Research On Several Key Problems Of High Speed Optocoupler And Construction Of Its Testing System

Optocoupler is a kind of unidirectional isolation device for forward transmission and reverse isolation.It is widely used,especially in the field of high reliability.It came into being in the 1960 s.Because of its small size,good isolation,high reliability and low cost,it has attracted great attention since its inception.After decades of development,optocouplers are becoming more and more integrated,faster and faster,higher and higher isolation voltage,smaller and smaller devices,and richer products.Due to the technical blockade from foreign countries,the research work of domestic optocoupler has been progressing slowly.Especially in the key index of speed,domestic optocouplers are at least 4-5 times behind foreign products,so they still mainly use foreign products in various fields.In order to realize the localized substitution of optocoupler,our team has carried out in-depth research and experiments on optocouplers in recent years.Firstly,we explored the principle of signal transmission and isolation of optocoupler and established the corresponding theoretical model.Then we put forward our own development ideas,designed a new photosensitive product and encapsulated a high-speed optocoupler with high reliability based on this new product.At the same time,we developed an automatic test system for this optocoupler based on the source meter and used a single chip computer to test the product.The reliability of the packaged products is also studied,and the methods to improve the reliability of the products are put forward.In the fabrication of high-speed optocouplers,there are three key problems: 1)the slow speed and low efficiency of the photoelectric conversion part;2)the photocurrent generated by photoelectric conversion is generally in the order of 10 ?A,which is far from the purpose of driving the back-end circuit,so it is necessary to establish a large bandwidth trans-resistance amplifier to convert the current signal into voltage signal;3)the lifetime of the optocoupler is relatively short.In view of the above three key problems,we propose the following solutions: 1)In the fabrication of photosensitive chips,we introduce two methods to improve the response speed.The first method is to use ultra-thin PIN junctions to ensure that the junction transit time of photocurrent is short enough,and at the same time,introduce micro-nano-porous arrays on the PIN junction to enhance the responsiveness of PIN junctions;the second method is to use light-dark combination.The diode array can cancel the "slow current" generated in the detector by the combination of the diode array and the differential circuit,thus realizing the high-speed conversion and transmission of the signal.2)In the CMOS integrated circuit part,a symmetrical structural transimpedance amplifier with wide bandwidth is proposed to realize the function of converting the photocurrent into voltage,and then through the operation amplifier,logic conversion and high-speed power supply circuit,the purpose of converting the photogenerated current signal into the output logic level signal is realized.3)Aiming at the problem of the lifetime of the optocouplers,we use accelerated lifetime experiment to study the performance of the device,reveal the influence of packaging technology on LED,and successfully locate the lifetime of optocouplers to the "soft breakdown" of the LED,and then we adjusted the packaging process to ensure the long-term reliability of the device.Finally,we test and evaluate the developed optocoupler.The measured product transmission speed can reach 50 MBd,and the quality reliability can reach the H-class standard of foreign products.It has become the first self-developed optocoupler with transmission speed of 50 MBd in China,and has achieved the goal of localization of ultra-high speed optoelectronic coupler.