Power supply oscillation - Page 1
Forum section: electronics / beginners

author: mrkva
author_rank: Unknown
timestamp: March 05, 2017, 12:32:45 am
content: Hello,I'd like to ask about a problem I am having.I've built a simple 8 channel amplifier based on pre-assembled modules from ebay:http://www.ebay.co.uk/itm/DC-12V-24V-100W-TPA3116DA-Mono-Channel-digital-Power-audio-amplifier-board-car/182187294854To power them, I bought this power–supply (I know it is slightly underpowered if 8x 100W is necessary).http://www.tme.eu/en/details/gst280a24-c6p/mains-power-supplies/mean-well/It is directly hooked up to the amplifier modules with short cables. Here is a picture to give you an idea:(brown and blue wires)When I power the whole thing up, the supply turns on for half of a second and then turns off. This continues as a "sawtooth" oscillation.First I thought there is a short somewhere or one of the modules is broken, but when I hook it up to my linear lab supply, there is no problem.Actually I tried hooking it up to a 24V/2A supply just as a test (actually made by Meanwell too), and that worked without an issue as well.The "standby" load is 0.25A, so nothing dramatic which that beefy supply couldn't handle.So it seems like this 24V/11.6A supply has something different about it, but can't really tell what it could possibly be. Any ideas?Thank you very much for any suggestions!

author: james_s
author_rank: Super Contributor
timestamp: March 05, 2017, 04:29:41 am
content: It may be that trying to charge up all those capacitors on the amp boards is tripping the overcurrent protection on the power supply. Just for testing you might try placing a low ohm resistor in series between the power supply and amps. Something around 1-5 ohms ought to do.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 05, 2017, 07:53:19 am
content: Some switched mode supplies also have a problem with too much low ESR capacity directly at the output. So some resistance / inductance in the wires to the modules could be a good idea - thinner, longer cables might be all it takes.

author: mrkva
author_rank: Unknown
timestamp: March 05, 2017, 12:20:14 pm
content: I see, thanks for the tips. It just boggles me that the "weaker" switch-mode supply didn't have any issues with powering the circuit. I though the only problem I can encounter will be meeting the minimal load requirement.

author: james_s
author_rank: Super Contributor
timestamp: March 05, 2017, 04:11:00 pm
content: The weaker supply may not have such sensitive over-current protection. Discharged capacitors appear as a short circuit for a very brief period when connected across a voltage source.

author: mrkva
author_rank: Unknown
timestamp: March 05, 2017, 04:20:49 pm
content: I found a 4.7power resistor in my junk bin and it seems to solve the issue. But it obviously heats up a bit.So I guess I need to find a lower resistance one which will work without starting the oscillation.

author: Audioguru
author_rank: Super Contributor
timestamp: March 05, 2017, 06:19:53 pm
content: You do not want a resistor in series to the power supply input on an audio amplifier.Did you look at the datasheet for the Texas instruments TPA3116D2 stereo amplifier IC (assuming the Chinese amplifier uses the real one and not a cheap copy)?Its output is 75W (not 100W) per channel with horrible 10% clipping distortion when its supply is 24V and its load is 4 ohms. Its power is about 40W with the horrible distortion into 8 ohms.With a 24V supply and a 4 ohm speaker its output at fairly low 1% distortion (but still audible) is only 60W.

author: mrkva
author_rank: Unknown
timestamp: March 05, 2017, 06:55:42 pm
content: Quote from: Audioguru on March 05, 2017, 06:19:53 pmHow do you propose I should stop the supply from oscillating on startup? Since that is the original question and resistor in series seems to help.Quote from: Audioguru on March 05, 2017, 06:19:53 pmSorry, but I don't see how that is relevant to the problem I am describing. The amplifier is not designed for hi-fi use. It will be used with 50W transducers, so I guess the overhead comes handy.

author: james_s
author_rank: Super Contributor
timestamp: March 05, 2017, 08:44:01 pm
content: My suggestion of the resistor was for testing purposes, not a permanent solution. You might be ok with a NTC thermistor inrush limiter, or you could use a resistor with a relay to bypass it, or you could reverse engineer the OC protection on the power supply and add a small capacitor between the sense pin and ground to provide a slight delay.

author: TimFox
author_rank: Super Contributor
timestamp: March 05, 2017, 09:00:27 pm
content: Typically, these power supplies have "foldback" current limiting, where the current limit value drops to follow the dropping output voltage on a short circuit. I have seen such systems oscillate with large load capacitance. Seehttps://en.wikipedia.org/wiki/Foldback_(power_supply_design)

author: Zero999
author_rank: Super Contributor
timestamp: March 06, 2017, 11:43:57 am
content: Quote from: mrkva on March 05, 2017, 06:55:42 pmHow about using a timer relay or a transistor to bypass the resistor once the capacitor is charged up?A MOSFET and RC circuit can also be used as a series resistor with a high value at switch on and low value after a few seconds.https://www.eevblog.com/forum/beginners/powering-a-dc-motor-with-plug-in-transformer/The above circuit was designed for 6V operation. If you want to run it at 24V, you'll need another 1M resistor between the gate and source pin, to limit the gate voltage to a safe level.When the MOSFET starts to turn on, the effective capacitance of C1 is increased due to the Miller effect, so the delay is longer then the RC time constant would suggest.RL is the circuit being powered.

author: james_s
author_rank: Super Contributor
timestamp: March 06, 2017, 04:42:29 pm
content: I think I would be inclined to use a zener diode to limit the gate voltage, otherwise yeah that looks like a good circuit.