Rev. C 09/12

8

TNY375-380

www.powerint.com

Applications Examples

The circuit shown in Figure 14 is a low cost universal AC input,

four-output flyback power supply utilizing a TNY376. The

continuous output power is 7.5 W with a peak of 13 W. The

output voltages are 3.3 V, 5 V, 12 V, and –12 V.

The rectified and filtered input voltage is applied to the primary

winding of T1. The other side of the transformer’s primary is

driven by the integrated MOSFET in U1. Diode D5, C3, R1, R2,

and VR1 compose the clamp circuit, limiting the leakage

inductance turn-off voltage spike on the DRAIN pin to a safe

value. The use of a combination Zener clamp and parallel RC

optimizes both EMI and energy efficiency.

Both the 3.3 V and 5 V outputs are sensed through resistors R6

and R7. The voltage across R8 is regulated to 2.5 V by reference

IC U3. If the voltage across R8 begins to exceed 2.5 V, then

current will flow in the LED inside the optocoupler U2, driven by

the cathode of U3. This will cause the transistor of the

optocoupler to sink current from the EN/UV pin of U1. When the

current exceeds the ENABLE pin threshold current, the next

switching cycle is inhibited. Conversely, when the voltage across

resistor R8 falls below 2.5 V, and the current out of the ENABLE

pin is below the threshold, a conduction cycle is allowed to

occur. By adjusting the number of enabled cycles, regulation is

maintained. As the load reduces, the number of enabled cycles

decreases, lowering the effective switching frequency and

scaling switching losses with load. This provides almost

constant efficiency down to very light loads, ideal for meeting

energy efficiency requirements.

The input filter circuit (C1, L1 and C2) reduces conducted EMI. To

shielding techniques in the transformer, reducing common mode

displacement currents, and reducing EMI. These techniques,

combined with the frequency jitter of TNY376, give excellent EMI

performance, with this design achieving >10 dBmV of margin to

EN55022 Class B conducted EMI limits.

For design flexibility, the value of C4 can be selected to pick one

of the three current limit options in U4. Doing so allows the

designer to select the current limit appropriate for the application.

•

Standard current limit is selected with a 0.1 mF BP/M pin

capacitor and is the normal choice for typical applications.

•

When a 1 mF BP/M pin capacitor is used, the current limit is

reduced, offering reduced RMS device currents and therefore

improved efficiency, but at the expense of maximum power

capability. This is ideal for thermally challenging designs where

dissipation must be minimized.

•

When a 10 mF BP/M pin capacitor is used, the current limit is

increased, extending the power capability for applications

requiring higher peak power or continuous power where the

thermal conditions allow.

Further flexibility comes from the current limits between adjacent

TinySwitch-PK family members being compatible. The reduced

current limit of a given device is equal to the standard current limit

of the next smaller device, and the increased current limit is equal

to the standard current limit of the next larger device.

Figure 14. TNY376PN, Four Output, 7.5 W, 13 W Peak Universal Input Power Supply.

D9

UF4003

U1

TNY376P

U2A

LTV817A

U3

L431

2%

C1

22

µF

400 V

C2

22

µF

400 V

C3

10 nF

1 kV

C4

10

µF

50 V

C11

47

µF

25 V

C14

100 nF

50 V

C13

10

µF

50 V

C9

1000

µF

10 V

C5

220

µF

25 V

C10

470

µF

10 V

C8

470

µF

10 V

C6

100

µF

25 V

C7

1000

µF

25 V

C12

220

µF

25 V

U2B

LTV817A

JP1

JP2

C5

330 pF

250 VAC

R1

100

Ω

R4

200

Ω

1/2 W

R6

20 k

Ω

1%

R5

1 k

Ω

R7

6.34 k

Ω

1%

R8

10 k

Ω

1%

R9

3.3 k

Ω

R2

47

Ω

R3

1

Ω

1/2 W

L1

5 mH

L2

3.3

µH

L3

3.3

µH

L4

3.3

µH

F1

3.15 A

85-265

VAC

+12 V, 0.25 A

+5.0 V, 0.5 A

+3.3 V, 0.5 A

RTN

-12 V, 0.03 A

L

N

D1

FR106

D2

FR106

D5

FR106

D6

UF4003

D7

1N5819

D8

SB340

VR1

P6KE180A

D3

1N4007

D4

1N4007

PI-4673-012009

T1

EEL19

1

N.C.

6

11

7

8,9,10

12

4

3

5

D

S

EN/UV

BP

TinySwitch-PK