paralleling switchmode regulators? - Page 1
Forum section: electronics / beginners

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: December 18, 2013, 05:46:31 am
content: I've just put together a little power supply, to be run off a surplus laptop power supply.It uses two premade LM2596 "modules", basically the same as these:http://www.ebay.com/itm/10-pcs-DC-DC-LM2596-Step-Down-Adjustable-Converter-Power-Supply-Module1-3V-35V-/130984316514?pt=LH_DefaultDomain_0&hash=item1e7f460e62I have 0.15ohm resistors on the outputs to balance the load, the original 10k pot has been removed, and replaced with a 4.7k pot, that runs both modules (is this an issue?)With the two regulators not connected in parallel, their outputs are identical to the 3rd decimal place!Problem is, that in parallel, with a 2amp load, one of them gets hot, the other appears to be at ambient.with a 3amp load, one regulator gets to maybe 70 degrees, the other is cooking at over 100.Why do they not match?I also removed the 220uF capacitors off the inputs of the board, and used a 470uF across both supplies...The case is just a tad cramped.... :p

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: December 18, 2013, 06:11:26 am
content: *sigh*by tying the potentiometer connections together, I was connecting the outputs together :p

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: December 18, 2013, 07:16:44 am
content: Alright, I dont think two regulators in parallel are going to do what I want.So, now I'm looking for a single switchmode regulator rated at about 5amps, through hole, so I can mount it to a small heatsink...LM2678 maybe? Although I don't think I have any

author: Jon86
author_rank: Frequent Contributor
timestamp: December 18, 2013, 07:33:18 am
content: There are thousands of 5A switching regulators, search ST or TI. Also, you haven't stated what your voltage and ripple specifications are.Don't be lazy.

author: Kremmen
author_rank: Super Contributor
timestamp: December 18, 2013, 07:33:33 am
content: Can you think of any reason why paralleling voltage sources might be a bad idea? Or putting current sources in series for that matter?Well, let me tell you. Check this pic:To help understanding, any practical source (of voltage or current) can be modeled as an ideal source combined with a source resistance.An ideal voltage source is capable of producing any current while keeping the voltage across its nodes unvarying, while a current source does the same for current at any node voltage.The internal resistance of an ideal voltage source is 0 and for an ideal current source it is infinite.For a practical voltage source, the source resistance is in series and for a current source, in parallel. The resistance may be frequency dependent but lets skip that complication for now. For current sources the source resistance is typically very high and for voltage sources it is the opposite, very low or at least on the low side. Any properly regulated voltage source is going to have a very low source resistance at least on low frequencies. Thus the Ri in the picture is going to be on the order of milliohms or so.An ideal source has the specified voltage across its output nodes, Vo. Alas, a practical source will only approximate this ideal, therefore one of the sources in the picture has the additional term v-hat. This signifies the difference that you always get due to adjustability or component variation. No 2 sources will have _exactly_ the same output voltage.Connecting 2 ideal voltage sources (ideal == Ri = 0) with different Vo will result in a circulating current of infinite magnitude. For practical sources the circulating current will be v-hat / (Ri1 + Ri2) but only if the sources are capable of operating in at least 2 quadrants, i.e. able to pass power in either direction while maintaining nominal output voltage polarity.Usually that is not the case with run-of-the-mill cheap PSUs. In that case, the one with lower Vo will just shut off and lets the other one do all the work. This will go on until the load current is high enough to cause sufficiently large voltage drop across the Ri of the active supply such, that the PSU output voltage V is lower than Vo - v-hat (or whichever unit produced the lower voltage), at which point the idle supply will wake up and starts to contribute.This is the fundamental reason for the issues you see. Paralleling voltage sources is basically a no-no. If you decide to do it anyway then make sure there is some way to arrange load sharing between the parallel sources. The classical way is by increasing source resistance by introducing a balancing resistor in series with the output of each source, and to combine only the other ends of the resistors. This will lower the current at which each PSU enters the active region while of course worsening the source resistance and resulting voltage regulation.Questions?

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: December 18, 2013, 08:11:19 am
content: Didn't have any ripple requirements reallyIts pretty much just a basic go to power supply, for testing battery operated devices, testing car audio gear such as headunits, testing computer fans, etc.I had 0.15ohm resistors to attempt to balance the load on the supplies...I don't think one supply got any load till the other was overloadedThis was just built using parts I already had, I currently have no job, so didn't want to spend any money on it, hence the two supplies in parallelJust plugs into an old laptop power supply, was testing using a 16volt, 4.5amp supply... I'd like to be able to get 4 amps out of this fairly reliably.

author: sleemanj
author_rank: Super Contributor
timestamp: December 18, 2013, 08:25:21 am
content: Thinking aloud. What about if you use a fixed resistor and an light-dependant-resistor in place of each of the original trimmers (which form a voltage divider with the 2596's ADJ pin at the mid point), then your panel mount pot can be used to drive two (as near to identical as possible) LEDs (powered from the raw input voltage) shining at the LDRs.This would form your voltage setting by adjusting one leg of each of the voltage dividers and keep each module separate until after your balancing resistors.

author: Kremmen
author_rank: Super Contributor
timestamp: December 18, 2013, 11:26:54 am
content: If you start to do circuit surgery the rather than that, just join the outputs but take the feedback signal of one regulator and feed it to both. That way the duty cycle of both regulators is controlled in synch and you achieve a reasonable load balancing. Not perfect but hopefully adequate.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: December 18, 2013, 09:02:31 pm
content: Quote from: Kremmen on December 18, 2013, 11:26:54 amI think that's pretty much what I ended up with...But it seemed to make the loading issue worse.I've now only got a single regulator in the housing, might do 2amps constant... was really hoping for more, as the main supply I normally use has 3amp currently limiting (but its rather big, and not very portable) so having something compact, of similar output would have been handy. (the 3amp supply will power up most 2000watt and under car amplifiers ok, to test rail voltages and such)

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: December 18, 2013, 09:52:25 pm
content: I'm going to try an external mosfet, as I already have some, also plan to put a decent sized heatsink on the back... I'll let you all know how it goes!

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: December 19, 2013, 01:10:07 am
content: I added an external mosfet, (IRFZ44N) and a bigger diode (MBR745) as per the schematic here:http://www.mikrocontroller.net/topic/237445Well, I think I did...It doesn't work.The output pin on the LM2596 simply outputs the full supply voltage, hence the mosfet simply outputs about 1volts less than this...The feedback pin on the LM, I tried connecting directly to the output, figuring this would regulate it to 1.2volts or so... well, with the full ~18volts on the feedback pin, it still didn't regulate...Hmm...Probably something I did wrong...

author: sleemanj
author_rank: Super Contributor
timestamp: December 19, 2013, 03:38:27 am
content: Quote from: AmmoJammo on December 19, 2013, 01:10:07 amEffectively what they are doing there is using the internal mosfet output of the 2596 to drive the gate of an external mosfet that can handle more current (sort of like a darlington pair), which then feeds the inductor, instead of the output feeding the inductor itself.The OUT pin will indeed be supply voltage, it's the frequency of the switching on that pin that changes, not the voltage. And similarly if you put put an external mosfet in there, it's not (so) important that the output voltage is lesser (except it will change the maximum voltage you can attain), it's again the frequency of it switching.As some of the posters on that page indicates, you may (probably) need a suitable gate driver between the output of the 2596 and the gate, the output not being able to drive it hard enough and so it not being able to switch quick enough.[ Of course this assumes you are using a well chosen mosfet, naturally of course you'll be wanting a P channel, and also naturally that's going to invert the switching, so you may need to invert the gate drive too... maybe? ]

author: free_electron
author_rank: Super Contributor
timestamp: December 19, 2013, 12:32:01 pm
content: Quote from: Kremmen on December 18, 2013, 11:26:54 amNo change in hell that works !Remember that these are switchers... Maybe if you also can sync their internal clock.That is why there are switching regulators that bring out their internal clock. You either connectect an rc network ( if master) or wire that signal parallel to the slaves.Then, and only then can you make this work. Without clock sync one switcher may start charging while the other is trying to take a reading or be in recirculation. You will get an intermix oscillation and all hell will break loose.Some of the simpleswitcher chips have that capability. They are typically th 7 pin devices. They bring out their clock so you can sync em up.I also never understood why people think that adding a 0.1 ohm resistor in series with the output would balance the load. That only produces heat. It doesn't do diddly squat.This is one of these 'tricks' that are based on completely wrong assumptions. They see this in acir uit where a bunch of power transistors are being paralleled, without understanding why it works there, and then it is applied elsewhere. Totally wrong.Even worse, with modern switchers that do current sensing through the coil you can't pull off trickery like that ! Those switchers have internal sample and hold circuitry and depending of the state of commutation these things sense the back emf and the current flow. That feedback pin is not just 'voltage' but monitors the dynamic signal across the inductor.If you have a switching regulator that has internal synchronous rectification you can bet all your money that this is the topology used. Try to parallel feedback there and you will have a smoldering crater where that chip used to be.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: December 22, 2013, 04:52:19 am
content: Popped it. Lol. Was running a 12volt screen from a single regulator. Need more current!

author: Kremmen
author_rank: Super Contributor
timestamp: December 22, 2013, 09:30:30 am
content: Quote from: free_electron on December 19, 2013, 12:32:01 pmProbably not without clock syncJust an idle thought because most likely nothing else will work either.QuoteSure. Been there, done that.QuoteWithout knowing the switching logic it is hard to comment (and i haven't checked and am not going to). But you are most likely correct.QuoteHere i do wish to argue the point. If you artificially increase the source resistance then it is a fact that the output voltage (after the resistor) will be more dependent on the output current. This is a basic circuit fact and has nothing to do with the nature of the actual source whether a switcher, linear, battery, whatever.QuoteWell, that might be the case or then not. I didn't check this particular chip because i wasn't that interested. As we know there are lots of different control strategies. Anyway, it has nothing to do with the above point. The switcher works as it works. It only sees its own output voltage, not that on the other side of the balancing resistor and that is the point.If you check my first post you will note that i do consider this paralleling a bad idea from the start and i still do. If one wants to do it anyway then this is one way how it sort of can be done. Not that the result is great as far as regulation goes. But if you say it won't work then you will have to come up with a better explanation why.