AN1296

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Doc ID 8159 Rev 3

3/8

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The valve insulation is not sufficient: in this case, the valve oil winding insulation breaks

down when the switch turns on, i.e. when the whole surge voltage is applied across the

valve. The load then behaves like a short-circuit. The ACS current is no longer limited

and could reach up to 120 A (refer to Figure 3). The conduction losses could then be so

high that the die silicon could melt or its bonding wires could fuse and cause the

destruction of the device package. In practice, as the test is done with the complete

equipment including clamping devices or filters, this behavior does not occur.

Figure 3.

3 kV surge with an insufficiently insulated valve

Vout (200 V/div)

Iout (50 A/div)