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Questions: MPPT Controller Design / Dig. controllable BuckBoost Converter 60V 5A

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OwO:

--- Quote from: Wi_Zeus on November 08, 2019, 11:36:54 am ---This approach only works with an existing converter IC, if I understand correctly? If I can find a suitable boost converter IC (because that is what we really need), I might look into this.
Could you point me towards resources explaining the use of dithering in this use-case?

--- End quote ---
It is preferable to use a buck IC that supports input voltage feedback because otherwise you will need a feedback inverter circuit, which has to be carefully designed to give the correct voltage setpoint range and also not affect loop stability (it should have a gain of 1 and not too much delay). There are off the shelf "MPPT" charger ICs like the CN3722 or CN3791 which already have the feedback polarity reversed and can take feedback directly from the input. Note that these are NOT true MPPT controllers - they use a fixed voltage setpoint which has to be adjusted to the panel you are using. However all it takes is a MCU adjusting the feedback bias (the same way as in the micro mppt schematic I showed) to add true MPPT.

Another approach is to implement the buck controller in the MCU. You would use a PWM output to drive a half bridge driver (ncp81151) which then drives MOSFETs. Normally I wouldn't recommend this approach because it's very difficult to implement a current mode controller (you need cycle by cycle current limiting, which requires very fast current sensing and ADCs, as well as careful software design to bound latency), but in this specific case (solar charging) you don't need to implement a current mode controller and duty cycle mode may be sufficient (solar panel is itself current limiting in the event of faults, but you need to assess whether the stored energy in the capacitors is enough to do damage).

Either way you probably want synchronous buck for the best efficiency, which means a 2-MOSFET half bridge. However it is dangerous to simply drive the bridge on or off because you could short the output to ground which might be a battery. You need to use a bridge driver with diode emulation mode. If you read the ncp81151 datasheet carefully you will see the control logic level is 5V, but there is a mid-state where the chip enters diode emulation mode. Therefore you most likely want to toggle the control pin between 2.5V and 5V (and never 0V). I would probably design some capacitor coupled circuit with diode to drive the ncp81151 that will fail safe in the event the MCU locks up.

Wi_Zeus:

--- Quote from: OwO on November 08, 2019, 02:11:58 pm ---
--- Quote from: Wi_Zeus on November 08, 2019, 11:36:54 am ---This approach only works with an existing converter IC, if I understand correctly? If I can find a suitable boost converter IC (because that is what we really need), I might look into this.
Could you point me towards resources explaining the use of dithering in this use-case?

--- End quote ---
It is preferable to use a buck IC that supports input voltage feedback because otherwise you will need a feedback inverter circuit, which has to be carefully designed to give the correct voltage setpoint range and also not affect loop stability (it should have a gain of 1 and not too much delay). There are off the shelf "MPPT" charger ICs like the CN3722 or CN3791 which already have the feedback polarity reversed and can take feedback directly from the input. Note that these are NOT true MPPT controllers - they use a fixed voltage setpoint which has to be adjusted to the panel you are using. However all it takes is a MCU adjusting the feedback bias (the same way as in the micro mppt schematic I showed) to add true MPPT.

Another approach is to implement the buck controller in the MCU. You would use a PWM output to drive a half bridge driver (ncp81151) which then drives MOSFETs. Normally I wouldn't recommend this approach because it's very difficult to implement a current mode controller (you need cycle by cycle current limiting, which requires very fast current sensing and ADCs, as well as careful software design to bound latency), but in this specific case (solar charging) you don't need to implement a current mode controller and duty cycle mode may be sufficient (solar panel is itself current limiting in the event of faults, but you need to assess whether the stored energy in the capacitors is enough to do damage).

Either way you probably want synchronous buck for the best efficiency, which means a 2-MOSFET half bridge. However it is dangerous to simply drive the bridge on or off because you could short the output to ground which might be a battery. You need to use a bridge driver with diode emulation mode. If you read the ncp81151 datasheet carefully you will see the control logic level is 5V, but there is a mid-state where the chip enters diode emulation mode. Therefore you most likely want to toggle the control pin between 2.5V and 5V (and never 0V). I would probably design some capacitor coupled circuit with diode to drive the ncp81151 that will fail safe in the event the MCU locks up.

--- End quote ---

Thanks for the elaborate explanation but I need a boost converter ;)

OwO:
Same principles apply, but boost is much harder if you want to do synchronous. I haven't found any bridge driver ICs that can do diode emulation on the top MOSFET. If the output is minimum 26V then a schottky diode won't be too bad.

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