Understanding H bridge with NMOS

[johnjoejohn]

I want to use nmos to drive a brushless motor but not quite understand it. I read a lot out there but still a bit confused.
My confusion is the charging and discharging of the bootstrap capacitor. My understanding is that you have to turn on the lower mosfet, give it a little time to charge before you turn on the top mosfet on.

For a 3 phase brushless motor, I will use 3 high side low side driver, so do I charge all 3 bootstrap capacitor at once??

So correct me if im wrong:
If I have 3 half bridge, call it A (A high and A low), B (B high and B low), C (C high and C low), and the sequence for the motor is lets say (just making it up)
A-high, C-Low
B-high, C-Low

First I would have to
1 ) turn A-low on because first state is to turn on A-high and it needs to get its voltage higher.
2) Then wait for like 10ms.
3) Turn off A-low.
4) Now I can turn on A high, and C low.
5) Now next state is B-high and C low so I would have to turn on B low to charge.
6) turn off B low
7) now B high should be able to turn on.

???

 

[crutschow]

Please post a schematic of your proposed circuit including the bootstrap capacitors.

 

[johnjoejohn]

Its just a standard H bridge using N channel mosfet. I just need to know the steps as I described in original post.
Basically it all boils down to, when do I give time to charge the bootstrap capacitor, every commutation step, or just once on the first commutation?

 

[crutschow]

Your steps do not appear to correspond to a usual bootstrap sequence is why I asked the question. Normally the bootstrap capacitor is automatically topped off every cycle during the time the high-side MOSFET is off. How are you planning on charging the bootstrap capacitor?

The bootstrap capacitor is not charged and discharged every cycle. It's used to boost the voltage of the high-side N-MOSFET and only the gate capacitance is discharged each cycle, not the bootstrap capacitor. The charge drawn from the bootstrap capacitor is only that required to charge the MOSFET gate capacitance and turn it on.

 

[johnjoejohn]

Maybe you mis read my original post, im not saying those are the steps im actually doing, I was just giving an example of how I THINK I would have to do to be able to drive the motor with it because I don't understand it.
I understand that you the bootstrap cap is used to boost the voltage of the high side and that's also where im confused, because from what I read from other sources, you have to first turn on the low side to allow the boostrap cap to charge so it canboost the voltage on the high side when you actually turn the high side on. From your post it seems like I don't need time to charge the bootstrap cap and I can just start switching the high side???

And if I do have to charge the cap first, do I have to allow time for it to charge every commutation? Or initially charge all 3 bootstrap cap (3 pair of half bridge, each half bridge has its own driver) and then for the rest of the commutation cycles, idont have to charge again.??

 

[crutschow]

Normally the bootstrap cap is automatically charged when the transistor is off. Thus you don't need to add additional time for that.

Edit: If you are concerned about the initial conditions, the bootstrap cap will be charged after the first few cycles. If it's not charged for the first cycle that just means the transistor won't turn on fully for that first few cycles, but that's normally not a problem.

You seem to be speculating about an imaginary circuit that somehow does what you want. That's why you need to come up with a specific bootstrap circuit to have a proper discussion about bootstraping. If you need a circuit for that purpose, then we can also discuss that.