The reliability of an optocoupler can be increased by employing a burn-in procedure.
Power supplies, home appliances, and battery chargers for smart phones are all examples of applications that need galvanic isolation between two circuits. Galvanic isolation can be achieved using transformers, capacitors, or optocouplers. Optocouplers offer many advantages, including the elimination of impedance mismatching and excellent noise immunity with high isolation voltage in a small package.
Structure of an optocoupler
The simplest optocoupler consists of an infrared LED optically coupled with, but electrically isolated from, a phototransistor. When the LED emits light, current will flow in the phototransistor proportional to the light intensity.
There are two types: DC input optocouplers have one LED on the input side and therefore conduct current in only one direction. These are commonly used in switching applications. AC input optocouplers have two LEDs connected in reverse parallel allowing current flow in both directions resulting in half waves of an alternating input signal.
Choosing the right optocoupler
One important factor is isolation voltage, which is determined by creepage, clearance, and insulation thickness (the package, so to speak). Different package sizes and types with various leadframe options (DIP4, SOP4, LSOP4, THT, or SMT mounting packages, etc.) allow engineers to pick suitable components for their applications.
An important parameter describing an optocoupler’s performance is the current transfer ratio (CTR), which is defined as the ratio of the current flowing through the LED, IF and the current flowing through the phototransistor, IC.
To build stable applications when designing with optocouplers, it is important to understand that the CTR value is affected by the ambient temperature and that it degrades over time.
The Würth Electronics optocoupler portfolio provides customers with the ability to choose ratios from 50% to 600% depending on the application.
Ensuring long life
One of the main considerations in circuit design is expected lifetime, which is based both on the product itself and on its constituent components. Components can fail completely or degrade in performance over time. For optocouplers, performance in the form of the CTR degrades over time depending on the operating conditions.
Since the lifetime of optocouplers can exceed several decades, an accelerated stress test is performed using increased operation conditions. When testing optocouplers with increased temperature and current, the degrading mechanisms occur much faster than they would under normal operation conditions with smaller temperature and lower current. The main thing to note is that CTR degradation can be reduced by reducing the operation temperature and driving forward current of the LED.
Some suggested design guidelines to increase the lifetime of optocouplers are (1) decrease the effective operating time of the optocoupler, (2) decrease the operating diode current and power dissipation from the LED, (3) minimize peak transient currents through the LED, and (4) adjust the duty cycle of the LED to keep the average current low.
Additionally, in the case of safety-critical products, such as devices with medical application, the reliability of the optocoupler can be increased by employing a burn-in procedure. However, to avoid damage of the devices, the burn-in parameters should be kept below the absolute maximum ratings.