Optocoupler with 5.3 kV isolation — active and available
The Vishay SFH618A-2 is a single-channel phototransistor optocoupler in a 4-DIP through-hole package, rated for 5300Vrms isolation between input and output. It accepts a DC input on the LED side and switches a phototransistor on the output, with a current transfer ratio (CTR) binned between 63% and 125% at a forward current of 1 mA. This CTR range is the key parameter for the design: it defines how much collector current the output transistor can sink for a given LED drive, which sets the pull-up resistor value and noise margin in digital isolation or the feedback gain in a switched-mode power supply.
The -2 CTR bin (63% minimum, 125% maximum at 1 mA) is a middle-grade selection — tighter than the broad -1 bin, but not as high as the -3 or -4 bins in the SFH618A family. A higher CTR lets you drive the LED with less current, reducing power draw on the primary side, but the trade-off is a wider spread that can complicate worst-case margin analysis. The 55 V output breakdown and 50 mA continuous output current give enough headroom for driving a logic gate input or a small-signal MOSFET gate through a resistor.
Temperature range and isolation rating
The 5300Vrms isolation voltage is a reinforced-grade rating, suitable for mains-isolated feedback in off-line power supplies and for safety-barrier applications that require a 5 kV minimum. The 4-DIP package with 7.62 mm pin spacing provides the creepage distance needed to maintain the isolation rating across the board.