What extra components are necessary to bypass a boost converter when the input voltage exceeds its regulated output voltage?

Question

I've made a USB-C power supply that's powered by a wind turbine. It contains a boost converter and a separate buck converter.

I'm aware that there are several buck/boost USB-C power supplies out there, but none of them operate down to a low enough voltage for my liking (3 V preferred). Because of this, I elected to make my own that involves a TPS61178 boost converter which operates down to 2.7 V. The boost converter output directly powers the USB-C buck converter, which can accept voltages up to 35 V.

My current issue is that the maximum input voltage for the buck converter is 20 V. So, on a windy day, if the turbine output voltage exceeds 20 V, I want to be able to bypass the boost converter completely, and power the buck converter directly from the wind turbine.

I've also noticed that the boost converter operates more efficiently when it's configured for smaller input voltage ranges, such as 3-12 V for example, instead of 3-20 V. There's overall less heat loss when the window is smaller.

Because of this, I want to take advantage of operating the boost converter with a smaller input window, but will need to design some sort of bypass for when the input voltage rises outside of the window.

Here's my main question: What components would I need to correctly make a bypass circuit? I figured a Zener diode would be involved at the very least.

The TPS61178 also has an enable pin, which I suppose I could switch-off with a transistor when the input voltage exceeds the Zener voltage.

I've included a diagram of the boost converter below. The OUT1 and PGND1 pins power the boost converter directly (not included). The wind turbine outputs 3-phase AC, so that's why the boost converter has a bridge rectifier at its input.

Also, here is a link to the TPS61178 datasheet: https://www.ti.com/lit/gpn/tps61178

One final thought: It dawned on me that the TPS61178 might be able to safely bypass in its current state already, but I can't tell. If that were the case, then am I over-thinking this whole thing? The datasheet mentions over-voltage protection on page 15, but it's not very detailed regarding input voltages.

Any help is appreciated, thank you!

Answer 1

What components would I need to correctly make a bypass circuit?

A boost converter would naturally disable the switching when the input rose to a value slightly above the desired output level. And, this will naturally happen when the input voltage does do this. It will happen because basically the synchronous diode inside the chip (in the form of a MOSFET) will pass higher-than-expected input voltages to the output. My edits in red: -

So, this is worth a try to see what happens.

the TPS61178 might be able to safely bypass in it's current state already, but I can't tell. If that were the case, then am I over-thinking this whole thing?

You might be but, then again (and we cannot know until you try it), the output disconnect MOSFET (U2 in your diagram) may try and shut down on output-over-voltage. I mean it's there for output under-voltage when there is too much load current (or a short circuit load) and not to prevent output over-voltage but, you can never tell. Try it and see but, try it carefully and incrementally until you are sure it will be OK.

The datasheet mentions over-voltage protection on page 15, but it's not very detailed regarding input voltages.

Just double check that what you hope to see on the output does not exceed that limit.