Rectifier with extremely low voltage drop - MOSFET? - Page 1
Forum section: electronics / projects

author: Didix
author_rank: Regular Contributor
timestamp: March 16, 2015, 05:31:20 pm
content: Hi thereI have an appliance (Ap) and a DC powersupply (PS). I need to use exactly this PS.The PS shows a ripple that I would like to get rid of.From my basic understanding I would now use a rectifier diode and a decoupling Elco.Unfortunately, the PS delivers 8.8VDC and the Ap shuts down as soon as the Voltage drops below 8.5V.Therefore, I cannot use a rectifier diode.Now I was thinking to use a N-Channel MOSFET with a very low RDS (Resistance Drain to Source).Unfortunately, MOSFETS hav a back diode.Well, I was thinking to put ten MOSFETS in series to get a reverse voltage drop of 7V.I think, there are better solutions to do that, aren't there?-DidixPS: I never have used MOSFETs before, so please excuse my newbie question.

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 16, 2015, 06:25:22 pm
content: Frequency and voltage(peek to peek) of the ripple?Mosfet will not help you. Depending on Vpp and F a power inductor with a capacitor might help.

author: Didix
author_rank: Regular Contributor
timestamp: March 16, 2015, 06:33:15 pm
content: 50Hz, 1V

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 16, 2015, 06:35:15 pm
content: Quote from: Didix on March 16, 2015, 06:33:15 pmAnd your PS is not broken ?

author: Didix
author_rank: Regular Contributor
timestamp: March 16, 2015, 06:46:06 pm
content: Sorry, I guess I am describing the problem the wrong way. I thought that would be an easy question ..Let's start over more precise:My appliance is a Lumix GM5.I made two measurements when the camera is started up. Green ist Voltage and white is current (measured over a shunt and amplified)The first image shows the curves when the camera is connected to my Lab PS.https://www.flickr.com/photos/didix57/16614941845/The second image shows the curves with the described PS.https://www.flickr.com/photos/didix57/16427932570/

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 16, 2015, 06:48:35 pm
content: And where is the 50Hz and 1Vpp?note: and what are you trying to filter and why?

author: Didix
author_rank: Regular Contributor
timestamp: March 16, 2015, 06:57:22 pm
content: your criticism is absolutely justified - mea culpa!As I stated before, I thought, the problem could be described with a simplification.Why is the current dropping? Wouldn't it be better to do something against that drop?

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 16, 2015, 07:06:23 pm
content: Quote from: Didix on March 16, 2015, 06:57:22 pmYou mean the voltage drop?It seems rather insignificant to me. Just a PS with some internal resistance. Nothing unusual there.The drop is only 0.1V. If you worried about that, don't be. Otherwise, use thicker cables, for example.

author: Didix
author_rank: Regular Contributor
timestamp: March 16, 2015, 07:28:48 pm
content: if you compare the two graphs on the time scale between 3.2 and 3.3 seconds on the one with the lab PS and between 4.3 and 4.4 with the dedicated PS,you recognize, that the white curve (current) when connected to the lab PS is much smoother than the one with the dedicated PS.The current is only for about 40% of the time on the level that is provided by the lab PS.Then the current drops to half and even less.It is doing this all 10ms. ( that's why I first guessed 50Hz).Ok, my camera works, but somehow this worries me and I would have preferred to do something against these 'current holes'. Therefore the idea with the Elco.Regarding your idea about the thicker cables: I use always the same cables. Also for the measurments on the lab PS.

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 16, 2015, 07:41:56 pm
content: Well, the PS has a slightly higher internal resistance that your lab PS.IMHO 0.2V drop on 9V (=2,2%) is nothing to worry about. And some other filtering, looking at the current graph.Where a you measuring, on the PS side or on the device side?Where is the ground from the probes connected?

author: Didix
author_rank: Regular Contributor
timestamp: March 16, 2015, 07:51:04 pm
content: I have a shunt on the high side near the camera.I use an opAmp as preamplifier (100x) and a NI USB-6001 as logger.I am not worried about the small voltage drop.This is partly accounts due to the shunt.What is irritating me is the fact, that the measurement of the current with the dedicated PS shows that deep drops all 10ms (although keeping the voltage) whereas the current from the lab PS stays stable.

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 16, 2015, 08:26:17 pm
content: Perhaps the output elco of the PS is not as big as the one in the Lab PS?Have you tried putting a elco on the camera side?

author: Didix
author_rank: Regular Contributor
timestamp: March 16, 2015, 08:28:25 pm
content: An Elco only does not help. The PS is switching. Therefore I had the idea of the diode.But the voltage drop is to high.How can I do this with an MOSFET?

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 16, 2015, 08:30:17 pm
content: Quote from: Didix on March 16, 2015, 08:28:25 pmand a power inductor in series?QuoteYou can't.

author: Didix
author_rank: Regular Contributor
timestamp: March 16, 2015, 08:49:03 pm
content: Well, I am afraid of using an inductor. I have never done so before and I fear a Voltage peak that goes above 9V and might kill my camera ...--- it's time for me to go to sleep now. Thanx a lot so far. I should be back here in two days ---

author: nctnico
author_rank: Super Contributor
timestamp: March 16, 2015, 08:58:05 pm
content: Just connect a capacitor parallel to the power supply or camera. No need for extra diodes.

author: eneuro
author_rank: Super Contributor
timestamp: March 17, 2015, 10:53:15 am
content: Quote from: Didix on March 16, 2015, 05:31:20 pmAC mosfets switch blocks AC currents and mosfets body diodes are not issue no moreHowever, we need two of them in series, so slightly higher RDSON, but we can use a few AC mosfets switches in parallel and with four mosfets we'll block AC current at similar RDSON in both directions and get rid of its internal body diodes

author: dexters_lab
author_rank: Unknown
timestamp: March 17, 2015, 03:01:52 pm
content: am i understanding this right?you bought a non-panasonic power adaptor for your Lumix camera and your trying to fix an issue where there is excessive ripple on the output of that adaptor?you either need a filter capacitor large enough or, if it was me... i would just buy a genuine panasonic power adaptor

author: wraper
author_rank: Unknown
timestamp: March 17, 2015, 03:08:44 pm
content: BTW this issue does not have anything to do with rectifier used.

author: Didix
author_rank: Regular Contributor
timestamp: March 18, 2015, 06:29:51 pm
content: Hi therethanx for constructive feedback!To get things straight ...I have a Lumix GM1 and a GM5, which I want to use for timelapse.Therefore I bought on ebay a DMW-AC8 and a DMW-DCC15 clone (Switching PS and DC coupler to put in to the camera).Works perfect and the timelapses get great.Therefore I do not have a problem and I do not need a new PSThen I was interested in knowing how much current the camera sucks for startup, taking a picture, saving, etc. (well, I am a curious person)Doing so I recognized those "current drops" all 10ms shown in this curve (white is current measured over a shunt, green is voltage)https://www.flickr.com/photos/didix57/16427932570/Comparing with a lab PS shows a different curve (much smoother).https://www.flickr.com/photos/didix57/16614941845/Being curious I am now wondering ...what causes the "current drops"?how could I get rid of those?Concerning the second question: an rectifying diode and a decoupling elco solves that problem in a linear PS. So why not trying here?Unfortunately, the PS delivers 8.8VDC and the camera shuts down as soon as the Voltage drops below 8.5V. So a normal diode does not solve the problem.Now I was thinking to use a N-Channel MOSFET with a very low RDS (Resistance Drain to Source).Unfortunately, MOSFETS have a back diode where the current would flow back from the elco.Well, I was thinking to put ten MOSFETS in series to get a reverse voltage drop of 7V.I think, there are better solutions to do that, aren't there?Quote from: eneuro on March 17, 2015, 10:53:15 amSorry, I am very new to MOSFETs and I cannot follow you. Although googling for "AC mosfet" I could not get further ..Quote from: dexters_lab on March 17, 2015, 03:01:52 pmexplained above ..Quote from: nctnico on March 16, 2015, 08:58:05 pmI think I tried this, but it did not change anything.I will make some more measurements next weekend.Quote from: wraper on March 17, 2015, 03:08:44 pmWell, I thought, that a diode, that lets current to an elco and avoids current to flow back is called a rectifying diode.Then I have read about "active rectifying with mosfets with low voltage drop" and thought, this could maybe solve me quest.So the main question is still open: how can I build a low drop rectifier (or diode) with a MOSFET?-Didix

author: JohnnyBerg
author_rank: Frequent Contributor
timestamp: March 18, 2015, 06:35:54 pm
content: Quote from: Didix on March 18, 2015, 06:29:51 pmSo, you loose energy in the wiring and internal resistance of the supply, and going to use a rectifier (of whatever sort) to compensate for that. Where does that energy come from?

author: Didix
author_rank: Regular Contributor
timestamp: March 18, 2015, 07:06:51 pm
content: Quote from: JohnnyBerg on March 18, 2015, 06:35:54 pmSince the PS is able to deliver 900mA (look at the graph at around t=4.48s) I would expect, that it could deliver enough energy at around t=4.3s to an elco to fill the gap?-Didix

author: TerraHertz
author_rank: Super Contributor
timestamp: March 18, 2015, 11:57:42 pm
content: Quote from: Didix on March 16, 2015, 06:46:06 pmYour lab PS is a well filtered, probably linear supply. And you're probably also using decent gauge wires to the camera - or at least to the point where you measure voltage droop with current.The PS that's giving you worries, is a crappy cheap switchmode, that's barely able to supply the needed current, has a nearly too-low voltage, and probably has the thinnest possible output wires. Probably also poor common mode switching noise filtering.Hence the serious switching noise and voltage droop on current peaks at your measurement point.Why don't you just buy a decent PS, with the right voltage, generous current capacity margin and big filter cap? And make sure the cables are not crap?

author: dexters_lab
author_rank: Unknown
timestamp: March 19, 2015, 09:18:07 am
content: Quote from: Didix on March 18, 2015, 06:29:51 pmif the camera is working just fine... why worry about it?

author: Tim F
author_rank: Regular Contributor
timestamp: March 19, 2015, 02:01:59 pm
content: All you need to solve that is more capacitance on the output of the PS and/or thicker gauge wiring between the PS and camera. You don't need a diode....but pretty much this:Quote from: dexters_lab on March 19, 2015, 09:18:07 am

author: eneuro
author_rank: Super Contributor
timestamp: March 19, 2015, 03:44:59 pm
content: Quote from: Tim F on March 19, 2015, 02:01:59 pmMaybe it could work longer?We have outdoor light bulb which shines, but... not too long, so... probably soft start could help a lot especially at winters, so since each light bulb is worth $1 if cheap easy soft start will extend its life span worth to try and good experiment for science too, while probably it is too often switched on/off in cold temperatures and it that is why it fails at higher rates.So, if something works it doesn't mean it can perform better and longer in better power supply conditions

author: janoc
author_rank: Super Contributor
timestamp: March 19, 2015, 03:50:26 pm
content: Quote from: Tim F on March 19, 2015, 02:01:59 pmConsidering that the voltage out of the PSU isDC already(albeit with ripple superimposed), I do wonder also what all those people talking about diodes and rectifiers wanted to achieve with them ...The OP needs either a better PSU (buying a clone switching PSU from eBay?) or stop worrying about non-issues ...