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VTL5C1 Datasheet(PDF) 42 Page - PerkinElmer Optoelectronics |
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VTL5C1 Datasheet(HTML) 42 Page - PerkinElmer Optoelectronics |
42 / 76 page ![]() 37 Characteristics of Analog Optical Isolators Voltage Rating The maximum voltage rating of the output element (photocell) applies only when the input is off. Two different kinds of dark current “leakage” characteristics are observed in photocell output elements. Figure 4 shows the soft breakdown found in lower resistivity materials. With no input, if the applied voltage is suddenly increased from zero to V1, the current increases along section ‘a’, with the steepness depending on the rate at which the voltage is increased. If the voltage is now held at V1, the current decreases along curve ‘b’ and stabilizes at a much lower value. If the voltage is again increased, the next section of the curve is traversed with the current dropping along curve ‘d’ in time. This process can be repeated until the reverse current becomes so great that the cell burns up. The maximum voltage rating for photocells with this soft reverse characteristic is based on a safe steady-state power dissipation in the OFF condition. Figure 4. Breakdown characteristics of photocells with low resistivity photoconductive material. Higher resistivity photoconductive materials do not show the reverse characteristics of Figure 4 to any significant degree. As voltage is increased, the dark current increases, but remains very low until breakdown occurs. The current then increases in an avalanche fashion resulting in an arc-over which causes the cell to be permanently damaged (shorted). The dielectric breakdown voltage is approximately 8 - 10 kV per cm of contact spacing for materials with this type of reverse characteristic. Photocells have 0.16 - 0.5 mm electrode spacing so the maximum voltage ratings typically fall into the 100 - 300 volt range. The high voltage capability of photocells suggests their use as the series pass element in a high voltage regulated power supply. Voltages up to 5 or 10 kV can be regulated but the current should be limited to 1 or 2 mA. The isolated input element greatly simplifies the circuit design and the single output element avoids the need for voltage and current sharing components. Power Rating Photocells are primarily used for signal control since the maximum allowable power dissipation is low. Typically, the steady-state output current should be kept below 10 mA on catalog LED AOIs because of the small size ceramic used in the output cell. However, the surface area is large compared to similarly rated transistors, so AOIs withstand significant transient current and power surges. Power ratings are given in the catalog and are typically a few hundred milliwatts, but special AOIs have been made with power dissipation ratings as high as 2.0 W. Life and Aging Life expectancy of an AOI is influenced both by the input and output devices. Isolators which use an LED have long life since LED lifetimes are long: 10,000 to 200,000 hours, depending on the application. LEDs normally show a decrease in light output for a specified bias current as they age. The photocell output elements in AOIs show an increase in output resistance over time as they age. With a continuous input drive current and with voltage bias applied to the output, the output resistance will generally increase at a rate of 10 percent per year. The aging rate is lower with intermittent operation. Figure 5 shows the trend line for output resistance under typical operating conditions. Other AOIs using different photoconductive materials show similar trends. Figure 5. VTL5C3 Life Test. |
Similar Part No. - VTL5C1 |
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Similar Description - VTL5C1 |
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