# How do you drive the gate of a MOSFET in an inverting buck-boost converter?

I'm currently working on implementing a buck-boost converter for MPPT of a solar cell, the gate of the transistor is to be fed 0-3.3 V PWMlogic and I am planning on using a gate driver IC to combat the gate charge and shift the logic to appropriate voltage levels. However, the MOSFET source is exposed to negative voltage due to the inverting nature of the buck-boost topology.

In my case, for a certain irradiance level, the voltage of the panel at the maximum power point is ~16 V, at which the output voltage is brought to ~-20 V. I think using PMOS is part of the solution here, but finding a gate driver IC that can output negative bias is proving difficult; and I can't reference the gate to the output voltage since it is constantly subject to change. If there's something I've missed or misunderstood, please feel free to post. Any help would be appreciated.

simulate this circuit – Schematic created using CircuitLab

However, the mosfet source is exposed to negative voltage due to the inverting nature of the buck-boost topology

In the configuration you have chosen, the internal body diode of the p-channel MOSFET will always be conducting current from the supply into the inductor = short circuit scenario.

Internal body diode: -

Image from What is the MOSFET body diode. This will make your circuit like this: -

If there's something I've missed or misunderstood, please feel free to post. Any help would be appreciated.

You could swap the positions of source and drain to get this configuration: -

This has got be be an easier way to drive the p-channel MOSFET.

• No, I'm not going to mention that because although, with care, that will work (the body diode is in the correct direction), the source will be at the inductor end and, we are back in the scenario that was posing the original problem to the OP @RoyC but, please feel free to cover that as an answer. Commented Mar 6, 2023 at 13:33
• @RoyC that won't work because the body diode will be forward biased. So, it's probably best that you just go straight ahead and leave an answer because I don't like being piggy in the middle. Commented Mar 6, 2023 at 13:41
• @RoyC any chance you could share your solution to this implementation with an NMOS FET? I would certainly be interested in the benefits of using an n-channel. Commented Mar 6, 2023 at 19:02
• @DomJP in case you don't get an answer from Roy, I'll point out that the only way an n-channel device can work is with the source connected to the switching node and, that would lead you back to the problem raised in your question --> how do you control the gate when the source goes negative etc. (unless Roy has a cunning plan). Commented Mar 6, 2023 at 20:04
• @RoyC you might be missing the point that for an inverting buck-boost regulator, the source voltage becomes well-and-truly negative in value and that regular MOSFET drivers won't work (read the question). Of course a high-side NMOS can be used in question 487819 when the topology is buck but, because an inverting buck-boost is quite different in the inductor voltages, you can't (unless you have a better cunning plan). Commented Mar 7, 2023 at 12:34