Re: SMPS noise
From: Terry Given (my_name_at_ieee.org)
Date: 02/24/05
- Next message: Treeline: "Re: Are 5 GHz telephones safe?"
- Previous message: Treeline: "Re: Are 5 GHz telephones safe?"
- Maybe in reply to: Joerg: "Re: SMPS noise"
- Messages sorted by: [ date ] [ thread ]
Date: Fri, 25 Feb 2005 12:48:59 +1300
QQ wrote:
> Hi,
>
> I am experiencing a problem with some DC/DC non-isolated buck
> converters using LM2743, on a otherwise digital board (16 layer) and am
> grateful for any help offered.
>
> I have designed a number of voltage mode DC/DC converters on a board.
> They are split into - a group of 3 (G3) that are close to one another
> and another group of 5 (G5) which are close to one another. They all
> run off a 12V power rail. All regulators are running at 350 KHz
> switching frequency approximately (the resistor that sets this does not
> gaurentee that the frequency will be 'exactly' set).
>
> The problem is - one of the regulators in G3 and one of the regulators
> on G5 have a certain amount of low frequency noise (50mV max -
> periodic, 4 KHz freq , but not sinusoidal, rather irregular shaped)
> that in turn has the normal switching noise (15mV) of the buck topology
> riding on it - as measured with a BNC cable probe, on the output caps -
> a 'null test' was performed too and I am quite confident that the noise
> is real.
>
>>>From what testing I have done so far, I have observed some symptoms:
>
> During all tests the noisy G3 rail is isolated from everything else on
> the board. It had a second noise filter inductor due to an ultra low
> noise requirement, which I removed for the testing. Not looked at the
> noisy G5 rail as of yet. The noisy G3 rail is 1.2V and the one
> immediately next to it is 1.8V. The noisy G5 rail is 5V.
>
> - When the other regulators in G3 are turned off, the low freq noise on
> the noisy regulator in G3 is reduced and changes shape but does not go
> away altogether.
> - When all other regulators on board are switched off the LF noise on
> the noisy regulator in G3 goes away altogether.
> - If I change the freq of the noisy G3 regulator to 300 kHz or 400 KHz
> while keeping switch freq on the other regulators 350 KHz as before, LF
> noise on the noisy G3 regulator goes away
> - If I change the freq of all regs to 300KHz the noise reappears on the
> noisy G3 regulator.
> - I tried lifting the SGND (connected by a thin net to PGND as of
> now)pin of the chip and connecting it to another ground point - this
> doesn't help. I tried air wiring the Vfb net too. This doesnt make the
> LF noise go away.
> - I checked that the Vcc power (5V) to the switching controller is
> clean. This is a differnt Vcc from the noisy 5V regulator mentioned
> earlier.
> - Basic rules such as short fat planes for the noisy power portion,
> AGND isolated from DGND etc. are followed. I am not an expert though
> and may have violated some not-so-basic rule of layout.
> - If I look at the switching node on a scope I see that the duty cycle
> has a jitter of about 50 ns though the frequency is very constant. When
> all regulators (except the noisy G3 reg) is switched off, then the duty
> cycle jitter becomes much lower - about 10ns.
>
>
> Based on this my main conclusions (which may be incorrect) are:
>
> - The problem is not loop stability. When all regs except the noisy G3
> reg is switched off, the LF noise dissapears. Also the noisy G3 reg is
> isolated from the board which means that its load is not changing
> depending on whether the other regs are on or off. I havent done a load
> transient test yet though.
>
> - I am thinking there is somehow noise injected from a nearby regulator
> on the PWM ramp inside the controller chip because the LF noise is
> periodic and depends on whether a nearby controller of the nearly same
> freq is on or off. I am rationalizing the fact that changing the freq
> of the noisy reg wrt to the others means that their respective PWM
> ramps change their phase rapidly (convolve?) wrt to each other which
> makes the 'noisy' overlaps last for a shorter time.
>
> Is this a fair conclusion? If so how do I reduce this interference and
> how do I methodically find the source of this noise? What is the most
> likely entry point for such noise. I can re-lay out the board if
> necessary.
>
> I am sorry that I cannot publish the schematic and layout due to
> confidentiality issues.
>
> Thanks,
> QQ
>
In addition to the other pertinent comments, what about magnetic
coupling between your chokes? if possible, use closed magnetic cores (eg
toroids, pot cores or shielded drum cores). orienting adjacent magnetics
at right angles to one another will also help. Many smps chokes have
huge air gaps - ie use drum cores. Most of these are also available as
shielded designs - a ferrite sleeve is placed around the assembly,
spaced far enough away that it doesnt wreck the NIsat
Cheers
Terry
- Next message: Treeline: "Re: Are 5 GHz telephones safe?"
- Previous message: Treeline: "Re: Are 5 GHz telephones safe?"
- Maybe in reply to: Joerg: "Re: SMPS noise"
- Messages sorted by: [ date ] [ thread ]
Relevant Pages
|
