ir2110 negative bias

[khaled ragab]

i am asking about the zener diode Ds is it zener diode or not, design tips DT99-7 put a symbol of zener diode?! http://www.irf.com/technical-info/designtp/dt99-7.pdf figure 2
if it is a zener diode am i using it to keep a positive voltage difference between Vb and Vs (equal to zener voltage) to prevent negative bias?
if i use a voltage supply of 120 volt for driving the motor should the reverse zener break down voltage be greater than that as if the reverse voltage be greater than the zener break down voltage a current well flow and their is now limiting resistance to limit the current that passes through the zener diode?

 

[goldsmith]

Hi Khaled

am asking about the zener diode Ds is it zener diode or not, design tips DT99-7 put a symbol of zener diode?! http://www.irf.com/technical-info/designtp/dt99-7.pdf figure 2
if it is a zener diode am i using it to keep a positive voltage difference between Vb and Vs (equal to zener voltage) to prevent negative bias?
if i use a voltage supply of 120 volt for driving the motor should the reverse zener break down voltage be greater than that as if the reverse voltage be greater than the zener break down voltage a current well flow and their is now limiting resistance to limit the current that passes through the zener diode?

No that is not zener diode . that is a schottky diode .
Best Wishes
Goldsmith

 

[FvM]

The application note is very clear about Ds being a schottky diode. It's purpose is to prevent internal latch-up of the driver IC under special conditions discussed in the text.

Zener and schottky symbols are somewhat similar but can be clearly distinguished.

 

[khaled ragab]

thank you it is a silly mistake

 

[khaled ragab]

thank you for reply
in this design tips i don't know how it gonna be when Vs is equal to input voltage(high side mosfet is on) by putting the diode like that i make a path for the volt to recharge(the capacitor is charged by vcc through diode i gonna recharge it with the Vs value) the capacitor is it right?

 

[goldsmith]

in this design tips i don't know how it gonna be when Vs is equal to input voltage(high side mosfet is on) by putting the diode like that i make a path for the volt to recharge(the capacitor is charged by vcc through diode i gonna recharge it with the Vs value) the capacitor is it right?

Don't forget that , that diode is reverse biased at usual condition .
second : that capacitor ( bootstrap capacitor ) will be charged up to +VCC . hence for a mosfet best condition is 15 volts ( for a usual mosfet ) . hence it can be occupied up to 15 volts but gate voltage up to ground will be around VDC+15 ( drain voltage +15)

 

[khaled ragab]

ok but when the high side mosfet is switched on the voltage across the Vs pin will be equal to the input voltage (drain voltage) so the diode is forward biased so the capacitor will be charged again

 

[goldsmith]

Hi again
Yes you are right about recharging with VDC . let me confess a thing , i didn't see circuits with that diode , until know , hence i don't know what is happening exactly with that additional diode . all of the bootstrap drivers that i have used was without that . hence i think it is better that i don't tell anything and we can wait for FvM's reply as he is a pretty expert engineer that i knows here , i'm sure he know many things about it , and thus we both can learn more about that effect .
Regards
Goldsmith

 

[FvM]

I don't understand which problem you are referring to. The bootstrap capacitor will be only recharged when the low side MOSFET is on.

 

[goldsmith]

Hi Dear FvM
I think his mean is when H side is on , ( DS is short circuited , approx ) that diode will be connected to the +VDC and then capacitor will be connected to the positive HV rail supply . but let me think more ......
oops ! yes it won't have any path be charged with that voltage because it needs a path to ground . yes you are quite right . thank you for that . i think now original poster can understand it too .
Sincerely
Goldsmith