Galvanic isolation in a 4-SMD gull-wing package
The Vishay SFH6156-4T is a single-channel phototransistor optocoupler providing 5300 Vrms of galvanic isolation between a DC input LED and a transistor output. It comes in a 4-SMD gull-wing package for surface-mount assembly, with the output transistor rated for up to 70 V and 50 mA continuous. The -4T CTR bin guarantees a current transfer ratio between 160% and 320% at a forward current of 10 mA, which determines the LED drive current needed to saturate the output for a given load.
The 160% to 320% CTR range at 10 mA forward current is the key selection parameter. A higher CTR means the output transistor saturates with less LED drive, which reduces the input-side power budget and lets you use a smaller series resistor. The typical forward voltage is 1.25 V, so at 10 mA the LED dissipates about 12.5 mW. The typical turn-on time is 3 µs and turn-off is 2.3 µs, with symmetrical rise and fall times of 2 µs each. For a 10 kbps optocoupler interface those numbers leave comfortable margin; at 100 kbps the switching losses start eating into the output current capability. The 400 mV Vce saturation maximum at the rated output current means the output pull-up resistor sees a clean logic-low within 400 mV of ground.
The gull-wing leads are compatible with standard reflow profiles for 4-SMD packages.