Page 89 - Power Electronics Handbook

82 Power semiconductor control components
connected to a human operator, requiring effective isolation between the
two parts. Two main types of components are described here, optical
couplers and transformers.

3.4.1 Optical couplers
Optical couplers are made from a combination of an optical source and an
optical detector, both within a single package. The gap between the source
and the detector is usually totally enclosed for power semiconductor drive
circuit applications. Glass or plastic separators are used between the source
and detector, enabling them to be placed close together, so improving
coupling efficiency whilst still giving good isolation between the two.
Light-emitting diodes (LEDs) are invariably used as the optical source,
although many different types of LEDs are in use, the most common being
GaAlAs, which has an emission in the near infrared (750-850nm) and
GaAs having an emission in the inftared (940nm). The LED material
chosen needs to match the spectral response of the silicon photodetector,
shown in Figure 3.8, as closely as possible and also have good efficiency in
terms of light emission for current input. GaAlAs is the most popular
material since its band gap can be varied relatively easily, to modify the
emissions in the range 650-900nm, by varying the gallium-to-aluminium
ratio. GaAlAs also needs a low drive current, so that it is well suited to
being driven directly from low-power logic circuits.
The parameters of most interest in optical couplers are the isolation
between source and detector, the input-output current transfer ratio and
the speed of operation. The isolation resistance is of the order of 10’151,
and is usually higher than the leakage resistance between package pins on
the printed circuit board. Another way of expressing this isolation is by the

Wavelength (nm)
Figure 3.8 Spectral response of a silicon photodetector