5 W Universal Input Dual Output Isolated TNY266

03-April-2001

4 Circuit Description

This circuit was designed for Home Appliance applications. The design for this had three main drivers: low

cost, high ambient temperature operation and input voltage surge withstand.

There are two input protection configurations, "1" (2 kV surge) on page 4 and "2" (6 kV) on page 5.

Configuration "1" has a 33

Ω, 3 W fusible resistor (R1) which limits the 2 kV voltage surge current such

that the peak charging voltage on C2 does not exceed the breakdown voltage of U1 (TNY266). R1 also

functions

as

a

fuse,

opening

any

short

that

might

occur

on

the

primary

side.

(Pico II, series 263, Littelfuse or TR5, series 370, Wickmann) can be used (if R1 is unavailable in low

wattage to ensure fusing).

-

Configuration "2" has two 47

Ω, energy rated resistors (R7, R8), which, along with the varistor RV1,

form a voltage divider. The life of RV1 is endless if its energy rating is not exceeded (see Fig. 8.7.1).

The energy rated resistors R7, R8 are not fusible and the short circuit current being limited (~0.9 A at

The efficiency of the 6 kV configuration can be improved at the expense of the total number of 6 kV surges

protection, by reducing the value of R7, R8 up to zero.

Downstream of the input protection circuits, the operation of the two configurations is identical.

In this Home Appliance application (refer to page 4 or 5 of this report), the AC input is rectified and filtered

by D1-D4, C1 and C2 to create a high voltage DC buss which is connected to T1. Inductor L1 forms a pi-

filter in conjunction with C1 and C2. The resistor R2 damps resonance in inductor L1. The frequency jitter in

U1 allows the unit to meet worldwide conducted EMI standards using a simple pi-filter in combination with a

small value Y1-capacitor C4 and a proper PCB layout. The built-in circuitry of U1 practically eliminates the

audio noise permitting the use of ordinary varnished transformers. VR1 and D5 form a clamp circuit that

limits the turn-off voltage spike to a safe level on the U1-DRAIN pin.

The secondary windings are stacked to improve the cross regulation. The 5 V winding is rectified and

voltage is determined by the sum of the voltage drops across the optocoupler U2 and the Zener diode VR2.

Resistor R3 (AC gain of the circuit) limits the current through U2, improving its response time. Resistor R4

output. A minimum loading is necessary on the two outputs to keep them within the specified limits.

The primary-to-secondary isolation is provided by using parts/materials (opto/transformer insulation) with

the correct level of isolation and creepage distances (opto slot/transformer bobbin). Also the C4 value

(while allowing common mode noise current path) has to keep the leakage current below the standard

(IEC950) accepted value.

been included in this circuit for troubleshooting convenience.

The board has a small, secondary side prototyping area for alternate voltage regulation control.

Page 6 of 32

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