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Power Integrations reserves the right to make changes to its products at any time to improve reliability or manufacturability. Power Integrations does

not assume any liability arising from the use of any device or circuit described herein. POWER INTEGRATIONS MAKES NO WARRANTY HEREIN AND

SPECIFICALLY DISCLAIMS ALL WARRANTIES INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A

PARTICULAR PURPOSE, AND NON-INFRINGEMENT OF THIRD PARTY RIGHTS. The products and applications illustrated herein (transformer construction and

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Figure 3. BP/M Pin OVP Function Shuts Down Power Supply.

Figure 2. Operating Efficiency vs. I

Figure 4. Conducted EMI Scan (Line): 115 VAC, Full Load.

When R8 (optional) is installed, it supplies current to the BP/M

pin,whichreducestheno-loadconsumptionofthesupply. When

R5(alsooptional)isinstalled,thecurrentitfeedsintotheEN/UV

pin of the IC enables the input under-voltage lockout function

(UVLO), and sets the start-up threshold at 65 VAC.

The output voltage is determined by the series sum of the

voltages across VR3, R6, and the LED in U2. The values of

R4 and R6 can be adjusted to fine tune the output voltage of the

supply, and a TL431 can be used in place of VR3, for tighter

output voltage regulation.

Key Design Points

• In the TinySwitch-III family of devices, the MOSFET

current limit can be selected from 3 levels by the value

of capacitance on the BP/M pin. This gives the designer a

number of options, which are shown in Figure 2. First, a

TNY279 could be used, with the current limit of a TNY278,

to obtain higher efficiency. Second, a TNY277 could be

used, with the current limit of a TNY278, to deliver the

same power at a lower efficiency. Lastly, a TNY278 could

be used, with the current limit of a TNY279, to extend the

power capability of the supply*.

• The UVLO function integrated into U1 only enables

MOSFET switching to occur once the current flowing

into the EN/UV pin exceeds 25 µA. Changing the UVLO

threshold voltage requires changing the value of R5 so that

25 µA flows through it at the desired threshold voltage.

• IfoutputOVPisnotrequiredandno-loadpowerconsumption

of 150 mW (max, at 265 VAC input) is acceptable, then R7,

D6, C6, R3 and VR2 can be eliminated and the bias winding

removed from T1, for an even lower parts count.

*Note: This supply was not thermally designed to continuously

deliver the extended power range that could be obtained by

operating a TNY278 with its MOSFET current limit set at that

of a TNY279.

72

70

68

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1

1.3 1.4

1.2

1.5

Output Current (A)

76

74

82

84

86

88

80

78

TNY279 (with I

TNY278 (with std I

TNY277 (with I

CEC/ENERGY STAR Minimum

TNY278 (with I

-20

-10

0

-10

20

30

40

50

60

70

80

0.15

1

10

30

Frequency (MHz)

0

250

500

15

0

Time (ms)