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2:47 AM
Anyway, I googled and my friend Wiki gave me a detailed explanation, with scary calculus stuff I sadly don't understand much. I vaguely remember that they are related to the S or Z transform. :)
(1) Boost Converter - Wikipedia

(2) Buck Converter - Wikipedia
I searched EE SE for "XL6009" found a lot of good Q&A. If you have not decided which boost converter to use, I would suggest to try this cheapy XL6009. I worry MT3608 is not powerful enough to drive a motor with steady current 200mA and start/stall current many times more than that.
4 hours later…
6:53 AM
Your SMPS seems very unusual than the cheapy SMPS hobbyists use. It datasheet seems to say that input can be AC or DC, and also gives the inrush current of 36A/24A. It also appears that the quite compact module has not snubber to suppress inrush current.
Snubber - Wikipedia

Apple iPhone charger teardown: quality in a tiny expensive package
7:11 AM
I am using a cheap AliExpress version of the RS SMPS Item 4 below.
(1) RS Pro LDE20-20Bxx AC/DC Converter 20W, AC-DC converter

(2) RS PRO, 20W Plug In Power Supply 5V dc, 4A, SMPS - HK$153

(3) RS Power Supplies Selection Guide - RS

(4) RS PRO, 240W Embedded Switch Mode Power Supply SMPS, 12V dc, Enclosed - RS £50

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