Can a DC-DC Switchmode supply cause a spike on the input supply voltage? - Page 1
Forum section: electronics / beginners

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: October 23, 2022, 09:51:35 am
content: If so, under what circumstances?Can a DC-DC PSU, whether it be something like an inverter, car amplifier, etc, generate (very briefly) a voltage on its power input terminals that's higher than the battery voltage?Trying to figure out two issues I've seen recently...Thanks!

author: Psi
author_rank: Super Contributor
timestamp: October 23, 2022, 10:20:18 am
content: It can yes.Especially if you have long wires from the power source to the DCDC and you don't have a large enough input capacitor physically close to the DCDC.Without a big enough cap you have high current switching pulses going through a long wire (inductor). So you can get high voltage ringing occurring on the input side.

author: MrAl
author_rank: Super Contributor
timestamp: October 23, 2022, 10:38:09 am
content: Quote from: AmmoJammo on October 23, 2022, 09:51:35 amDC DC converters draw current in pulses so for a time they will draw a large current and shortly later they will draw very low current. That means that any inductance (wires) connected in series with the input have the potential to generate back emf which will appear at the input to the switcher.Most DC DC converters have an input capacitor though which would help to mitigate this problem, but if it is not adequate there could still be spikes of higher voltage. The input caps have to be low ESR.If you can avoid long input wires that would be better but if not you may have to add a few extra input caps. They should be of the low ESR type. Caps with ESR too high dont help as much and maybe not at all or very little. The required ESR would be according to the max input current of the device.

author: Terry Bites
author_rank: Super Contributor
timestamp: October 23, 2022, 02:06:00 pm
content: All circumstances!It needs to be bought under control to avoid interference with other equipment.Its a good idea to add an LC section to smooth out the current drawn by the converter. To design of the LC section you need to know the switching frequency and the input impedance. Zin is Vin(max)/Iin(ave)Use MLCC cap(s) for low esr. The filter neeeds to go right at the converter's input. See application note 62AN101E from Rohm for instance.Check for large current loops in the input wiring. Use a common mode choke to close down current a loop.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: October 25, 2022, 11:00:53 am
content: Cheers for the replies!It basically confirms what I suspected, so a bit of background information!1000w pure sinewave inverter, connect it to a 120ah lithium "12v" battery, and the battery shuts down, disconnects the cells from the output.The same inverter works flawlessly off an AGM battery.You can also get the inverter to power up fine if the connect an AGM in parallel with the lithium, then disconnect the AGM after the inverter is on.Another situation.Trying to run a car amplifier off a modified server PSU.The power supply would go into protect and shut down, even though it can do 260 amps, more than any inrush current of any car amplifier I've looked at...

author: Infraviolet
author_rank: Super Contributor
timestamp: October 25, 2022, 02:34:07 pm
content: Yes, it can happen, LC spikes are one of the common ways if the switching regulator involves inductors and cermaic caps:Thishttps://www.pololu.com/docs/0J16/allis a good resource.Also good is a tip I was given on here, if you need to suppress these spikes but don't want to have to include (sometiems inconveniently large) electrolytic caps in a compact (in vertical height above board) design, you can fake a high ESR electrolytic by getting another ceramic and putting it in series with a resistor (1 to 10 ohm range often, but test for your application) and then using the pair as if they were an electrolytic.

author: MrAl
author_rank: Super Contributor
timestamp: October 25, 2022, 06:47:39 pm
content: Quote from: AmmoJammo on October 25, 2022, 11:00:53 amMany converters have a "soft start" mechanism because many switchers will draw a much higher current when starting up. The soft start allows them to come up to full operation after a short time but just enough to stop them from drawing a huge inrush current. You can measure this with a clamp on ammeter that can catch inrush current.The soft start was invented because of this, and if you ever looked at the inrush profile for some converters it would probably shock you because it can be 10 times higher than the normal input current. The input caps have to charge for one thing, but without soft start the converter itself could draw a very high current in order to get up to operating speed. So this might be what you are seeing.Sometimes you can start them up with a lighter load, and sometimes with a small resistor in series, then short it once it starts. You have to experiment with this though.You also have to be careful when connecting batteries in parallel or even in series. Some of these batteries can really explode.