Making sense of power protection circuit - Page 1
Forum section: electronics / projects

author: morci
author_rank: Newbie
timestamp: January 31, 2025, 11:44:05 am
content: Working at a project using the MA12070p class D audio amplifier chip I stumbled on thishttps://www.infineon.com/dgdl/Infineon-Class_D_audio_amplifier_MERUS_Evaluationboard_DEMO_BASSAMP_60W_MA12070-ApplicationNotes-v01_00-EN.pdf?fileId=5546d46272e49d2a017351e1a4545964evaluation board and, specifically on the "On/off switching, reverse polarity and hot-plug protection" circuit on page 10:This the description of the circuit:QuoteWhat's I'm perplexed about is the slow raise cited at the end.Specifically, I tired to simulate the circuit with KiCad, simulating a 12V power supply connected at time t=3s and the SW38 switch shortened at time t=6s, focusing at the output voltage and at the voltage at the gates for the MOSFETs.Geez. This thing transfers current to the load as soon as the barrel plug in inserted even when the power button is off!The reason for this behavior is clear: C13 sees a square wave input that is passed it on its other pin for a small amount of time, this enables the gates until C60 draws enough charge to lower the voltage under the gates' activation threshold.But this was not my main focus. The point here is that I only see square waves here. Under which circumstances should the aforementioned slow raise manifest itself?Please note that I’m a total newbie here: this is my first time using KiCad and a circuit simulator. I studied electronics in high school almost 40 years ago, but I switched to computer science and never looked back—until recently. While I understand the basic concepts, I have zero familiarity with the tools and some remote memories about electronics.

author: mtwieg
author_rank: Frequent Contributor
timestamp: January 31, 2025, 02:00:31 pm
content: You're seeing how simple load-disconnect circuits can fail to operate as intended under certain conditions. Often newbies will only look at this circuit with Vin being static, in which case it works fine, but as you can see it turns itself on when either side is hot-plugged.Removing C60 should resolve this issue. It's not clear what purpose it served in the first place.

author: morci
author_rank: Newbie
timestamp: January 31, 2025, 05:03:10 pm
content: QuoteI guess that's the reason C60 is there.But then again, I see no such slow transition here...I mean, this circuit comes directly from Infineon. I've also seen it copied and pasted in other projects as well. What am I missing here?

author: morci
author_rank: Newbie
timestamp: February 20, 2025, 10:00:46 pm
content: So I decided to redesign that thing from scratch. This is what I came up with:And it seems to work as expected:A question though: how do I turn this thing off? I mean, yes, pulling Vg high, but what's the best way of doing it?

author: thm_w
author_rank: Super Contributor
timestamp: February 20, 2025, 11:31:23 pm
content: When we talk about soft start electronics, its usually not in terms of multiple seconds, maybe some ms or 10's of ms. Try zooming in a bit.3s turn on is way too slow unless there is a very specific requirement for that. Maybe for motors you'd have a 1-3s soft start.

author: morci
author_rank: Newbie
timestamp: February 21, 2025, 12:23:06 am
content: That's not the point. Change a couple of caps values and you're fine. It was just to make it more obvious in the chart.And, by the way, the original circuit was not "soft-stating" even in the realm of the 10s of ms.That being said, I'd really like to understand how to switch it off now

author: thm_w
author_rank: Super Contributor
timestamp: February 21, 2025, 10:57:44 pm
content: Its definitely applying some soft start if I look at the microcap sim here, maybe 4ms or so.