Rated maximum voltage of the boot strap capacitor of a high-side mosfet driver

[eneuro]

Hello,
I'm looking for your opinion on rated maximum voltage of the boot strap capacitor of a high-side mosfet driver.
In this example (classic) bootstrap circuit

from this link:
http://electronics.stackexchange.com/questions/87650/what-must-be-rated-maximum-voltage-of-the-boot-strap-capacitor-of-a-high-side-mo
there is conclusion:

However, if your circuit shuts down with a load connected to the high voltage and source the cap will need to be rated at the full power rail because one side will be taken to the high power rail by the load and the other will decay to 0 volts.

No question about it that:

The reverse voltage rating on the diode needs to be at the full power rail by the way.

But, really do we need HV boot strap capacitor of a high-side mosfet driver?
Just put into simple circuit simulator 12Vcc and simulated overvoltage on half-bridge mosfet source in the middle with lets say 24V spike and no way there were no more than 12V on boost strap caps in high side 

Update: Ups, probably on this optoisolated gate driver PSU (left battery) should be also power rail rated diode blocking possible higher voltage spikes througth zeners...
 
In this simple H-bridge I plan to use a few the same simply optocoupler insulated low speed low current gate drivers and 16V 100uF boost strap caps since I do not know  how is it possible that those caps will be charged to higher voltage than 12V since I've additional 12V zeners 

Read quite nice intros howto calculate needed capacity, but there is nothing about boot strap capacitor rated maximum voltage 
http://www.ixysic.com/home/pdfs.nsf/www/AN-400.pdf/$file/AN-400.pdf

[JohnnyBerg]

Looks like a IRF2110 to me.
It is in the datasheet: Gate drive supply range from 10 to 20V

The reverse breakdown of the diode is High side voltage minus VCC

[nctnico]

The bootstrap capacitor should not need to provide a higher voltage than the maximum gate voltage. 20V is the maximum you can expect and often the voltage is much lower. One thing to watch out for is the dielectric of the bootstrap capacitor. If it is not NPO of COG the capacitance may drop radically with several Volt applied. So you may need a capacitor rated for 50V or more to meet the capacitance requirements.

[eneuro]

One thing to watch out for is the dielectric of the bootstrap capacitor. If it is not NPO of COG the capacitance may drop radically with several Volt applied.

Thank you for this hint 
I can easy put 25V caps in those cheap simply gate drivers, while it is not part of any critical systems and 50% duty cycle-no sinusoidal waveforms-pure ON/OFF 
Probably even 10uF caps are overkill there when estimated this needed capacitance and played a little bit in circuit simulator