Changing a Buck Converters Input Voltage Instantly (From 5V to 20V)

[rents]

Hi all! It's my first post, be gentle.

I'm currently designing a carrier board with usb power delivery for a FPGA system on module. i'm using buck converters that are working between 1000kHz-1500kHz switching speeds for different voltage levels i need. As far as i can understand when you first plug in your type c cable you get 5V from VBUS and then source and sink establish a communication and then you get your desired voltage. Let's say i have programmed my PD controller(i'm usng FUSB302) to give me the maximum voltage and current source can provide. And let's say that is 20V. My VBUS voltage is going to increase from 5V to 20V almost instantly. My question is would i get a voltage spikes at the output of the regulators that can damage components? How fast does buck converters react to feedback?

I made a circuit with a buck controller that can output 5V 5A and a power switch (basically a comparator shuts down a PMOS that is connected to VBUS and enables the buck controller when VBUS voltage is higher than 5.5V).And i was going to connect all of the other buck converters to that but it's messy, takes too much space and expensive. Is there any other solution that can look in to?

[thm_w]

Post the part number of the buck converter you are using.
In the datasheet there will be a specification called "Line regulation", that will provide some of your answer.

https://www.tek.com/en/documents/application-note/simplifying-dc-dc-converter-characterization-0

You say the Vbus will almost instantaneously switch from 5 to 20V, but in terms of time scale a regulator can deal with, it is probably relatively slow. As there is a regulator inside the USBC supply that is having to change from 5 to 20V.

The actual time might be in the USB spec somewhere.

[jwet]

In the USB PD Spec, the basic slew rate max is .03v/ms, so a 10v change takes somethings like .3 ms, not super fast.  Settling is about 300 mS, which is leisurely.  Like most USB stuff, I don't know how compliant commodity USB PD sources are but a decent power supply could easily handle that kind of a step.  There is a similar negative slew which can also create issues if your supply can't sink.

[David Hess]

The regulator's "line regulation" specifications cover what happens to the output when the input changes.

Current mode regulators have better, actually excellent, line regulation and as jwet points out, the USB specification is not particularly fast.  There should not be any problem.