RLC filters and DC-DC converters - Page 1
Forum section: electronics / beginners

author: ivan747
author_rank: Super Contributor
timestamp: December 07, 2014, 07:11:44 pm
content: Hello, I am designing a lab power supply (based on another design) with a preregulator consisting of a 700kHz DC-DC converter. So, the circuit is as follows: there is a DC-DC converter followed by an LDO, the DC-DC converter tracks the outut of the LDO and adds 1.7V to it. The max current output of the supply will be 3A. The converter outputs about 5 to 20mV of ripple (depends on how you measure it and whether it's simulated or not). This was unacceptable by me for a lab power supply, so I figured I needed a filter.With that in mind, I set myself the task of selecting proper component values for a filter that could take at least 20dB out of that. Being mostly self-taught in electronics, I hadn't studied LC filters that closely,so kinda struggled with the filters, but I got something more or less usable, it seems.I'd like you to please warn me if my solution is wrong. Remember this is going to go between the DC-DC converter and the LDO.So I made an RLC filter like this:Where R is 0.1 ohms, L is 1uH and C consists of four paralleled ceramic 10uF caps.That gives me a 25kHz cutoff frequency. It's not very damped the circuit, at a damping factor of 0.32. I simulated the thing in LTspice and it resonated at 23.8kHz, but the resonance had only 4dB of amplitude, so I kinda ignore it because the voltage regulator has good ripple rejection at that frequency.I decided that R had to be somewhere in that range because of the power dissipation. 0.3V*3A is almost 1W. In practice I am going to use a thick film SMD 2W resistor.For the inductor I am using this:http://katalog.we-online.de/pbs/datasheet/74404063010.pdfFor the caps, as I said, I am using four ceramic caps, close to the LDOs.The inductor and the resistor will be close to the DC-DC converter and the caps will be close to the LDO, is this okay?I'd like you to tell me if the filter is okay or if I am missing something:Can I trust the simulations?What is the proper way to arrange this on a PCB?Is this is a typical way to filter (and combat resonance in) a DC-DC converter? How doyoudo your DC-DC filtering?Thanks guys

author: Zero999
author_rank: Super Contributor
timestamp: December 07, 2014, 07:28:08 pm
content: One problem with that is if you connect a circuit to the output which oscillates at the resonant frequency, it'll ring.You could connect a resistor across the inductor to make it critically damped and the inductor will bypass the resistor at DC, thus lowering the voltage drop.

author: madires
author_rank: Super Contributor
timestamp: December 07, 2014, 07:28:53 pm
content: I'd just use a LC filter, maybe 10-20µH and 100 or 220µF (that's what my stomach says.

author: awallin
author_rank: Frequent Contributor
timestamp: December 07, 2014, 08:17:06 pm
content: Quote from: ivan747 on December 07, 2014, 07:11:44 pmquite often the problem with dc-dc converter or other switched mode psus is common mode noise.it can't be removed with any amount of filtering only on the output-'wire'. You need either a common-mode choke through which bot output and ground go. Or filter-caps across the isolation-barrier from both the output and GND to a known stable potential (case-ground/protective earth ?)check this:http://www.murata.com/~/media/webrenewal/products/emc/emifil/knowhow/26to30.ashx

author: ivan747
author_rank: Super Contributor
timestamp: December 07, 2014, 09:08:27 pm
content: Quote from: awallin on December 07, 2014, 08:17:06 pmSomebody mentions this in another thread, I remember. I learned the following:https://www.eevblog.com/forum/projects/is-this-board-solderable/msg557920/#msg557920Tim says that due to my DC-DC converter layout (which is posted right above the message I linked), my board can be easily filtered for common mode noiseSo that's a plus.Now... I need to filter both differential and common mode noise using separate filters right?If that's how it's done, I'm going to read up on differential filters too. Since it's a lab power supply, I have earth connections available, that won't be a problem (the idea is to connect both the positive output and the "ground" of the output of the DC-DC converter). Something I won't have available is a metal case, I already bought and decided on a small plastic case, but I could cover it with copper sheets somehow or make little copper shield cans for the board.Quote from: Hero999 on December 07, 2014, 07:28:08 pmLike the resign on the right on the attachment? O0 I think I'm getting the hang of it, for differential filters

author: ivan747
author_rank: Super Contributor
timestamp: December 07, 2014, 09:21:49 pm
content: I think having a perfectly damped circuit is not as helpful for me as having a circuit that is almost perfectly damped. The reason is, you get some resonance, but if that resonance is at a frequency that your regulator can handle, your regulator will get rid of it, or most of it. The main advantage of the not perfectly damped circuit is that the slope is higher, so when I get to 700kHz, I get more attenuation (which my regulator cannot compensate).

author: ivan747
author_rank: Super Contributor
timestamp: December 07, 2014, 10:09:41 pm
content: Well, I'm a bit confused with the common mode filter. Should it go on the input stage of the DC-DC converter? At the output of the DC-DC converter? At the output of the 60Hz transformer?No ideaEdit: I have no idea on what sort of frequency to design it for either. Can't I just get one of those IEC sockets that go into enclosures and have space for a fuse and have built in filters?I'm a bit lost here, and all the info I look for doesn't give any practical implementations or solid advice. Everybody explains what a common mode filteris, but no one describes where andhowto use them, what sort of component values to use, nothing.The most practical information out there is in regards to the kind of DC-DC converter you find in ATX supplies and laptop power bricks, the sort that plugs into the mains, but I can't seem to find anything for local regulation

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: December 08, 2014, 03:37:43 am
content: I have no idea where I got this Design Note from but I think it is right on subject. See attached PDF.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 08, 2014, 04:19:00 am
content: Common mode is a separate issue, and much more complicated (in your simulation, you're probably using GND = net '0', but in reality, there will be voltage superimposed on those 'ground' connections).Differential filtering alone is fairly easy: you pick a cutoff frequency and filter order that's sufficient to attenuate the noise as required, then dampen it so everything else is happy (mind the LDO may go unstable if you have even a well damped LC in its control loop bandwidth -- they're finicky buggers).The best way to design a power supply filter is to use the minimum LC necessary, then damp that with an R+C in parallel with the C. It's not quite as good as a critically (or over-) damped RLC, but doesn't cost any DC resistance.Beefy capacitors generally have nonzero ESR (electrolytics, tantalum and the like, but not ceramic or polymer types which have very low ESR), which can be harnessed for this purpose. Tantalum are preferred because the ESR is well controlled, whereas the electrolytic ESR varies strongly with temperature.Although such capacitors are commonly referred to as "bulk", they actually have little direct effect on the supply impedance -- high speed transients are supplied from the small (ceramic) bypass(es), while low frequency surges are supplied by the regulator itself. The bulk capacitor nonetheless has an important function in the supply network.Tim

author: Zero999
author_rank: Super Contributor
timestamp: December 08, 2014, 11:30:08 am
content: Quote from: ivan747 on December 07, 2014, 09:21:49 pmThat's true, the damping reduces the Q which reduces the attenuation but it becomes more susceptible to oscillation under transient conditions.Try connecting a load to the output and switch it with a MOSFET, close to the resonant frequency of the filter and see what happens.