Switch-Mode Power Supplies.

[Joe]

I recently came into possession of a 9V switch-mode 'wall-wart.' I found when connecting it to my bench amp that it caused a high-pitched whistle which was far more intrusive than mains hum.

I 'scoped it and saw this:-

   

Putting capacitors across it gets a nice flat line but doesn't seem to do much about the whistle and, having only the vaguest idea of how they work, I don't even know if this is safe to do.

I know they are used in audio gear so what sort of filtering is necessary? I have no pressing need of it but it certainly would be handy and interesting to fiddle with.

- Joe

[tiv48]

hi jo this is the same problem that i had with the pub landlords laptop psu
that turned out to be a lack of mains input filtering smps run at highish freqences and with out proper filters can radiate harmonics into the rf band you could try a ferrit ring on the output lead .and or an rf choke
smps caused me more headaches in tvs sat boxes ect than any other part of the circuit
rob t

[AlanBeckett]

Joe,
Just to clarify, is this an Audio whistle or radiated rf which you pick up on your P75?
Alan

[Joe]

It was audio as there wasn't a radio switched on at the time. I was trying it out on an amplifier and getting this whistle with my MP3 player plugged into it. Disconnecting that still left the whistle.

That 'scope trace is four-and-a-bit squares peak-to-peak (time) with it set to 5 ms/div and a volt high (5 x 200mV).

- Joe

[Skywave]

Putting capacitors across it gets a nice flat line . . .

(a) Would they be capacitors (a) across the d.c. O/P, or (b) across the input primary winding?
If (a) then there is a serious failure of the rectification and filtering of the d.c. voltage.
If (b) then there is severe harmonic distortion, probably caused by a partially-failed mains transformer or one with an inadequate core size.
The whistle is due to the high harmonic component of that waveform. Cases (a) and (b) will cause this.

Al.

[Joe]

The capacitors were across the final output.

The device in question is actually a PSU for a BT HomeHub and I'm wondering if most of the smoothing/filtering components are inside that rather than the power supply.

As it's one of these sealed-up things playing about within is difficult and working in close proximity to mains part is not to undertaken lightly either.

- Joe

[Skywave]

Thank you Joe for your reply: all is now clear.

I think you may well be right, viz. the PSU filter / smoothing / regulation components are physically remote from the transformer module. This seems to be a common practice these days, so that the manufacturer can select a mains transformer to suit the a.c. supply voltage in the unit's destination country, but retain the otherwise unaffected d.c. circuitry. Hence, manufacturing costs are reduced. Hewlett-Packard have been using this practice with their ink jet printers for many years now.

Al.

[Yorkie]

As to 'transformers' AFAIK, switched mode power supplies don't use a 'mains transformer' in the conventional sense to reduce the AC voltage from mains potentiial to that which is required. Basically, I think they usually consist of a bridge to rectify the incoming AC to DC, still at HV, then a smoothing cap, with the HV smoothed DC passing to a DC-DC converter, which switches at a frequency of 100 kHz to 1 MHz. A Pulse Width Modulator circuit then regulates the DC output voltage by adjusting the ON/OFF durations of switching transistor, so you get a short period of full on DC, then a period of full off, and the process repeats itself at the desired operating frequency to give the desired output voltage. A feedback circuit monitors the output voltage and adjusts the frequency of the DC-DC converter to stabilise the voltage -the power supply's pulse-width modulating (PWM) circuit comparing the supplied 5VDC output to an accurate Voltage reference so that an error-correcting feedback signal develops. This signal adjusts the relative ON and OFF durations of the DC-to-DC converter, thus holding the output at the required voltage. I believe that SMPSs are very fussy about having electrolytics with low ESRs, and the proliferation of SMPSUs in computers and as mains adaptors, which perhaps explains why ESR meters have come into vogue in recent years.

There's a simplfied explanantion of how SMPSs operate, which is pitched at about my level, here:

http://ecmweb.com/power_quality/electric...r_quality/

They tend to use toroids rather than conventional mains transfomers, which is a big advantage in terms of keeping down the weight, size and cost of the PSU. For such things as plugtop adaptors for say laptops, SMPSUs aren't not much bigger than the plugtop itself. I use one for an Apple Macbook Pro - I've never looked at the output on a scope but I can imagine it's not a pretty sight! Maybe I'll take a squint at it, if only out of curiosity:omg:

[Skywave]

Indeed, David, of course: a silly over-sight my me!
Apologies Joe.

Al.

[Yorkie]

Indeed, David, of course: a silly over-sight my me!
Apologies Joe.

Al.

No call for blush's or apologies Al - I only have the sketchiest knowledge of SMPSs. If we knew a bit more about SMPSs, we'd know whether the waveform that Joe sees means that there's a fault, or that it's working perfectly! To quote Erasmus of Rotterdam: 'In the kingdom of the blind, the one-eyed man is king!' (There have been some useful articles in electronics magazines about how to convert PC SMPSs to high current bench power supplies).