bootstrap circuit explanation

[azarutz]

Hi can any one explain me how bootstrap circuit works in high side mosfet triggering.I know for N-MOSFET gate source voltage should be 12V to be on. if it connected in high side how 12V is maintained across it because gate voltage should be higher than drain voltage. I have seen it with driver ic IR2110 . How the bootstrap cap triggers high side mosfet. I doubt in is capacitor stores (Vcc+Vdd) Vcc is IR2110 supply voltage . Vdd is mosfet drain voltage. or it stores only Vcc and blocks mosfet source potential . I confused it in this please help me.

 

[dick_freebird]

The bootstrap is for the flying supply, not the trigger. It
powers the source-referenced gate driver and part of its
level shifter, which are the trigger path.

 

[azarutz]

The bootstrap is for the flying supply, not the trigger. It
powers the source-referenced gate driver and part of its
level shifter, which are the trigger path.

Hi i didn't understand how the flying supply (capacitor) gives higher voltage (i.e gate_voltage+drain_voltage) ..

 

[azarutz]

somebody help me whether cap stores (gate+drain) voltage or it blocks source voltage(how) and only give gate voltage(ie turn on voltage +15 for n type power mosfet ).
thanks

 

[dick_freebird]

The bootstrap diode charges the fly-cap when the SW
node is at GND, with (Vcc-Vf). When SW flies up, the
charged cap travels with the source and the high side
gate driver rides with it, similarly source-referred. Like
a battery with a pretty short life-of-charge, but a
very frequent charging.

 

[azarutz]

The bootstrap diode charges the fly-cap when the SW
node is at GND, with (Vcc-Vf). When SW flies up, the
charged cap travels with the source and the high side
gate driver rides with it, similarly source-referred. Like
a battery with a pretty short life-of-charge, but a
very frequent charging.

How to select rating of the bootstrap capacitor . Is it 25V is enough or it should above drain voltage? and capacitance value?