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 V
1
, 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.
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 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.
Figure 5. VTL5C3 Life Test.
37
PERKINELMER [ PERKINELMER OPTOELECTRONICS ]
