03-April-2001

5 W Universal Input Dual Output Isolated TNY266

8.7 Surge Voltage Immunity (2 kV and 6 kV, 1.2/50

µs per IEC1000-4-5)

The surge protection for configuration ”1” (2 kV) and configuration ”2” (6 kV) is illustrated in the schematics

(pages 4 and 5).

R7, R8 limit the maximum surge current to approximately 50 A, the value at which the clamping voltage of

the varistor is characterized.(< 800 V). This voltage level was selected to ensure enough margin for the

diode bridge D1-D4. The 6 kV, 1.2/50

µs pulse at 800 V clipping level is approximately 100 µs (see Fig.

8.7.2). From the graph in Fig.8.7.1 it can be inferred that the unit will survive 10 k surges of 6 kV. Reducing

the value of R7, R8 would reduce the total number of 6 kV pulses the unit can survive.

Figure 8.7.1 - Varistor Life (Number of Surges) as a Function of the Rectangular Pulse Amplitude

and its Duration.

Figure 8.7.2 - Varistor Clamping Voltage.

VRV1

Varistor Clamp Voltage

Instantaneous Line Voltage

Page 23 of 32

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