Q1) The output caps on the EVM, C2, C3 and C4 are 35V caps. They have to be changed to higher than 42V caps for maximum cap voltage reasons. You can maybe swap out with 50V or 63V caps. Also Mosfets M1 and M2 are not rated for 42V Output. You will need to use 120V or 150V mosfets since each mosfet will have at least 84V across it and that is not including the spike during switching transitions. Same with caps C1 and C5. It is better use slightly higher voltage caps there also.
Q2) It is OK to apply separate Vdd of 13V as long as it is isolated from HV Primary side Input and can be referenced to the 42V Output’s Ground.
Please make sure that the external bias of 13V is always present and is never locked off under any condition. If it gets locked off then, it is needed to make sure that the LLC primarily controller gets turned off as soon as the 13V goes down. If this does not happen, then the Secondary mosfets can get damaged due to thermal runaway.
Another option is you can always use a linear regulator from the 42V Output to generate the 12V bias and use diode Oring between this 12V and the external 13V. So 2 additional diodes will be necessary along with a linear regulator. This will prevent the above condition from happening.
Q3) Even if there was a current sense and the MP6925 could actually turn the gate off, the internal diodes of the Mosfet would still conduct into a short or a overcurrent load as there is no way to control them. So it does not help in having a OC protection on the MP6925. It is better to turn off the Primary side LLC controller. Check out our LLC controllers like HR1211 or the HR1001x product family.
So it is not useful to have the overcurrent protection in MP6925 because of the following conditions:
- Internal Body diode of the Secondary mosfet fails as a short or,
- The Output Load is short circuited or overloaded. The Internal Body diode of the Mosfet will still conduct into the short even if we pull the Gate of the secondary mosfet low.
We also have current sensors, they can be used to sense the current and send a signal to the primary side through an opto–coupler to shut down the primary side controller.