8.1.4 High Voltage Testing

TI has adopted a partial discharge test criterion that conforms to the German VDE0884 Optocoupler Standards. This method requires the measurement of minute current pulses (< 5 pC) while applying 2400-Vrms, 60-Hz high-voltage stress across every ISO124 isolation barrier. No partial discharge may be initiated to pass this test. This criterion confirms transient overvoltage (1.6 × 1500 Vrms) protection without damage to the ISO124. Lifetest results verify the absence of failure under continuous rated voltage and maximum temperature.

This new test method represents the “state-of-the art” for nondestructive high-voltage reliability testing. It is based on the effects of nonuniform fields that exist in heterogeneous dielectric material during barrier degradation. In the case of void nonuniformities, electric field stress begins to ionize the void region before bridging the entire high-voltage barrier. The transient conduction of charge during and after the ionization can be detected externally as a burst of 0.01–0.1-µs current pulses that repeat on each ac voltage cycle. The minimum ac barrier voltage that initiates partial discharge is defined as the “inception voltage.” Decreasing the barrier voltage to a lower level is required before partial discharge ceases and is defined as the “extinction voltage.” The package insulation processes have been characterized and developed to yield an inception voltage in excess of 2400 Vrms so that transient overvoltages below this level will not damage the ISO124. The extinction voltage is above 1500 Vrms so that even overvoltage induced partial discharge will cease once the barrier voltage is reduced to the 1500-Vrms (rated) level. Older high-voltage test methods relied on applying a large enough overvoltage (above rating) to break down marginal parts, but not so high as to damage good ones. Our new partial discharge testing gives us more confidence in barrier reliability than breakdown/no breakdown criteria.