Transient/Surge when plugging in device - Page 1
Forum section: electronics / beginners

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: December 14, 2023, 03:48:30 pm
content: This switcher works great when it is "soft powered" on - in other words, connected to the supply prior to energizing. And I can specify that must be done to the end user, but of course, there will be those who do not, and then damage the device. It would be better to protect the device against this.The supply (outside of my control) is 24VDC, but it will float around 32VDC when it's unplugged. When hot-plugged, there is a transient that lets the magic smoke out of the switcher, which is rated at 42V absolute max. This image shows VIN on U1 while I take the power lead and jam it in to my powered-on bench supply at 32V with no current limit (well, 3.2A)I'm looking for feedback on how to resolve this. I have a few thoughts1. I can change the IC to a different product that will handle this better.2. The BD9G101G datasheet does not mention anything about this, however, the datasheet for the manufacturers evaluation board DOES say not to hot-plug the board or damage could occur. This makes me wonder if I should ditch the BD9G101G altogether (see #1).3. I'm skeptical if a PTC will handle this fast enough4. The startup delay I recently added (R13/C11) has no effect5. Perhaps a diode can suppress this fast enough6. Maybe my ferrite bead is causing issues - on my list to remove it and bridge pads, and run the test again

author: Siwastaja
author_rank: Super Contributor
timestamp: December 14, 2023, 07:32:01 pm
content: Seehttps://www.analog.com/media/en/technical-documentation/application-notes/an88f.pdfTLDR: add any cheap-ass 47uF or so electrolytic in parallel with C1.Also remove FB1. Chances are high it does more harm than good.

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: December 14, 2023, 07:51:16 pm
content: Thanks, this is a great read!I will try to bodge something on tonight and do some tests. Perhaps replacing FB1 with a 0.5ohm resistor can help as well for this run of boards.I am space constrained so I will try to find a capacitor that meets the right size.

author: johansen
author_rank: Super Contributor
timestamp: December 14, 2023, 09:57:24 pm
content: The inductance of the supply in series with the capacitance of the load can produce a voltage doubler effect. A lossy electrolytic is one solution.

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: December 14, 2023, 10:16:00 pm
content: I have two findings1. Replacing FB1 with a jumper actually improves the transient. Perhaps FB1 was not very nice.2. Replacing FB1 with a 1ohm resistor also helps a ton.The device needs only 0.3W. I have ordered some 0.5/1/1.5 ohm resistors in 0805 to replace FB1 (can do a bit more tests), and that will take care of this batch.I could redesign the board to use electrolytic, but this will increase the height of the board. I do have some height constraints, but this would also increase risks for shipping with a single electrolytic sticking out 8mm, when the rest of the board is more or less flat currently.Given the low power constraints of the board I am wondering if replacing FB1 with a resistor could be an acceptable solution?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 14, 2023, 11:06:24 pm
content: A TVS isn't a bad idea, but the regulator should be rated higher (50-60V would be fine, MP2456GJ-P for instance, assuming little current is required) to ensure TVS peak clamping voltage doesn't exceed rating.Electrolytic is also a good option, dampening the casual overshoot of ceramic capacitors * cable inductance, but does nothing for any other kind of surge, in case it should happen.Tim