I was building a bootstrap based gate driver for BLDC motor control application.From this application note (https://www.infineon.com/dgdl/Infin...N.pdf?fileId=5546d4626c1f3dc3016c47de609d140a) I came to know that charge pump is required for continuous turning on of high side MOSFET. They have demonstrated the same using a circuit also.But I need some clarity in this regard. Here it is stated the timer will output a 7.5kHz square wave with 15V amplitude.How is this 7.5kHz designed.?Does this value have any significance.I need to switch my device at 100kHz. Is this frequencies correlated.Also my Mosfet require a gate voltage of 6V only.So should i use 6V zener to achieve the required amplitude.

 

Also my Mosfet require a gate voltage of 6V only

What mosfet?

 

Here it is stated the timer will output a 7.5kHz square wave with 15V amplitude.How is this 7.5kHz designed.?Does this value have any significance.I need to switch my device at 100kHz. Is this frequencies correlated.Also my Mosfet require a gate voltage of 6V only.So should i use 6V zener to achieve the required amplitude.

The frequencies are not directly correlated. The timer runs to keep the capacitor charged. It gets pumped up, and then declines a little every time the gate gets recharged. Designed properly, the next pump pulse comes along in plenty of time to maintain a good voltage there. So the inflow of charge from the pump to the bootstrap capacitor does have to balance the outflow going to the gate. The frequencies do matter, just not directly. I mean, running it at 8kHz instead of 7.5kHz might slightly increase the voltage on the capacitor but it would not affect operation. Dropping the frequency far enough would let the capacitor voltage fall too far.

Note that the important voltage for the MOSFET gate is not relative to ground, it's relative to the source pin, which will be well above ground.

 

The755 charge pump circuit is only required if the top MOSFET needs to stay on for a long period (DC).
Otherwise the normal switching operation of the 2125 will keep the 100nF capacitor charged.

my Mosfet require a gate voltage of 6V only.So should i use 6V zener to achieve the required amplitude.

No, 6V is the minimum.
you don't need to limit the MOSFET Vgs to 6V, you only need to keep it below the Vgs maximum rating.

 

The755 charge pump circuit is only required if the top MOSFET needs to stay on for a long period (DC).
Otherwise the normal switching operation of the 2125 will keep the 100nF capacitor charged.

No, 6V is the minimum.
you don't need to limit the MOSFET Vgs to 6V, you only need to keep it below the Vgs maximum rating.

That's why I asked what mosfet. The 6V sounds more like a threshold voltage number not an "on "voltage.

 

I am designing using GaN device.The maximum gate source voltage required is 7V only

 

Hi all,
Thanks for looking at my post.I couldn't understand how the zener provides the voltage required for the timer in this case.Also,will the charge be provided only after bootstrap capacitor drains out?Else will it be having the possibility of overcharging the gate.

 

Here is the charge C calculator if you need it.
https://www.silabs.com/support/isolation-bootstrap-calculator
Max.

 

Do you need the circuit to apply a DC voltage to the gate of the GaN device or is it always switching?

 

The755 charge pump circuit is only required if the top MOSFET needs to stay on for a long period (DC).
Otherwise the normal switching operation of the 2125 will keep the 100nF capacitor charged.

That's true. It's not clear in the data sheet, or I just missed it, but it's very clear in the AN.

My post muddied the water because I thought keeping up with the switching was the goal of the bootstrap, and it's essentially the opposite situation. You're right, you only need the bootstrap if there's not enough switching going on for the 2125 to hold on by itself.

Do you need the circuit to apply a DC voltage to the gate of the GaN device or is it always switching?

He says he's running a BLDC motor, so yes, he potentially needs DC there.