I'd like to build a synchronous buck converter with about the following specs, notice that I want the ability for Vin = Vout, up to 100% duty cycle.
Vin = 12-100v
Vout = 12-18v
Vin >= Vout (minus the voltage drop from Rdson)
Iout = 40amp
F = 400khz
duty cycle = 0 - 100% (in the case of Vin = Vout, 100% always on)
1.5-2amp high side driver peak current (A guess based on some mosfets I've calculated)
Controlled by a dsp, using a PID routine.
Options and Questions:
If I want the possibility of turning on the high-side (transition from buck to an always on high-side), what are my options for driving this sync buck?
1. If I use a typical high-side driver that calls for a bootstrap capacitor (which I think would normally limit my duty cycle to somewhere well below 100%), can I replace Cboost with a dedicated power supply 10volts above Vout? Somewhere between 22-24 volts for the high side gate supply. How would I wire this, I can't find application notes that discuss such a use of a high-side driver? Can a high-side mofset driver be driven to 100% duty cycle with a dedicated Vboost supply?
2. If not, I have considered using a high-side driver for sync buck operation (with bootstrap cap), then when I want to switch to always on mode, I could have some op amp (with dedicated supply) power the gate. Of course the DSP would shut down the buck pwm and use a ttl output for the op amp. This complicates the design, I prefer option #1 above.
3. I looked for a charge-pump high-side driver that could run at 100% duty cycle, but didn't find anything. Perhaps this is an option if such a part exists.
Perhaps it would help to know why I want to do this:
Reason: At some voltage of Vin (perhaps near 13v), it would be more efficient to directly connect the Vin = Vout. I can limit the loss to Rdson of the high-side (plus overhead of gate supply and dsp), and skip the switching losses.
I'm obviously new to using high-side mosfet drivers with bootstrap capacitors, any help would be appreciated.
Thanks,
Marcus
Vin = 12-100v
Vout = 12-18v
Vin >= Vout (minus the voltage drop from Rdson)
Iout = 40amp
F = 400khz
duty cycle = 0 - 100% (in the case of Vin = Vout, 100% always on)
1.5-2amp high side driver peak current (A guess based on some mosfets I've calculated)
Controlled by a dsp, using a PID routine.
Options and Questions:
If I want the possibility of turning on the high-side (transition from buck to an always on high-side), what are my options for driving this sync buck?
1. If I use a typical high-side driver that calls for a bootstrap capacitor (which I think would normally limit my duty cycle to somewhere well below 100%), can I replace Cboost with a dedicated power supply 10volts above Vout? Somewhere between 22-24 volts for the high side gate supply. How would I wire this, I can't find application notes that discuss such a use of a high-side driver? Can a high-side mofset driver be driven to 100% duty cycle with a dedicated Vboost supply?
2. If not, I have considered using a high-side driver for sync buck operation (with bootstrap cap), then when I want to switch to always on mode, I could have some op amp (with dedicated supply) power the gate. Of course the DSP would shut down the buck pwm and use a ttl output for the op amp. This complicates the design, I prefer option #1 above.
3. I looked for a charge-pump high-side driver that could run at 100% duty cycle, but didn't find anything. Perhaps this is an option if such a part exists.
Perhaps it would help to know why I want to do this:
Reason: At some voltage of Vin (perhaps near 13v), it would be more efficient to directly connect the Vin = Vout. I can limit the loss to Rdson of the high-side (plus overhead of gate supply and dsp), and skip the switching losses.
I'm obviously new to using high-side mosfet drivers with bootstrap capacitors, any help would be appreciated.
Thanks,
Marcus