a 3amp 5volt switching power supply - Page 1
Forum section: electronics / general

author: slburris
author_rank: Frequent Contributor
timestamp: September 17, 2010, 08:28:38 pm
content: I picked up a 5V 3A power supply board from Sure Electronics. It's basedon the LM2596 Simple Switcher from National Semiconductor. The costat Sure was about same same as if I just bought the LM2596 alone here inthe US, sadly. So, for about the same cost, I get all the parts for the powersupply, plus a PIC16F57, crystal, and a header for connecting to the PIC:http://www.sureelectronics.net/goods.php?id=1139So let's see what I actually got, board photos attached. OK, first thingwhich strikes me are the diodes at the top and bottom of the inductor,3 each. Looking at the schematic, it appears the design parallels 3 1ampdiodes to reach a 3 amp specification. Really? What, does a 3 amp diodecost more than a penny or two more? Does this even work? Here's theschematic and manual:http://www.sureelectronics.net/pdfs/PS-SP12118_Ver1.0_EN.pdfI'd expect paralleling diodes to be a bad thing. Invariably, they willhave different forward voltage drops, and the one with the smallest dropwill take the most current. Off to the 1N5817 datasheet I got. Hmm,so as current increases, the forward voltage increases. I *think* thatmeans we'll be OK as far as runaway goes, because the diode with the lowestdrop will carry more current, the Vf will increase, then another diode willbecome the lowest.Still, I would have just used the right diode....Soldering looks mostly OK, except you can see a solder blob on pin 4 ofthe regulator chip. And what are those jumper wires? Well, they aren'texactly jumper wires. They actually seem to be embedded in the board.Looking at the back of the PCB, we see even more of these "wires".Never seen this before -- is this an attempt to draw heat away from theregulator? Don't know. It may not be apparent from the photos, butthe cloudy area around the wires in the back is where the solder maskhad been scraped off the PCB.So I've got another China product with some, uh, unusual design characteristics.Next up, I'll have to power this up and do some measurements.Scott

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: September 17, 2010, 08:36:08 pm
content: QuoteDo not start acting like Simon does , or else I will shoot my self ..Move forward on testing , this is an more interesting part..

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: September 17, 2010, 09:03:01 pm
content: I have purchased a number of items from Sure and found them to b e well made, functional and some of them truly innovative designs. I'm a satisfied customer.Jon

author: slburris
author_rank: Frequent Contributor
timestamp: September 17, 2010, 09:28:18 pm
content: Quote from: Jon Chandler on September 17, 2010, 09:03:01 pmI've purchased mostly dot matrix displays and controllers from them in the past. This board is one of several things I recently boughtand I'll put up some kind of review for all of the items I got in a recent purchase.Scott

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 17, 2010, 10:52:32 pm
content: Nothing wrong with parallelling diodes of the same type - Although Vf may differ slightly at low loads, the increase in Vf with current will tend balance them. The only thing to watch is that they are reasonably well thermally coupled, as if one can get significantly hotter it may tend to hog current.The only reason I can think they did this is inventory - e.g. they already had the 1A parts in stock, or use them on other products.

author: slburris
author_rank: Frequent Contributor
timestamp: September 18, 2010, 03:36:51 am
content: Quote from: mikeselectricstuff on September 17, 2010, 10:52:32 pmSo I guess I can see paralleling 3 diodes on input to prevent reverse polarity problems.But they are also paralleling the catch diodes of the switcher. Doesn't that mean theswitcher sees 3x the capacitance on the wrong side of the inductor? Maybe theLM2596 is really tolerant of such things -- I don't know I've never designed with thatparticular chip.Doing a little googling I've found all opinions from "Don't ever do that" to "it works just fine".When I do some testing, I think I'll measure the temperature of the diodes and see howbalanced or not they are in real operation.Scott

author: Zero999
author_rank: Super Contributor
timestamp: September 18, 2010, 06:52:04 am
content: There are two diode banks: one for reverse polarity protection and the other is part of the buck regulator.Paralleling diodes is a bad idea because of the thermal issues described above, if one is hotter than the other more current will flow through it, which is likely to happen because the diodes have different characteristics and are not well thermally coupled (i.e. on the came piece of silicon). If diodes need to be paralleled, derating or using current sharing resistors is a good idea. In this case resistors would cause extra losses so I'd derate and use four 1A diodes orthree 1.5A diodes, if it needs to pass a 3A.

author: Simon
author_rank: Unknown
timestamp: September 18, 2010, 07:12:34 am
content: Quote from: Kiriakos-GR on September 17, 2010, 08:36:08 pmhey whats that supposed to mean, no please don't shoot yourself, I'd miss arguing with you

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: September 18, 2010, 04:20:56 pm
content: Quote from: slburris on September 18, 2010, 03:36:51 amBut a 3A diode would probably have a similar capacitance to 3x 1A diodes.

author: jahonen
author_rank: Super Contributor
timestamp: September 18, 2010, 06:11:02 pm
content: I wouldn't worry about the diode capacitance. If you are worried about current sharing (it is quite legitimate thing to worry about, since hottest diode will carry the most current), then you can probably squeeze suitable 3 A diode (like MBRS 340) in place of those 1 A diodes. Somebody might do a measurement how well the current is actually shared between the diodes. To get maximum current through the diodes, the duty cycle should be small and load current maximally high, this drives the converter to continuous mode and maximizes the current through the diodes.Regards,Janne

author: Zero999
author_rank: Super Contributor
timestamp: September 18, 2010, 06:29:40 pm
content: Quote from: jahonen on September 18, 2010, 06:11:02 pmThat would be tricky, since breaking the circuit and putting a meter in series will increase the resistance of the diode being measured and reduce the current. Even using a hall effect won't be fool proof for the buck diodes because the inductance will increase the impedance to the high frequency signal.

author: Unknown
author_rank: Unknown
timestamp: September 18, 2010, 08:11:02 pm
content: Quote from: Hero999 on September 18, 2010, 06:29:40 pmUse a µCurrent. Adding a low-value shunt in series with each of the diodes would make it even, but may cause the unbalanced distribution to go away (like emitter resistors). The Tek A6302 (a DC-50MHz hall effect/transformer current probe probe) specifies an insertion impedance of max. 0.05ohm at 500kHz, decreasing linearly to about max 0.01ohm at 150kHz. Most of the switching harmonics are probably well below 500kHz (switching frequency appears to be 150kHz), so I don't think it's significant at 3A. For best effect, you should probably get three probes and clip them around each of the diodes.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 18, 2012, 03:23:43 am
content: sorry to bump. i was looking for hp psu schematics manual. but paralleling diodes current mesurement issue raised. so i just bump instead of creating a new thread since it might be more realted here...Quote from: Hero999 on September 18, 2010, 06:29:40 pmcant we measure each diode temperature to predict which one carry the most current?P=VI, temp=degC/W?

author: IanB
author_rank: Super Contributor
timestamp: February 18, 2012, 03:29:51 am
content: Three diodes in parallel with have much better heat dissipation than a single diode. More surface area, more lead area, more board area next to the leads. This might be an intelligent design decision rather than a cost saving effort. In fact, I can't see how three diodes are cheaper than one...

author: ciccio
author_rank: Frequent Contributor
timestamp: February 19, 2012, 02:26:47 pm
content: Quote from: slburris on September 17, 2010, 08:28:38 pmI do this regularly when high current will flow through the track, or when a low impedance is required.I've seen this in many power PCBs, from power supplies to audio amplifiers: don't trust on the current capabilities of PCB's tracks: half a mm of solder has always a lower resistance than a thin copper track.Look at the solder side of the single-sided PCB in a PC power supply.Best regards

author: wkb
author_rank: Super Contributor
timestamp: February 19, 2012, 05:56:21 pm
content: Quote from: ciccio on February 19, 2012, 02:26:47 pmAnd if I need really high-current capability: take the insulation off some mains installation wire, 1.5mm2 or 2.5mm2 solid copper works fine once soldered onto a PCB track. Not the most beautiful thing in the world (neither is drowning PCB tracks in solder imho) but it fixes the issue once and for all.

author: Unknown
author_rank: Unknown
timestamp: February 19, 2012, 09:27:38 pm
content: Quote from: ciccio on February 19, 2012, 02:26:47 pmJust barely. Resistivity of solder is about ten times worse than copper. So half a millimeter of solder is better than 1 oz of copper, but worse than 2 oz of copper. Of course the resistance of the solder is in parallel with the copper, but it doesn't make as much difference as you might think.