Why does a charger/SMPS makes buzzing sound with no load? - Page 1
Forum section: electronics / beginners

author: Darkwing
author_rank: Contributor
timestamp: October 28, 2015, 11:09:49 am
content: Hi folks!Title says it all. I noticed that chargers like the iPad charger or several other USB chargers of all price ranges make a slight buzzing sound when there is no device/load plugged to them, or when the device is fully charged. You can hear it when you put your ear near to the charger in a quiet environment.What causes this noise? Is it cheap electronics, built quality or what?Noise means that something must be whiggeling at a rate from 3000 Hz to 10000 Hz, otherwise I wouldn't be able to hear this high noise. What could be whiggeling and why?Thanks!

author: firewalker
author_rank: Super Contributor
timestamp: October 28, 2015, 11:22:28 am
content: With no load the power converter will go in stand by mode. Some ics will turn down the switching frequency to kHz range, so you can actually hear the switching frequency.Alexander.

author: con-f-use
author_rank: Unknown
timestamp: October 28, 2015, 12:07:05 pm
content: As to what's generating the noise. It's either transformer noise or ceramic capcitors that vibrate. Nothing to really worry about unless the charger gets hot or it didn't behave like that from the beginning.

author: fivefish
author_rank: Frequent Contributor
timestamp: October 28, 2015, 12:23:28 pm
content: Usually the inductors are buzzing (due to inductor construction). Some even make a "frying bacon sound".Some of the better made (more expensive) shielded inductors don't have this problem, while the cheaper ones do.

author: Richard Head
author_rank: Frequent Contributor
timestamp: October 28, 2015, 12:31:52 pm
content: The control loop goes into a state refered to as "Hic-cup" mode. The voltage control loop reduces the duty cycle to the minimum that the control chip can produce after which the pulse terminates. The result is that it takes several clock cycles for the output voltage to drop below the voltage setpoint, during which time the PWM is inhibited. As soon as the output voltage drops below the setpoint the PWM again generates a pulse that it too wide so the output voltage overshoots again, and the cycle continues. This causes the power to be transfered perhaps every ten clock cycles compared to once per cycle. The magnetostriction in the core is what causes the audible noise and this occurs only when the power pulse occurs. So if it's only pulsing every tenth cycle and the switching frequency is nominally 100khz then you will hear the 10khz pulse frequency. If you apply even a very light load the condition disappears.Many current mode control chips such as the UC3844/5 etc require a small RC to be placed between the current sense resistor/transformer and the current feedback pin. This RC contributes to the minimum pulse width that the circuit can generate. A small amount of pre-load can eliminate the problem.

author: Darkwing
author_rank: Contributor
timestamp: October 28, 2015, 01:56:50 pm
content: Thanks so far for the explanations!How would you explain the following behavior: when there is no load and the noise occurs, I unplug the charger from the power socket. The noise continues for a little while, changes its frequency, going higher and higher until it stops after a couple of seconds.How would you explain this effect?Thanks again!

author: Richard Head
author_rank: Frequent Contributor
timestamp: October 28, 2015, 02:04:13 pm
content: Geez, I don't know. I'd have to get the scope out. Probably something to do with the input voltage dropping.

author: wraper
author_rank: Unknown
timestamp: October 28, 2015, 02:22:36 pm
content: Quote from: Darkwing on October 28, 2015, 01:56:50 pmWhile voltage on the mains side capacitor is dropping, controller increases pulse frequency to keep the same output voltage.

author: mariush
author_rank: Super Contributor
timestamp: October 28, 2015, 02:34:44 pm
content: The voltage coming from your mains socket is rectified to DC voltage using a bridge rectifier, so your 230v becomes Vin x 1.414 or about 320v. But it's not a constant 320v, just like the AC is a sine wave going up and down 50 times a second (50 Hz mains frequency), so will the DC voltage after the bridge rectifier, 100 times a second the voltage will go from 0v to 320v and back.To prevent this and to keep the voltage as close as possible to the maximum all the time, an input capacitor is used. It charges with energy and then when the voltage from bridge rectifier goes down towards 0v, the capacitor provides the missing energy.The chip that converts the high voltage to lower voltage is design to work with a wide range of input voltage (let's say from 60v to 350v) and when the input voltage changes, it simply adjusts the number of pulses sent through the transformer to the low voltage side, to keep the output voltage within the desired range. These pulses are normally at high frequencies, like 60-100 kHz (thousands of times a second) so even if these on and off cycles would cause components to vibrate (or expand and shrink), it would happen at those high frequencies so you won't hear (you don't hear anything above 15-18 kHz)When you don't have anything connected at the output, the chip goes into a low power mode, it keeps doing its job because some components on the low voltage side have some losses, so some energy will be wasted inside the circuit, but the converter chip runs at lower frequencies which can become audible.In such scenario, that input capacitor will have a lot of time to charge up with a lot of energy and the voltage will stay close to the maximum (300v and higher).When you pull the adapter from the wall socket, the energy is still in that capacitor but it's no longer replenished and it will slowly be consumed by the converter chip and the components on the low voltage.As the voltage on that capacitor goes down from 300v+ to under 60v or so, the consumption on the low voltage side remains the same (very small) so the converter chip will have to slowly adjust (increase) the frequency of pulses to keep the output voltage to the same levels. That's when you hear the noise changing.Normally, discharge resistors are used wherever there's high voltage.. such resistors are placed across the leads of large capacitors and are basically there to discharge such capacitors to safe voltages (let's say less than 10-20v) in a short amount of time, let's say 5-10 seconds. Your adapter/charger probably has no such resistor or a very poorly chosen one, allowing for the product to stay working for a period of time that's longer than necessary after being pulled from the mains socket.

author: Darkwing
author_rank: Contributor
timestamp: October 28, 2015, 03:50:20 pm
content: That perfectly does make sense, thanks!Quote from: Darkwing on October 28, 2015, 01:56:50 pmSo I'd assume there is nothing wrong with the bleeder resistors in the devices tested.The noise stops after about 10 sec.Again: thanks a lot!Great explanations!

author: kkamel
author_rank: Contributor
timestamp: March 03, 2023, 06:10:47 am
content: Is it possible to reduce this problem by listening to which component making the buzz and add a bead of silicon?

author: EEVblog
author_rank: Unknown
timestamp: March 03, 2023, 06:53:44 am
content: As others have said, various components can emit audible noise caused by excitation when driven at certain audible frequencies. Transformer and their laminted cores are classic, inductors, and even ceramic capacitors can all do this.