PWM noise through the PSU? - Page 1
Forum section: electronics / projects

author: Things
author_rank: Regular Contributor
timestamp: September 07, 2014, 10:48:56 am
content: So I'm driving about 15A worth of 12V fans using PWM and some FET's. I've already incorporated a lowpass filter on the fan power to get rid of the PWM buzzing, but now it seems my open frame switchmode PSU is buzzing like mad as a result of the PWM too. I've tried adding about 8800uF of extra capacitance to it's output but still have the issue.Anyone have any suggestions how I could quieten the PSU (it's fine with a constant load, just not PWM)? Have also opened it up and added a bit of extra glue to the large output inductor to no avail.Cheers,Dan

author: penfold
author_rank: Frequent Contributor
timestamp: September 07, 2014, 12:05:18 pm
content: You'll need a filter inductance to get the current ripple down, capacitors are rarely effective without an inductor

author: DanielS
author_rank: Frequent Contributor
timestamp: September 07, 2014, 12:55:20 pm
content: It would help to have a schematic of exactly what you have done so far. Otherwise, all we can do is take random shots in the dark.In a typical PWM regulator, you would usually have an inductor and free-wheeling diode (or synchronous rectifier) as part of the output circuit to smooth current out. Without it, you are shorting the source into a capacitor and that will generate nasty current transients.

author: Things
author_rank: Regular Contributor
timestamp: September 07, 2014, 01:05:59 pm
content: Scuse the crude diagram, don't have access to my desktop, but basically this (times 3):It is being controlled by an ATMega328/Arduino, so PWM is running at ~490 and ~970hz (unfortunately I can't take it any higher).The fans are nice and quiet now, but the PSU sounds awful. Seems to be most noticeable with this SMPS though, running it from a computer PSU is fine.

author: DanielS
author_rank: Frequent Contributor
timestamp: September 07, 2014, 02:48:17 pm
content: What you want to do is something more like what I attached.The inductor prevents instantaneous current changes and the diode (a Schottky type with adequate ratings for your application, which would be 20V and 20A) provides a path for the inductor current to go through when the MOSFET opens.At only 1kHz though, that inductor would need to be rather large. Most of Atmel's microcontrollers have built-in counters that can be configured as basic PWMs. You might want to look into those if they are not already tied up into other stuff.

author: diyaudio
author_rank: Frequent Contributor
timestamp: September 07, 2014, 05:21:48 pm
content: Quote from: Things on September 07, 2014, 10:48:56 amYou have access to a scope ? Measure the ripple frequency, design a simple LC second order LP filter (High value C) low turn count inductor (L) that should do the job, you need an inductor to squash out that harsh ripple, a capacitor alone wont do the job if the ripple current to high.so much fun basic stuff to experiment with an calculate.

author: johansen
author_rank: Super Contributor
timestamp: September 07, 2014, 05:52:11 pm
content: yeah what do you think happens when you have a capacitor charged to 6 volts and then you connect it to your power supply through a .25 ohm resistor.hmm 24 amps inrush current?no wonder its buzzing.use an inductor and a diode wired up like a buck converter.or get fans with a pwm input signal.

author: Things
author_rank: Regular Contributor
timestamp: September 07, 2014, 11:40:28 pm
content: Decided just to switch back to a computer power supply for this. Decided I was going to be pushing the open frame SMPS a it too hard for my liking. Computer PSU is silent

author: salbayeng
author_rank: Frequent Contributor
timestamp: September 08, 2014, 01:23:55 am
content: As other contributors have noted, you really need an inductor and circulating diode, If the fans are brushed DC , then the internal inductance is sufficient.It's entirely inappropriate to blame the PSU , and attempt to "fix" it , when the problem could be solved by learning how to apply PWM correctly.

author: Things
author_rank: Regular Contributor
timestamp: September 08, 2014, 01:27:51 am
content: This PSU has some other weird things going on too, but it's not surprising for an eBay cheapy. The large output inductor is just 2 toroid cores stuck ontop of each other and wrapped as one large toroid. It also makes a loud clicking noise when you turn it off at about 10hz or so.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 08, 2014, 03:25:29 am
content: The power supply is doing exactly what it should, for what you are demanding of it. A pulsating load, at a rate well below the cutoff frequency of the output filters (~kHz), is causing it to throttle up and down repeatedly. This probably isn't a reliability problem, but evidently there are inductors or capacitors or something in there which are audible under this stress.It's probably making a fair bit of supply voltage ripple too. At medium frequencies, the power supply is unable to control its output very tightly -- if it tried, the excess gain, combined with the phase shift of the output filters, would cause it to oscillate. At low frequencies (including DC), the regulation is as stable as it can get, probably within 10mV over the load range. But at those frequencies inbetween, it can potentially be much worse.Your only solution is to avoid doing that in the first place -- use a much higher frequency (>10kHz?) and an LC filter at the fan. As long as the PWM remains constant, the only byproduct will be switching ripple, which the power supply can filter out (but more importantly, you'll need to filter it locally so you don't interfere with other things; but that's easy because an electrolytic or a few will do the job nicely).Tim