Multiple SMPS's on a  PCB sync'd up.....to reduce electrical noise? - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: July 28, 2020, 09:48:22 pm
content: Supposing there are multiple SMPS’s on a PCB which also contains noise sensitive signal and comms circuitry. Will it help to have all the SMPS’s on the PCB synchronized with each other? When I worked in an Automotive place they used to sync all the SMPS’s on a PCB, and it was said to be to reduce noise problems.SMPS Sync'ing is kind of not great, because it means that the switching instants are all lined up, and kind of add up with each other...however, if they are not sync'd, then every now and then you get the different SMPS's switching say about 100ns apart from each other, which causes a very harsh, high frequency noise "explosion". (Sync'ing avoids this) Would you agree?

author: schmitt trigger
author_rank: Super Contributor
timestamp: July 28, 2020, 09:56:54 pm
content: What you are describing is called interleaving.And yes, it is a technique to avoid the input current's leading edge of all the different converters not aligning with each other.

author: trobbins
author_rank: Frequent Contributor
timestamp: July 28, 2020, 11:40:53 pm
content: It could depend on what influence you are trying to minimise. Is it the influence of one switcher's noise on another switcher (as it could corrupt regulation sensing parameters at a particular time in a switchers cycle, and so perturb the outputs of other switchers), or is it the influence of all the switchers on some independent circuitry (which you have not described how they could be perturbed/influenced).For example, dithering the switching frequency of a switcher is one technique that can reduce noise experienced by other circuitry - but whether that is a benefit or not depends on the 'other circuitry'.

author: ocset
author_rank: Super Contributor
timestamp: July 29, 2020, 06:38:49 am
content: Thanks, yes, we are worried about the switchers interfering with sensitive comms and control circuitry on the same board.Interleaving, yes.....we would entertain that, though here i was just speaking of sync'ing all the switchers up so they switch together. Interleaving needs a little more circuitry and we aren't convinced its worth it....if sync'ing alone does the job, then we'll settle for that. If the SMPS's are jittered, then we won't be able to sync them or interleave them.But anyway, it appears that concensus so far is that sync'ing multiple SMPS's doesnt make them less electrically noisy to other circuitry (eg comms and control) on the same board?

author: tom66
author_rank: Super Contributor
timestamp: July 29, 2020, 08:36:49 am
content: There are a number of switching converter ICs that support what you describe via a SYNC pin - one benefit can be simplifying EMC filtering components as you can optimise your filter for one frequency instead of switchers operating at many distinct frequencies.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 29, 2020, 08:45:18 am
content: Quote from: treez on July 28, 2020, 09:48:22 pmNoo! The whole idea of syncing is toavoidthis. You sync them, but introducephase shiftbetween them, so that they never switch at the same time, not even accidentally. If left non-synced, who knows if parasitics make them sync up with no phase shift for worst EMI performance. Or then again, they free-run and sometimes create a higher combined pulse.Many switch mode ICs that have sync inputs/outputs already phase shift the output by 90 or 180 degrees (look at the datasheets) so that you get the phase shift just by connecting the sync pins directly. But obviously, the correct amount of phase shift depends on the number of converters you are running, so the integrated amount of phase shift isn't necessarily correct for all applications.If the converters share a DC link, this obviously reduces ripple voltage and EMI on that link, as well as decrease capacitor heating and possibly allow you to use smaller bus capacitance.

author: Berni
author_rank: Super Contributor
timestamp: July 29, 2020, 09:07:11 am
content: Witch a bunch of different switchers its more that synchronizing them makes them act more consistent.It reduces the impact of noise of one switchers on the others regulation loop. The noise they all make together is also consistent since they are not drifting in and out of phase where the noise sometimes cancels out and sometimes adds up. Such constantly varying noise tends to be more annoying to deal with than a constant even noise. The peaks might reach a lot higher and the noise might demodulate itself in some nonlinearity into wobbling DC voltages in sensitive circuits.The sort of evenly spreading out by phase (say 4 switches being 90 degrees out) is typically just used when multiple switchers are paralleled up. This is very common on PC motherboards and graphics cards where they need a say 1.2V 150A power rail. Its difficult making a single switcher for this much current so they use multiple parallel power stages to spread the load. Because the input to them is usually 12V this means they run at a very low duty cycle of about 10%, this makes for a very intermittent current flow of high current trough it and hard work for the capacitors, so spreading it out over 4 8 12 or even 16 phases can make a huge difference in the required capacitors, also helps with EMI since switching 150A all at once at 100s of KHz is sure to sing like hell.EDIT: Oh and about reducing the EMI of switchmode converters is very dependent on the PCB layout. You can throw all sorts of filtering and fancy phase trickery at your switchmode design, but if you screw up how the grounds are connected up in the switcher and everything else you can still have a EMI nightmare that barfs out RF.

author: nctnico
author_rank: Super Contributor
timestamp: July 29, 2020, 12:05:29 pm
content: Quote from: Berni on July 29, 2020, 09:07:11 amIndeed. With sensitive circuitry behind an isolated SMPS I usually add a common mode filter to the output to get rid of noise being pushed out of a DC-DC converter.

author: Berni
author_rank: Super Contributor
timestamp: July 29, 2020, 12:50:56 pm
content: Quote from: nctnico on July 29, 2020, 12:05:29 pmOh yes isolated DC/DC can cause some major common mode havoc as the whole isolated side jumps up and down with the switching frequency due to the capacitive coupling in the transformer windings.In once case of EMI testing a product having such a DC/DC i had to turn on the internal attenuation on the spectrum analyzer to keep it from getting its input overloaded. Then also found out that a portable FM radio looses reception if placed within half a meter of the thing (I tried that because i seen a massive spike right in the FM band).

author: dmills
author_rank: Super Contributor
timestamp: July 29, 2020, 04:28:21 pm
content: One useful trick with switchers and sensitive things where there are data converters in play is to sync the switchers to a harmonic of the sample rate.This will then result in the switcher noise aliasing to a DC offset at the output of the converter that is usually easier to deal with then spurious tones that can move as the switching frequencies drift. Well worth the time on things that matter.

author: ejeffrey
author_rank: Super Contributor
timestamp: July 30, 2020, 03:21:44 pm
content: The main benefit to synchronizing without multiphase is that it avoids generating low frequency beat notes which can be rectified by any nonlinearity in the circuit. This can generate audible noise or cause interference to other parts of the system. It means your ripple currents add coherently which isn't great but with unsynchronized converters they will sometimes line up anyway so it doesn't actually make things worse in that regard.Multiphase switching is obviously better as some of the ripple current is cancelled making filtering easier, but plain synchronization is still a good idea if you have multiple regulators with the same nominal frequency.

author: free_electron
author_rank: Super Contributor
timestamp: July 30, 2020, 03:49:22 pm
content: Quote from: treez on July 29, 2020, 06:38:49 amThere's multiple ways to mitigate this- spread spectrum switcher. This one does not run at a constant frequency but 'wanders' in a band. that advantage is that you spread the noise energy so you don't have a sharp peak somewhere.- multpihase switchers. this reduces input noise as well as output noise. and ripple. think about it : when you rectify with a single diode vs a bridge , vs a 3 phase bridge .. some advanced switchers for large loads can have up to 8 or more phases. especially when having to deliver large currents at low output voltage ripple becomes an isse. modern cpu cores running at 1 volt don't have whole lot of wiggle room ...whatever you do : do not synch switchers to each other. all you do is creating more 'hammering'. that all go boenk at the same time ...some swithcers have sync pins but they still include spread spectrum and they have a mechanism , through the sync pin , to make sure they stay apart from each other.

author: fourfathom
author_rank: Super Contributor
timestamp: July 30, 2020, 04:52:36 pm
content: Quote from: dmills on July 29, 2020, 04:28:21 pmA very long time ago I worked on a digital PBX (telephone switching gear) where they had locked the switching supply to the system sampling frequency, in order to reduce this type of noise. Unfortunately, while this did indeed clean up noise overall, some channels became vulnerable to noise caused by phase modulation of the switched waveform. Essentially that channel became a synchronous demodulator of the phase and amplitude shifts as the switcher was compensating for lime/load variations. As I recall I ended up synching the switcher to a different clock, letting the noise turn into a high-frequency beat which we could filter out, and I also cleaned up some layout issues to reduce the noise injection. Fun problem!

author: mark03
author_rank: Frequent Contributor
timestamp: July 30, 2020, 04:56:42 pm
content: If your concern is the comms circuitry sharing the same PCB as the switchers, I should think the point of syncing the supplies is not to sync them with each other, but rather to sync all of them with the sampling clock of the comms subsystem. This of course implies that the supplies will be synced with each other, but that is a side-effect, not the main issue. (Indeed, they could be synced with varying phases, as long as they are all phase-locked with the comms clock.)

author: ejeffrey
author_rank: Super Contributor
timestamp: July 30, 2020, 07:15:10 pm
content: Quote from: free_electron on July 30, 2020, 03:49:22 pmSyncing can be fine, especially if you have multiple relatively low power regulators for different voltages such as small POL regulators. Yes, ripple current will be higher, but 5 2 watt switchers synchronized is no worse than a single 10 watt switcher and few people would insist that a 10 watt supply use a multi-phase switcher as a general rule. Synchronization prevents low frequency beating which is still a big advantage even at low power.Interleaving is for sure better but if your choices are free running at the same nominal frequency vs. locked, I would pick locked every time.If you are talking about multiple high power switchers or especially a single rail with multiple switchers in parallel then you absolutely want interleaving.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 30, 2020, 08:11:44 pm
content: What I missed? Why would you sync the converters up but not interleave them at the same time? It's all about giving a phase shift, or a time shift to the sync signal you already have. Many switcher already add that delay for you.

author: Berni
author_rank: Super Contributor
timestamp: July 31, 2020, 05:46:01 am
content: For most of the cases it doesn't matter anyway.If there is switching noise getting into sensitive circuits that usually points to some other design issue. Next to no noise should be making it to the sensitive circuitry in the first place rather than trying to make this noise less harmful.You can make much more of an improvement using shielding, good PCB layout, filters, linear post regulation etc... If that is still not enough you can do some trickery to switchers like messing with the bootstrap capacitor to slow down the edges on its switch to trade off some efficiency for less EMI.I have seen switcher designs that blast our EMI like crazy and some switcher designs that a spectrum analyzer sitting right next to it can't even tell the difference between it being turned on or off.Sure in some niche examples with very sensitive circuitry there might be some benefit to synchronizing them, but its going to be very rare. (Apart from those high power parallel multiphase switchers where this makes a huge difference)