capacitordc/dc converterisolation
I've been looking at board mount isolated DC/DC converters to use in an application. I found this rather reasonably priced for my needs. Here is what I'm confused about, what is C103 doing here? Isn't it breaking the isolation?
Or, is it because it's DC it won't break the isolation?
One class of devices that may be worth a look at are the integrated TVS devices used for USB protection. They include a TVS and current limit resistor but might be a bit light-on for lightning protection if that may be an issue. One example is the Semtech STF203 series available from Digikey at reasonable low-volume prices.
It sounds like a device similar to the TXB0108 would work for your needs.
It has an output enable pin which places all of the pins in high impedance state, otherwise it is a bi-directional signal translator.
Best Answer
It will not unduly compromise the galvanic isolation or safety because it is is an XY rated safety capacitor. It's also low capacitance (less than 1 nF) so the impedance at mains frequency is rather high and the resulting current will be low, even if L and N were accidentally reversed and a path was present from -DC OUT to earth.
Although the working voltage rating is only 250VAC it will withstand 4kV to meet the safety agency requirements. Do not substitute another type of capacitor without doing a proper engineering (safety) analysis.
The purpose of C103 is to conduct noise caused by the internal switching back to the input. If you leave C103 out, the output will have high frequency noise on it relative to the input (and thus relative to earth).