I would like to ask for a recommendation for the Desat diode for ADuM4135 as follows.
1. What are the required specifications? Is STTH112 suitable for the 400V DC link voltage?
2. Can I connect the Desat diodes in series in order to reduce the Vce threshold?
The main requirement for the desat diode is to be able to absorb the full reverse voltage of the DC link plus any overshoot in the IGBT collector-emitter voltage, plus fast reverse recovery time to avoid excessive switching loss. The STTH112 is more than sufficient - you probably could manage with a 600V device.
You could connect them in series but I wouldnt really recommend it as you will be vulnerable to nuisance tripping of the DESAT due to noise. If the IGBT is desaturated it will rapidly develop 9V, and changing this will not make a huge difference to the protection.