nF

200 V

7,8

11,12

1

2

NC

T1

EF25

C11

1 nF

250 VAC

C1

100 nF

275 VAC

F1

3.15 A

TOPSwitch-JX

L

N

12 V, 2.5 A

RTN

6

4

C3

82

µF

400 V

C4

4.7 nF

1 kV

VR1

P6KE180A

85 - 264

VAC

D

S

C

V

F

X

CONTROL

R1

5.1 M

Ω

R3

10 M

Ω

R2

5.1 M

Ω

R15

14.3 k

Ω

1%

R4

10 M

Ω

C20

33 nF

50 V

NC

Input Voltage (VAC)

85

230

Full Load Efficiency (%)

81.25

86.21

Average Efficiency (%)

85.13

No-load Input Power (mW)

60.8

115

83.94

84.97

61.98 74.74