Frequently Asked Questions about Opto-couplers
Quick Link Reference
SPICE models for the optocouplers are not available individually, but they can be accurately modeled by using a current dependent current source in series with a high voltage diode. This will approximate the optocoupler function.
Answer: Over time the gain on the OC100HG will decrease as rapidly as 30% over a three-hour time frame when running both LEDs at the full rated current of 100mA.
Lower operating levels of LED current results in a slower decline in opto-coupler gain, but a gain drop has been observed in even very low LED current levels over time.
Solution - MI suggests using the OC100 or OC100G opto-coupler as a replacement. Gain degradation in the OC100G is less severe over time, but the initial gain is less than that of the OC100HG. Coming Soon - Comparative Gain Degradation Graph
Recommended Replacement - We recommend the as a replacement for applications that do not require high gain.
Answer: The delay time between the LED current and reaction time of the photo diode is listed as 2µs in the data sheet. It is defined as Ton and Toff.
Of more concern is the capacitive load. The output current supplied by the photo-diode is dependent on the LED current, and the gain of the opto-coupler (refer to data sheet). The opto-coupler acts like a current source. The switching time will depend on how long it takes the current output from the opto-coupler to charge the load capacitance.
Question: What is the thermal resistance of the module to air?
Answer: The thermal resistance of the module to air through the epoxy body is very poor. The best heat path is through the diode leads. Thermal resistance from the photo diode junctions to lead length is 6°C/W at 0.10 inch, and 12°C/W at 0.20inch. This assumes both leads are attached to an infinite heat sink.
Question: What is the impact of ambient light on the opto-coupler?
Answer: Incident light will increase the baseline leakage current in the photo diode. To minimize the baseline leakage, shield the opto-coupler from stray-light sources. If possible, test your circuitry in darkness and ambient light to determine whether ambient light will increase the leakage current to unacceptable levels.Question: I recently installed six optocouplers in a two-channel push-pull circuit. I checked the LEDs with an near-infrared viewer before applying high voltage and noticed there s a lot of light coming out of the clear housings. It seemed like a significant amount, compared to ambient light.
Will painting the outside of the devices black, or wrapping them in electrical tape cause problems? (electrical breakdown). The concern is the potential for optical cross-talk while operating at 6kV preventing the "off" arm from completing turning off.
I also noticed that there are two small holes in the housing (not near any leads) Do these need to be left uncovered?
Answer: Cross-talk can be an issue with these devices. Because they are high voltage, many end-users will encapsulate the optocouplers (and surrounding circuitry) with an epoxy or coating to further insulate the optocouplers. This epoxy is usually opaque, so it eliminates the cross-talk.
You can paint or otherwise coat and protect the optocouplers, but you will want to do some testing after the parts are coated to ensure the coating does not cause any arcing around the part at your applied voltage levels. Some customers paint the outside of optocouplers with a white paint to have more complete internal reflection in the optocoupler, increasing the gain of the device.
The two holes you mentioned are just depressions in the epoxy that are there to assist in centering the LEDs during molding. They are not significant to the operation of the optocoupler. They can be oovered up if you like.
Question: Are the OC100 and OC100HG opto-couplers RoHS compliant?
Answer: The OC100HG is RoHS compliant. The OC100 is not. The OC100 uses a non-compliant LED. Unfortunately, a compliant LED is not available. If RoHS is a requirement, we recommend the use of the OC100HG.
Question: Is it okay to put two 25KV OC-250 opto-couplers in series to get a 50KV opto-coupler?
Answer: We don’t recommend it. By connecting two OC-250s in series, the volts-per-mil stress between the LED and the diode in the second OC-250 in series is greater than 350V/mil. 350V/mil exceeds design parameters for the molding material used in the OC-250.
Question: : Is it possible to get a 2W 10KV opto-coupler in the OC-100 package?
Answer: In order to build a 2W opto-coupler, the number of LED’s would have to double, and there would have to be a way to remove the heat from the high voltage diode. It is not possible to do all of that and keep the package size the same.
Question: Does the gain change with temperature?
Answer: Gain changes over temperature. Gain decrease as temperature increases. It is linear over the temperature range of 25C to +150C.
Question: What is the current gain of the device? Can it be increased?
Answer: The current gain of the device is 1/1000. This is the ratio of the output current (diode leakage current) divided by the forward current through the LED(s).
CTR = 100mA = 10E-06 =. 001 = .1%.
Can the current gain be increased?
No. The current gain is fixed.
Question: What is the power gain of the OC-100 device?
Answer: The power gain is 2. This is the ratio of the output power of the photo diode divided by the input power to the LEDs.
Question: What is the minimum gain of the OC-100 device?
Answer: Gain is determined by output/input, and in this case, is also dependent on the level of applied reverse voltage. Defining "gain" as LED current/Optocoupler current, the following graph illustrates gain at two different levels of Vrwm.
For example, at Vrwm = 1KV, and I(LED) = 30mA, Gain = .00077 = (23uA)/(30mA). Similarly, at Vrwm = 4KV, and I(LED) = 30mA, Gain = .00087 = (26uA)/(30mA)
Question: What is the forward current (If) range through the OC-100 LED? What about current through the OC025?
Answer: For the OC-100, each LED is rated at 100mA.
Question: Is the Vf rating for the LED for the whole assembly, or for each LED?
Answer: The Vf rating is for each LED shown on the schematic.
Question: Can the LED current range be increased?
Answer: No. The manufacturer of the LED determines the LED current range. The end user must not exceed the maximum current as specified by the manufacturer.
Question: Do both of the LEDs in the OC-100 have to be working in order to get an output from the photo diode?
Answer: No, but the output (Ir) of the photo diode is decreased by one half if only one LED is working.
Question: Do I have to run the photo diode at 10KV or 25KV?
Answer: No. The 10KV rating for the OC-100 and the 25KV for the OC-250 are maximum ratings.
Question: Can the potting material be changed?
Answer: No. The potting material has been specially selected to maximize the optical transmission of light from the LED to the photocopies.
Question: Can the package styles be made smaller?
Answer: No. The package sizes have been optimized to accommodate the various LEDs and photo diodes.
Question: Are custom configurations available? If so, what are the costs and how long will it take?
Answer: Custom electrical configurations are not available.
Custom mechanical changes to the lead configuration or package size will require a new mold (upwards of $10K) for the OC-100 style, and a new shell for the OC-250 style.
The packages can only increase in size.
Custom mechanical changes can take upwards of twenty weeks depending on vendors, yields, and schedules.
Question: What kind of processing is available?
Answer: Processing should be specified by the customer and reviewed by engineering. Most processing should be done on the final assembly. Diode processing is limited due to requirements for use in an onto coupler.
| | | | | | | |
|Last Update: 23 Mar 2015|