Darlington (Swithing): The "Power Pass" Laughing. The Driver Crying to Death. - Page 1
Forum section: electronics / projects

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 29, 2011, 07:10:05 pm
content: tonight i like being fooled by a circuit. tested my "novel" switching voltage regulator at 0.63V (average, noise still there) supplied from 12V LA battery. short circuit the output using DMM Ammeter drawing 0.32A (figured out the DMM is 2ohm load inside), wait for a while, to my surprise the driving transistor T2 is boiling (i put some flux on the heat sink pad of both T2 and T3 of the 13007 power transistor, yes 2x13007 didnt i say its novel?), after a while of head scratching, testing and error'ing, i figured out the main current path as in the red line in the diagram by using my 2nd DMM ammeter. now, the classic told me most of the current should flow through the lowest level of transistor (power pass) ie T3, but not in this case. so its still a mystery to me, does anyone know whats the explanation of this magic? (i'm sure there's more power electronician out there)1) faulty component? (but they are ok, ON'ing/passing CE and OFF'ing based on the NPN BE potential)2) no limiting resistors at driving transitors emitter? (2x drivers here,(novel?)then why the current not going through T1 (higher driver, leftmost small C945) and blow the shit out of it?)3) bad design? (still please explain the reason if you can, or just simply the unhelpful... "its a bad design" comment)thank you to all eevblog community. cheers

author: Simon
author_rank: Unknown
timestamp: December 29, 2011, 07:37:20 pm
content: hm, why are you using a 3 stage darlington ? what is the base drive voltage ? it does seem like the main transistor is not passin collector current, maybe something to do with main supply voltage ?Why not use a mosfet ? if you need so much current gain

author: qno
author_rank: Frequent Contributor
timestamp: December 29, 2011, 07:56:19 pm
content: Your problem is the CE voltage. When driving the 13007 the CE voltage drops to 0.7V.This only works if the emitter is connected to ground.Try to layout this configuration for PNP transistors. 1 power transistor will be enough as long as the steering circuit can provide 5 to 10 times Ibase. With low hfe power transistors this will be a problem.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 29, 2011, 08:09:45 pm
content: Quotebecause i dont have middle class transistor that can drain 2A max for the main pass (main pass speced to draw 2A from base while draining 10A output). so i used "in stock" another 13007, and since i cannot drive it directly from the switcher (i dont want to damage the switcher), so for the safe side, i have another C945 driving, thats the idea.Quotefull 12V as in diagram, i tried to lower it down using divider in every stage, the result is, not full 12V output (minus whatever v drop in the darlington), 6V driving volt means 6V max output, sighthis attempt is done while trying to lower the noise, not tried at the 1st stage driver (middle 13007) i dont have high power resistor for the divider.Quoteexactly! this is the magic i'm asking why.Quotesupply voltage is using a "very stable and powerful" LA 12V battery (blown my DMM fuse by shorting it), also tried with 40Vpp 8A max transformer half bridge rectified and smoothed using 3300uF.Quotesorry friend i dont quite understand it. can you explain more why? the way i see, the middle driver got more drop compared to the main pass, so current should still got less resistant going through the main pass T3, but not in this case.Quotedo you mean NPN power transistor steered/drived by PNP? do you have quick example?

author: Simon
author_rank: Unknown
timestamp: December 29, 2011, 08:12:19 pm
content: hang on, are you using npn transistors for a high side drive ? not a good idea I think unless you have worked it out carefully. why not use pnp devices, or better still one single mosfet

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 29, 2011, 08:28:57 pm
content: Quote from: Simon on December 29, 2011, 08:12:19 pmyes its all npn in the diagrami dont have P channel mosfet, from what i studied, for common collector design (load connected to emitter) we should use NPN or N-channel mosfet. and most power darlington i saw is using both NPNas in the picture cut from a datasheet below. should give a try for 70ohm setting, i thought its for pull down purpose only necessary for mosfet.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 29, 2011, 08:42:47 pm
content: and for another information, if i drive for larger current, say 1-3A, the T3 is boiling as expected. so for high power job, the main pass is doing its job as it should, but not for low volt low ampere like this (my case at least). and i probed some slight rippling on the LA battery following the switcher on-off state, not sure how this can affect the situation. and every line got noise based on this switching, this is the current task for me to tackle

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 29, 2011, 10:02:28 pm
content: since power master advisors cannot see the reason clearly, i made another tinker attempt, swapping between the power transistors from stock, it turned out to be... different 13007 spec between brand (maybe different internal impedance or such) or half-faulty component i guess. but they are all seem working, regulating etc, took me some minutes meditating which one is the culprit, but my suspect is the (initially from above diagram) T2 is half-faulty ie build lower internal impedance, hence current tends to flow through it. using another 13007 resulting in "as expected" result for the 0.63V 0.32A test (T3 boiling first). anyway, the internal operation of such semiconductor will always a mystery to me, no need for me to dig to such level unless someone else interested. so i guess mystery solved.edit: took one good night stand just for thisvery unproductive!

author: amspire
author_rank: Super Contributor
timestamp: December 30, 2011, 12:03:24 am
content: Mechatrommer,I think we need a lot more information.What is the intended output current?What is the resistance of the intended load?What pull up resistor is on the LM393 output, and what voltage is the pullup resistor connected to?What is the switching frequency?What is the ON to OFF ratio from the LM393? If it was fully on, then something is not right. If all the transistors were fully ON, then you would have over 4A into the multimeter.A P_Channel mosfet would probably be a much better solution - much greater speed, much lower ON voltage, and it is turned ON by the LM393 going LOW rather then going HIGH. This suits the LM393 as it has no active pullup. It has no pullup at all, so there always has to be something like a pullup resistor to the positive rail. If you want output in the Amps range, the 13007's will need heatsinks. A suitably low ON resistance mosfet will probably hardly get warm even with no heatsink, and you could use a SMD MOSFET if you like.Also a dumb question, but I better ask. Are you sure that both the 13007's are connected correctly? If the emitter and collectors are swapped, they still function as transistors but the current gain is really bad (probably less than unity in the >1A range), and the transistors will break down somewhere above 9V. This combination of low gain and low breakdown means lots of room for confusing results. So if you are looking at the front of the transistors with the leads down, the base is on the left, emitter on the right and collector in the middle. This means that in your circuit, the center leads of both 13007's are connected together, and the center leads go to 12V. Is that right?Richard

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 30, 2011, 12:53:24 am
content: What is the intended output current? = originally... 8-10A, but in reality (DMM ammeter shorting, 2ohm load/shunt inside) it only can draw 3-4A.What is the resistance of the intended load? = 0-infinityWhat pull up resistor is on the LM393 output, and what voltage is the pullup resistor connected to? = 10k pull up to 12Vsupply (datasheet says 15k) not much difference i believe.What is the switching frequency?:depends on the load, Vref level vs Vsupply and some mystical stuff which i dont know, probing sometime showing 60KHz, sometime 3MHz, up down etc. pls dont be mistaken this with the normal/standard switching voltage regulator (fixed frequency, varying duty cycle), this design is based on "novel" topology that i've not seen, just an astral projection into my mind. do i talk too much?What is the ON to OFF ratio from the LM393? = what?Quote from: amspire on December 30, 2011, 12:03:24 amyou are right, if i fully ON the darlington (tie the input to Vsupply, at shorted output, DMM registered 3-4A, but not when switching/regulated < Vsupply. i believe the LM393 is not fully ON at some regulation far less than Vsupply.Quote from: amspire on December 30, 2011, 12:03:24 ami totally agree, the problem is, i havent found P-MOSFET in my stock.edit: pmosfet + comparator switching = bad oscillation.to get you the feel of it, i've provided some pictures, bear in mind, no simulation involved (it just failed miserably), this all the real thing. and the dso capture showing Vsupply from 20V supply (transformer rectified), not the 12V LA battery from original post, i've changed the setup.thecircuit.jpg: the real circuit on breadboard1-5vreg.png: regulating at ~5v2-10vreg.png: switching to ~10v3-1.5vreg_1.png: 1.5v regulation (unseen 7mV-1V switching noise in there for high impedance load)3-1.5vreg_2.png: closer look (timebase) for 1.5v regulationby the way, as i said, mystery solved, its variation in manufacturer's production, even with the same transistor name (13007, D13007K) i've swap between T2<->T3 place, its now working as it should.

author: amspire
author_rank: Super Contributor
timestamp: December 30, 2011, 01:21:43 am
content: The 13007 really couldn't be a worse choice for your application. It is a 700V transistor, means it is pretty bad for low voltage use, and there is no way it can be switched at 1MHz, let alone 3 Mhz. It is a fairly large die transistor (which means high base-emitter storage charge that makes it hard to switch quickly) and to achieve the high voltage, it has to use a very low doping in the collector - lots of resistance. High voltage also means really lousy current gain.If you want to use these transistors, add base-emitter resistors to all the transistors to discharge the B_E junction when turning off. You need something like a 10 ohm resistor on the output transistor, 47 ohms on the second 13007. The first transistor is fine. You also want to limit the maximum switching frequency to well under 100kHz. 20 KHz maximum would be better, as anything faster and you will get horrendous switching losses.If you have some lower voltage PNP power transistors, use a 2 stage PNP darlington, and you will get better results.If you need the 3MHz, then forget this design. Bite the bullet and order some mosfets as it is the only thing fast and efficient enough.Richard.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 30, 2011, 01:36:11 am
content: sorry forgot to tell... yellow trace = LM393 (switcher) output, blue trace = output voltage. i wish i could show Vsupply trace in the same screen, i only have 2ch scope. its 20V rectified/smoothed supply (of course rippling with switcher, i've tried reduce it so much, wish me luck). thanks for your advice richard.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 30, 2011, 04:40:24 am
content: and lastly, the finale, the most mystical. i didnt have enough time to react, got panic which cable to disconnect, changing to ohm range and turning off the DMM quickly didnt help, so the starboard arm got torn to pieces, the port arm experiencing 99% degree burnt (you know? newbies mistake, changing the range to V while the probe at A, i should have just disconnect the battery cable, but cant since its already burning), luckily, no passive elements injured. the morale, get a good rest before you get tired. this concluded my tinkering today, errr... from last night?

author: amspire
author_rank: Super Contributor
timestamp: December 30, 2011, 05:40:56 am
content: That looks like a lot of current through those leads. More then 10A I think.You should never really use the current range of a DMM as a load - it will just wreck the meter. Especially if the power source is a battery.If you don't have any power resistors, then there are substitutes. A 240V/100W incandescent light bulb is about 40 ohms and at 12V it is totally impossible to damage. Several in parallel make a safe load.

author: wkb
author_rank: Super Contributor
timestamp: December 30, 2011, 08:57:00 am
content: Quote from: amspire on December 30, 2011, 05:40:56 amThat grey 'box' looks like a dryfit lead-acid battery to me. Short circuit currents of these batteries are >> 10A.At least put some currently limiting in the battery circuit (resistor, fuse). As used now this is plain dangerous.The 'funniest' thing I ever saw was a piece of soldering wire managing to short out a battery like this. The solder melted instantly, leaving a trail of droplets.

author: Zero999
author_rank: Super Contributor
timestamp: December 30, 2011, 11:05:20 am
content: Agreed, it could've been far worse, a fuse is a must and make sure it's sized according to the cable to avoid the meltdown you've just experienced.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 31, 2011, 07:55:06 am
content: thanks for the advice. i have 50W 12V incandescent (tungsten? not sure) light bulb for testing. but since i keep changing here and there, circuit and load, this is what happended. i should have thought the fuse through battery idea.

author: Zero999
author_rank: Super Contributor
timestamp: December 31, 2011, 03:18:40 pm
content: With that size cable and breadboard, the fuse should be no more than 2A, 1A would probably be fine. If it's for experimental use, a polyfuse would be a better choice. If you need more than a couple of amps, use thicker wire and either make a propper PCB for the high powered part, or connect the components together with thick tinned copper wire.

author: Mechatrommer
author_rank: Super Contributor
timestamp: December 31, 2011, 08:07:24 pm
content: Quote from: Me!!! Me!!!wrong! i managed to find pmosfet from my stock, J327. with LM393 comparator as switcher, its oscillating like hell, 2Vpp is totally unacceptable. i have to agree (saw) with Hero's point now on the other thread. always fascinated seeing things going smoke. J327 is dead now (+another LM393 [mysterious death]), dont mind some bulging on the breadboard, will be a good reason for next ebayingi learnt many things... happy new year!

author: SeanB
author_rank: Super Contributor
timestamp: January 10, 2012, 07:47:47 pm
content: When you get the next mosfet place a gate resistor of 100-220 ohms in series with the gate, using the shortest possible lead length to the gat. A good addition would be a ferrite bead slipped over the gate lead as well. This will stop the mosfet from acting as a very good RF oscillator during switching and when it is on. That is most likely what killed the original. As well place a 15V zener diode across the gate to prevent the gate voltage from going too high and causing the mosfet to punch through the gate oxide.will drive large capacitive loads.I have fried a few mosfets this way, leaving these out. The best driver for a mosfet in low voltage applications ( where the supply is between 5-15V) and you want a low cost switch that is not going to go much above 100kHz is to use the low cost 555 timer as a buffer. 300mA drive and will drive large capacitive loads. Still needs a stopper resistor, but fast turn on and fast turn off are easy without oscillation.