Ok I found the issue, it was not due to the power supply. The card does not like if we open all tuners at the same time, FW loading of all tuners must be done in sequence.
I am using the latest git version of the open source drivers, on kernel 4.4 and 4.11. I noticed that my card with hang (I get timeout on all tuners) when I try to use tuner number 4 )(/dev/dvb/adapter4).
another one, could you please confirm if the external power is mandatory ? I tested the card without it, and it looks like some tuners are stuck in the IOCTLs...