Optoelectronic Devices & Galvanic Isolation

Optoelectronics couples electrical carriers to photons. That conversion can emit, sense, communicate, or isolate. The same optical path that breaks a DC conductive connection also introduces efficiency spread, aging, delay, capacitance, and insulation constraints.

Emitters and detectors #

An LED is a forward-biased junction in a direct-band-gap material. Electron–hole recombination releases photons whose energy roughly sets wavelength. Drive it by current, not by assuming a fixed voltage. Optical output, wavelength, efficiency, thermal resistance, and lifetime all vary with current and junction temperature.

A photodiode creates photocurrent proportional to incident optical power over its linear range. In photovoltaic mode it operates near zero bias for low noise; in photoconductive mode reverse bias reduces junction capacitance and speeds response at the cost of dark current and bias-related noise. A transimpedance amplifier converts current to voltage.

A phototransistor adds current gain but sacrifices linearity and speed, especially if driven into saturation. Photoresistors can be sensitive and simple but are slow, nonlinear, and material-dependent.

Isolation is a system property #

flowchart TB
  IN["input domain"] --> LED["LED<br/>electrical → optical"]
  LED -. "insulation barrier" .-> DET["detector<br/>optical → electrical"]
  DET --> OUT["output domain"]

For a phototransistor optocoupler, current-transfer ratio is

$$CTR=\frac{I_C}{I_F}.$$

CTR has broad part-to-part spread and changes with LED current, temperature, aging, collector voltage, and saturation. A design must close at minimum CTR at end of life and maximum CTR where turn-off or bias matters. Digital optocouplers and isolated drivers may specify propagation delay, pulse-width distortion, common-mode transient immunity, and fail-state behavior instead.

Working voltage, isolation test voltage, creepage, clearance, insulation material group, pollution degree, surge category, and reinforced/basic classification answer different safety questions. A high one-minute test voltage does not automatically establish continuous working voltage for a product.

Reading the signal path #

For a slow analog channel, budget LED nonlinearity, CTR drift, detector noise, and feedback. For a logic channel, budget thresholds, propagation skew, output topology, startup state, and edge asymmetry. For power switching, add common-mode $dv/dt$, gate-driver supply behavior, Miller turn-on, desaturation or overcurrent response, and the energy spent during isolation delay.