Gate driver charge cap for buck converter: whats the voltage it needs to handle? - Page 1
Forum section: electronics / projects

author: Odd
author_rank: Regular Contributor
timestamp: March 12, 2023, 12:01:53 pm
content: Please see attached datasheets."Cboot" capacitor may be 0.15uF / 150nF - but is there a voltage requirement?VIn is 25v

author: georges80
author_rank: Frequent Contributor
timestamp: March 12, 2023, 04:17:58 pm
content: Datasheet is missing a bunch of info. But for similar style buck drivers, it'll be whatever the Vbst is above Vsw. A 50V x7r ceramic will do just fine and be a common/cheap part. Voltage across that cap should likely be < 10VIf you want more complete specs, check similar topology buck drivers from mainstream suppliers, e.g. google and read datasheet for something like the AP62200.cheers,george.

author: Odd
author_rank: Regular Contributor
timestamp: March 12, 2023, 04:51:15 pm
content: Thank you.I am asking because I have observed a odd effect that destroys those controllers, and I don't know why.My load is MCU, opamps pulling about 15mA /5V from it , fed at 25V (input capacitor is 4.7uF , output is 10uF,100nF)If I just barely touch the Vin with a 25v source at a rate of 5-10Hz (just imagine poor connection/bounce during) - than there is a chance the regulator IC will be destroyed, apparently overshooting and killing med MCU as well.I can't figure out why "bounce" on input is so catastrophic.

author: georges80
author_rank: Frequent Contributor
timestamp: March 12, 2023, 08:26:40 pm
content: Put an electro at the input of the buck. What you are seeing is the LC effect of the wiring to from your supply to the buck, combined with ceramics at the input. Undamped spike that blows the buck and then pumps high voltage through to blow you uC.Something like a 10uf or so electro/tantalum will 'dampen' the LC effect at the input. i.e. you want some ESR in your input cap.A ceramic (say 10uF) with a series 0.47ohm (to give the ceramic some ESR) can also be used in place of the electro/tantalum.cheers,george.

author: Odd
author_rank: Regular Contributor
timestamp: March 12, 2023, 08:38:35 pm
content: I have this (attached) 47uF close to the switchmode, this is after 4cm of 0.4mm wide PCB trace. Would you say I still should add a resistor?

author: georges80
author_rank: Frequent Contributor
timestamp: March 12, 2023, 09:12:00 pm
content: If the cap you have is ceramic, then no, it won't dampen the LC effect.Read this ->https://www.analog.com/media/en/technical-documentation/application-notes/an88f.pdfcheers,george.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 12, 2023, 10:33:42 pm
content: Scope the input. Then put on a TVS, or electrolytic, or generally any R+Cb (with Cb >> C and R ~ sqrt(L/C), where L is the supply inductance, say from stray wiring length, ~0.6 uH/m) to clamp the input inrush/overshoot.With a linear capacitor, the maximum overshoot is twice the input, but ceramic lose value as voltage rises, making the pulse much sharper and taller.Tim

author: Odd
author_rank: Regular Contributor
timestamp: March 13, 2023, 07:07:34 pm
content: Thank you guys, learning is fun

author: MathWizard
author_rank: Super Contributor
timestamp: March 15, 2023, 03:29:01 pm
content: I was looking at 1N54xx general rectifier diodes, and they list the lead length under max current and reverse current. So there's PN-junction capacitance, maybe pF-nF and very little inductance from the leads, I would think p-nH, I can't remember the eq.Unless you were using these around switching PSU's, would the inductance ever matter with a few A at 60Hz ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 15, 2023, 03:34:39 pm
content: Quote from: MathWizard on March 15, 2023, 03:29:01 pmNot at 60Hz as such, no. Maybe a small amount during reverse recovery, but that should still only be a few A/us and not much ringing (and the mains or transformer inductance dominates over lead length).Mainly they note it for purposes of heatsinking i.e. it's an ideal sink beyond that length, so RthJA is dominated by lead conductivity.Tim