13.8v/30A PS problem - Page 1
Forum section: electronics / projects

author: marvkaye
author_rank: Contributor
timestamp: February 17, 2014, 11:59:19 pm
content: Hi guys, I'm hoping someone can help me figure out what I'm seeing with this crazy one-hung-lo PS I picked up. It's a Goldsource () 13.8vdc/30A supply. I bought it primarily to use as a bench supply to drive my RC hobby LiPo chargers. The first one they sent me died after 3 uses, but I discovered that the bottom of the case was bent, it looked like it had been dropped, so could be something was smashed in shipment and it was just a matter of time before it went TU. I never bothered to try and troubleshoot it, as when it died I called the dealer I got it from and they stood by their 1-year warranty, no questions asked, and just sent me a replacement. Before the first one died, however, during a typical charging session I noticed that just one of the 6- 2N3055's on the unit would get really hot, the others remained pretty much near ambient. Now I've got the replacement unit and lo & behold, the same one of the 3055's is like toast, the rest only heated by it, the closest one warmer than the others, but all closer to ambient than that one "firecracker" (IR thermometer says it's 99C, the one right under it on the same heat sink is 57C, the other 2 I can get at on a different heatsink, about 26C, just a tad over ambient). I need to pull the rear heatsinks in order to get to the 2 inner 3055's, and to be able to take some voltage readings to see if I can figure out what's going on. The schematic specific to my unit (almost, mine has 6- 3055's, the schematic only shows 4) is at the bottom of the attached page... if you look in the table for VX2 you'll see the 30-amp unit requires 6X2N3055. I'm a little confused by the schematic, as it shows the emitter of V26 connected to the bases AND emitters of all the 3055's... makes no sense to me at all, I'm thinking it's a misprint. They also don't specify a part # for V26, but the topology looks identical to the middle schematic where V2 is a 2N3055 so is that what V26 is??? Hmmm... The temperature readings were taken when I was getting a stable 13.79VDC @ 3A at the output. I can only imagine what would happen were I to try and load it to its rated limit... any suggestions on where to start figuring out how to get the heat & load equally shared among the 6-2N3055's would really be appreciated. If I can get this one squared away I'll dig into the first one and try to resurrect it as well. TIA <marv>

author: Andy Watson
author_rank: Super Contributor
timestamp: February 18, 2014, 12:11:47 am
content: 2N3055s do not have much gain and therefore require a substantial base drive current - that's why you have a 2N3055 driving the output transistors.But the real problem with the circuit is that it is poorly designed - there should be some resistance in the emitters of the output transistors, otherwise the warmest transistor will conduct the most current, will get warmer because it is taking the most current, will take more current because it is warmer, etc., until it melts.Edit: Oops, misread,...QuoteI suggest the circuit is wrong.

author: marvkaye
author_rank: Contributor
timestamp: February 18, 2014, 12:46:38 am
content: Thanks for the pointer, Andy... so let me see if I understand.... assuming I'd like to see max 5A across each 3055, Ohm's law says 13.8V @ 5A requires a 2.76ohm limiting resistor in series with each 3055's output, correct? 69 watts??? whew! If I've got this right it looks like Mouser has some 3ohm @75 watts available @ $11.90, call it 12 bux each. Jeez.. that's 3/4 of what I paid for the stupid PS. However, if this solves the problem.... or am I all screwed up and I've missed the point completely?? Sorry, I'm not an EE, just a hobbyist who's trying to get a clue. I appreciate the help. <M>

author: Andy Watson
author_rank: Super Contributor
timestamp: February 18, 2014, 12:58:20 am
content: The usual method is to put a small resistance in each emitter. The idea is to swamp the effects of temperature change. It needs to be large enough to have an effect, but not so large that you lose all your power. I have an ancient TTI power supply with multiple 2N3055s in the output. It uses 0R68 in each emitter - and they seem to be pretty reliable.

author: dannyf
author_rank: Super Contributor
timestamp: February 18, 2014, 01:15:31 am
content: 0.22 - 0.47ohm are pretty common. Bigger ones are less efficient and reduces the output capabilities of each transistor.You could also think about mosfets here - they are more resistant to thermal run-away. However, you lose some headroom for that.

author: guipoletto
author_rank: Contributor
timestamp: February 18, 2014, 01:17:28 am
content: I could be wrong, but you may be able to use standard 75W halogen lamps from car headlights. The only drawback would be the lamps lifespan.

author: Psi
author_rank: Super Contributor
timestamp: February 18, 2014, 01:24:25 am
content: The point of the emitter resistor is so an increase in current from thermal runaway causes an increase in voltage across the emitter resistor which raises up the emitter voltage.Since you turn on a transistor using a voltage between the base and emitter if you raise the emitter voltage up then the control signal on the base is now smaller (from the transistors perspective) so it conducts less which in turn reduces the current.When you have multiple transistors sharing a load this balances them out because if any one transistor gets hot from excess current its emitter resistor causes it to turns off a little making all the others work harder instead.

author: dannyf
author_rank: Super Contributor
timestamp: February 18, 2014, 01:35:57 am
content: You can use light bulbs. They introduce an element of non-linearity, but it is welcome here.

author: marvkaye
author_rank: Contributor
timestamp: February 18, 2014, 01:59:27 am
content: I think I'd rather use proper resistors instead of light bulbs. I would prefer to use 0R33 resistor as even at full power (5W per 3055) I'd only be losing 1.65V. My charger cuts out if the supply drops below 12v, so with 0R47 resistors I'd be limited to about 23A out of the supply. Not that I expect to drive this thing that hard most of the time, but I'd sure like to have the ability to do so if required. If the 0R33's will work I'd much rather use them. Do you think they'll get the job done for me? <M>

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: February 18, 2014, 02:04:05 am
content: Hi,If you get about 100-200mV on the emitter resistors at the rated current it will force the sharing. So 0.05 to 0.1 Ohms will be fine.Unless I missed something, I don't see any current limiting in the schematic.Jay_Diddy_B

author: marvkaye
author_rank: Contributor
timestamp: February 18, 2014, 02:27:07 am
content: You guys are the best. Got some resistors ordered, should be a happy camper in a few days. I'll let you know how it worked out... will send pics of the mod. Thanks so much!!<marv>

author: VK5RC
author_rank: Unknown
timestamp: February 18, 2014, 02:33:31 am
content: I recently built a 30V 30A linear psu for anamateur radio transmitter, used 2n3055, x12, if you look at the heat dissipation, junction to case (i think. 5 degree C/W) you are pushing it for 5A each I would suspect (depends on voltage drop across transistor). 0.2ohm helped get all the trannnies working together with a bigheat sink with fan. The data sheet is helpful. I suspect it isn't designed to be at 30A for too long.

author: marvkaye
author_rank: Contributor
timestamp: February 18, 2014, 03:53:27 am
content: Thank you for the input, VK5RC, more stuff to think about. I hadn't considered the drop across the 3055's... collector side is about 29v, so it's considerable. I ordered 0.22ohm 15W resistors, way overkill at lower output, and an adequate cushion for them, at least, if I do find myself pushing it. Once I get the resistors installed I'll do some additional testing to see what I might need to do to improve the heat dissipation... I'm not averse to adding more heatsinks and sticking a fan on it if required. Once I get the new one balanced I'll dig into the first one, see if I can find and repair its problem and then duplicate the resistor installation... I ordered enough to do both of them, JIC. Come to think of it, I seem to remember reading 28+ volts at the output when it failed... that would suggest one or more of those 3055's are shorted collector to emitter... I'll bet it's at least the one that was hot. Now I'm anxious to get it out of the locker and see if that's it. Fun stuff.

author: harry h
author_rank: Contributor
timestamp: February 18, 2014, 06:21:36 am
content: Thought I would share what I am using for my RC charger. I put together (2) used DPS-600PB server power supplies that I got on Ebay for $18ea. Running them in series for 24v - 47A output. Been going strong for about a year and a half with moderate usage.Harry

author: poorchava
author_rank: Super Contributor
timestamp: February 18, 2014, 09:49:58 am
content: 30A linear supply....Have they ever heard of SMPS?

author: peter.mitchell
author_rank: Super Contributor
timestamp: February 18, 2014, 10:09:37 am
content: Quote from: harry h on February 18, 2014, 06:21:36 amI can vouch for that.Server PSUs are now my go-to source of high current high reliability power supply.

author: VK5RC
author_rank: Unknown
timestamp: February 18, 2014, 11:44:44 am
content: I know it was crazy idea, it was an interesting exercise. I have had a bit of SMPS hash getting into things so thought I would try to avoid esp as it is going to be part of an Earth-Moon-Earth transmitter-receiver system, 160W RF out at 1296MHz, with input 0.4dB noise factor 30dB gain pre-amp into Icom 910H.As an amateur I spend a lot of my time cursing noisy wall warts and their harmonic emissions drifting up and down the bands.I agree quietening a SMPS would have been a LOT easier but I did learn a lot in the process!Who said what doesn't kill you makes you stronger.

author: dannyf
author_rank: Super Contributor
timestamp: February 18, 2014, 12:10:23 pm
content: QuoteProbably not at 30a territory.

author: marvkaye
author_rank: Contributor
timestamp: February 18, 2014, 01:30:53 pm
content: Quote from: GEuser on February 18, 2014, 07:39:55 amActually it's one of the output trannies that's getting hot. The PS has 4 heatsinks that make up the rear of the cabinet... it looks like V26 is mounted by itself on one of them, and then the other 3055's are mounted 2 per each on the other 3 heatsinks. The one with V26 on it is underneath/behind the heatsink with the hot 3055, making V26 rather inaccessible without removing the outer pair of heatsinks. Consequently, V26 could be getting hot as well... but until I can get at it to take some temperature readings I just don't know. Anyway, I'll attach some photos that show the layout when I add the resistors.Quote from: GEuserThanks... wish I'd known to think about the resistor for V26, I'd have added it to my Mouser order. I've got a great electronics surplus place close by... I'll have to stop there and see if they have anything appropriate... can I assume a similar value is required there (0.1-0.2 ohms)?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: February 18, 2014, 03:54:23 pm
content: Hi,One more thought for you:Not all 2N3055s are created equal. There is a large variation from one manufacturer to another. Opening the transistor and looking at the size of the silicon die can tell you a little about the quality.You may want to consider using 2N3772 from a distributor like Digkey or Mouser.Jay_Diddy_B

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 18, 2014, 05:53:07 pm
content: Quote from: dannyf on February 18, 2014, 12:10:23 pmSince the high current draw is only when transmitting, you could have a linear supply hold up the rail for receiving and only enable the switching supply when transmitting. (That assumes it's a half duplex setup.)

author: burbeck
author_rank: Newbie
timestamp: February 18, 2014, 07:07:11 pm
content: hithe short across VX2 transistors B and E aint helping ha ha, got to be a error.agreed that the outputs need sharing resistors on the emiitters 0.47R or so.and when you have got it working ok, sharing the load etc, then a good modification would be to add some form of current limiting.

author: SeanB
author_rank: Super Contributor
timestamp: February 18, 2014, 07:26:02 pm
content: I would change the 2N3055's to 2N3773's and buy a better diode than that load of parallel 1N540x diodes. A GBPC5006 50A bridge for $3 is cheaper than the 2 diodes, and mounts easier to a big heatsink. 0R1 5W resistors will do as ballast resistors. If you are going to draw high current a lot a small 24V fan fed from a point by C5 will help keep the inside cooler.

author: VK5RC
author_rank: Unknown
timestamp: February 19, 2014, 10:50:25 am
content: Since the high current draw is only when transmitting, you could have a linear supply hold up the rail for receiving and only enable the switching supply when transmitting. (That assumes it's a half duplex setup.)"Unfortunately" I have built the beast, all `10+kg of her, she doubles as a great room heater!She is a bit of a dinosaur design wise using a LM723 driving the 2n3055s. I tried using 0.1 ohm balancing resistors but kept blowing 2n3055s; 0.47 ohm worked. I used inductor to slow the in rush current into the filter caps on start up along with a PTC resistor on the mains side of the toroids.She will pump out 20-35V at 30A , the 12x 2N3055s get pretty hot, about 90 C by IR probe, even on a fan vented heatsink.At present she is not connected up, when testing into a dummy load boy did the shack get hot. After 20mins I surrendered.As a bit of icing I used a LT6106 high side current sensor amp, read by an Arduino , which also reads the voltage and runs an LCD. The output is relay controlled by the Arduino which has a 'crowbar' function for over voltage and over current.

author: marvkaye
author_rank: Contributor
timestamp: February 25, 2014, 08:52:21 pm
content: Well, I performed the mod and thought I'd share a couple photos as promised. I anchored the aluminum plate that holds the ballast resistors to the front panel and one of the heatsinks at the rear, which allowed the wiring to be easily accomplished as well as help keep things neat on the inside. I did a little test with it the other day, didn't push it hard, but at 3A all the 3055's stayed so near to ambient that I couldn't tell if there was a hot one in the bunch. Prior to the mod even at 3A there was that one that would get to 100C+ while the others stayed right near ambient. Given the marked difference I'd have to say that the modification was a success and thank all of you who shared your suggestions once again for your help. You guys get my vote. <M>