In modern power electronics, optocouplers play a crucial role when it comes to galvanically isolating control and load circuits. But how exactly do these components work? What makes them so indispensable and how do ageing and stress affect their service life? This article sheds light on the design, areas of application and the most common degradation mechanisms of optocouplers and shows why regular testing of these components is essential.
1. Function, design and area of application
Various components can be used for galvanic isolation. An optocoupler is suitable for the transmission of certain signals. It essentially consists of a light emitting diode and a photodiode or phototransistor. The photodiode becomes conductive when a sufficient number of photons strike its semiconductor surface. This converts an electrical signal into a light signal, which in turn excites the counterpart and switches it through. An insulator is often used between these two components when the separate power supplies have too large a potential difference.
In power electronics, such optocouplers are often used in combination with a gate driver to control power transistors (e.g. IGBTs). The control and load circuits are thus electrically isolated from each other. The driver often consists of a complementary output stage with bipolar transistors or MOSFETs (see Figure 1).
Figure 1: Left: Internal structure of an optoelectronically isolated gate driver, Right: Internal structure of a package with LED, photodiode and isolator [1]
2. Life expectancy of optocouplers
The current transfer ratio (CTR) is an important parameter for determining the progressive degradation of an optocoupler. This value is defined as the ratio between the LED forward current IF and the output current Io [2, 3]:
With:
K: transmission factor of the optical path
Rph: sensitivity of the photodetector (defined as electrons of the photocurrent per photon)
η: quantum efficiency of the emitter (defined as emitted photons per electron at the input current) as a function of the forward current iF, the temperature T and the time t
β: gain of the output as a function of the photocurrent ip, the temperature T and the time t
If this ratio is set in relation to the original value CTR0, a change over time can provide information on the ageing process.
Each factor in the above equation is subject to some degradation:
Transmission factor
Photodetector sensitivity
Quantum efficiency
Gain
The greatest dependence on the lifetime of an optocoupler is in the quantum efficiency region. Over time, the photon output of the light emitting diode decreases due to a reduction in the recombination possibilities in the depletion zone. [3, 4]
The mean time to failure (MTTF) for optocouplers is primarily determined by the current density JF and the junction temperature TJ [2, 3]:
With:
α: Combination factor of a technology constant and a correction factor
JF: current density at the diode [A/cm²]
EA: activation energy [eV]
kB: Boltzmann constant [eV/K]
TJ: Junction temperature
The temperature has an exponential and the current density a reciprocal-square factor on the mean time to failure.
3. Failure mechanisms of optocouplers
Different consequences can be derived from the possible degradation effects:
Increased power dissipation in the driver (and therefore increased heat generation)
Loss of individual switching pulses (and therefore possible occurrence of inadequate switching states)
Restriction of reliable on/off switching via the driver circuitry
In principle, the failure mechanisms in optoelectronic components should be seen as signs of ageing. However, some manufacturers are now investing in accelerated ageing tests. The temperature and current are usually manipulated to such an extent that accelerated ageing can be claimed [2].
For example, the Würth acceleration factor is described as follows:
With:
AF: Acceleration Factor [h]
Itest: Forward current used in stress test [A]
Inorm: Typical field use forward current [A]
Ttest: Temperature used in stress test [K]
Tnorm: Typical field use temperature [K]
EA: Activation energy [eV]
kB: Boltzmann constant [eV/K]
N: Exponent, here: N = 2
For example, an ageing period of approximately 25 years of operation can be simulated with an operating time of 1000 hours, a test temperature of 110 °C (at a normal temperature of 80 °C) and a test current of 30 mA (at a normal current of 5 mA).
4. Conclusion
In order to switch the power electronics reliably, it is essential that the driver circuits are functional. This includes the optoelectronic elements, which are put through their paces at Eternus Technology GmbH.
Current and temperature are important factors in the progressive degradation of these components.
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[1] A.S. Kamath et al. (2024): Understanding Failure Modes in Isolators, 2024 in Isolation Interface Group Texas Instruments.
[2] D. Köck (2021): Lebensdauer von Optokopplern, Application Note von Würth Elektronik, AN0006.
[3] T. Bajenescu (1995): CTR Degradation and Ageing Problem of Optocouplers, Conference: Solid-State and Integrated Circuit Technology, 1995 4th International Conference CH-1093, DOI: 10.1109/ICSICT.1995.499774.
[4] E. Kiljo [2020]: Optocoupler Breakage in Frequency Converter Use, Master-Thesis from Aalto University, School of Engineering.