Connection from power transistors to printed-circuit board

hi group,

i'm planning to build my own power amps and i've narrowed down my choices t o the LM3886 and one of the LMEs (49810 or 49811 or 49830), but i'm very mu ch tempted with the LMEs (main two reasons: higher audio quality and config urable power -- that is, i'm not stuck with 70W; i can choose the right tra nsistors and the right supply voltages and i can build a 50W amp, or 100W,

300W, etc.)

however, i don't have any experience building amps with discrete power tran sistors, in particular TO-3 transistors needing a heat sink that could make it difficult to solder the transistor pins directly to the printed-circuit board.

my question: what is the recommended practice? send thick wires (say, up to 4 or 5 inches long) from the transistors to the printed-circuit board an d nothing else? place small capacitors (say, 1nF or 100pF --- or even les s??) in the board or at the transistor pins (or both?) to prevent oscillati on? if so, capacitors connected how? base to gnd and emitter to gnd? col lector to base and base to emitter? (and collector to base?)

if right at the transistor pins, then how? three capacitors, one between e ach pair of pins? then send the wires to the printed-circuit board?

anything else in addition to (or instead of?) the capacitors?

or is all this hopeless and the only option is soldering the transistors di rectly to the printed-circuit board?

any advice or pointers (for example books that cover these practical issues ) will be much appreciated!

thanks,

-Zico

Reply to
zico
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i'm planning to build my own power amps and i've narrowed down my choices to the LM3886

** Long established and well proven power amp IC with full SOA protection - not to mention very low cost.

and one of the LMEs (49810 or 49811 or 49830),

** Fraid that POS has no SOA or VI limiting circuitry.

Means the output devices and probably the fancy IC will be destroyed by the briefest short on the speaker line.

Really dumb choice.

.... Phil

Reply to
Phil Allison

to

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good points.

since i had already almost completely decided in favor of the LMEs, i didn' t want to spend much text in the reasons; but one additional disadvantage i n my mind for the 3886 is the heat sink. i haven't been able to find any su itable heat sink in my local electronics store or online, other than hackin g up a CPU heat skink (as in, a *computer CPU* heat sink), but it sounds al l messy. with the LMEs, i can choose among a ton of available TO-3 heat si nks.

a little help here with the acronyms, please? POS i guess stands for "piec e of s***t"? SOA i guess "safe operating area"? what about VI?? (just vo ltage-current?). so i guess in general you're pointing out the lack of bui lt-in circuitry to protect against short and transient overloads?

anyway, three things:

(1) wouldn't a fast-action fuse at the very output stage do just fine? i mean, of course, it is very inconvenient, but still, a short on the output is not part of what one expects to occur frequently, mainly given my second point:

(2) the amps are going to be embedded in the speakers, with its outputs fi xed (most likely soldered) to the speakers. in other words, the chances of a short while the amp is operating are virtually null.

(3) what about the shutdown circuitry? as i understand from the specs, it has an internal temperature sensor that shuts down, but also an external c onnection with which you can shut it down --- would it be a matter of placi ng a voltage comparator at each of the output feedback resistors (the 0.22 ohms in the suggested diagram in the datasheet) and have that one trigger a , say, 1-second pulse going to the shutdown input pin?

any other reasonably simple ways to add short / overload protection for the LME and for the output transistors?

tnaks,

-Zico

Reply to
zico

You can get TO-3 sockets, or just the individual-pin variants (whatever they're called), but I've usually just soldered directly to the pins. For the case, a lockwasher-type solder tab works fine. Don't worry about short wire runs to the PC board. You shouldn't need any extra caps.

I've mostly built power amps using only discretes, with the exception of one design that used a high-voltage Apex TO-3 that I ran on +300 and -15 V for a special job. In that case, I did include the TO-3 right on the PC layout. The TO-3 pins were long enough to go through the heat sink and board and solder normally. Likewise, the TO-3 mounting screws passed through the heat sink and PC. This type of mounting means you need a lot of "empty" board space around the TO-3... no problem in this single-device application, but in a more-normal amp with multiple devices I'd certainly have put them off-board.

Best regards,

Bob Masta DAQARTA v7.50 Data AcQuisition And Real-Time Analysis

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Scope, Spectrum, Spectrogram, Sound Level Meter Frequency Counter, Pitch Track, Pitch-to-MIDI FREE Signal Generator, DaqMusiq generator Science with your sound card!

Reply to
Bob Masta

good points.

since i had already almost completely decided in favor of the LMEs, i didn't want to spend much text in the reasons; but one additional disadvantage in my mind for the 3886 is the heat sink. i haven't been able to find any suitable heat sink in my local electronics store or online,

** FFS - you can use just about any flat surface piece of aluminium extrusion you like.

with the LMEs, i can choose among a ton of available TO-3 heat sinks.

** Huh?

TO3 pack transistors are as obsolete as DOS.

a little help here with the acronyms, please? POS i guess stands for "piece of s***t"? SOA i guess "safe operating area"? what about VI?? (just voltage-current?). so i guess in general you're pointing out the lack of built-in circuitry to protect against short and transient overloads?

** For all the expensive TO3 darlingtons YOU have to add.

(1) wouldn't a fast-action fuse at the very output stage do just fine?

** No, a dead short would destroy ALL the output devices in 5 mS.

(2) the amps are going to be embedded in the speakers, with its outputs fixed (most likely soldered) to the speakers. in other words, the chances of a short while the amp is operating are virtually null.

** When voice coils burn and fail, they end up going to a near short.

But having the amp permanently wired to the load is a big plus for an unprotected output stage.

(3) what about the shutdown circuitry? as i understand from the specs, it has an internal temperature sensor that shuts down, but also an external connection with which you can shut it down --- would it be a matter of placing a voltage comparator at each of the output feedback resistors (the

0.22 ohms in the suggested diagram in the datasheet) and have that one trigger a, say, 1-second pulse going to the shutdown input pin? ** A dead short would destroy ALL the output devices in 5 mS.

You sure YOU can engineer a schemes to stop that ?

YOU cannot even engineer a simple heatsink or see how easy it is to mount TO3s through a PCB.

... Phil

Reply to
Phil Allison

On Thursday, February 20, 2014 8:37:46 PM UTC-5, snipped-for-privacy@mailinator.com wrote :

to the LM3886 and one of the LMEs (49810 or 49811 or 49830), but i'm very much tempted with the LMEs (main two reasons: higher audio quality and conf igurable power -- that is, i'm not stuck with 70W; i can choose the right t ransistors and the right supply voltages and i can build a 50W amp, or 100W , 300W, etc.)

ansistors, in particular TO-3 transistors needing a heat sink that could ma ke it difficult to solder the transistor pins directly to the printed-circu it board.

You can get connectors for TO-3 packs.. (I entered TO-3 connector into digi key.)

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p to 4 or 5 inches long) from the transistors to the printed-circuit board and nothing else? place small capacitors (say, 1nF or 100pF --- or even l ess??) in the board or at the transistor pins (or both?) to prevent oscilla tion? if so, capacitors connected how? base to gnd and emitter to gnd? c ollector to base and base to emitter? (and collector to base?)

At audio frequencies a few inches of hook-up wire is fine.

each pair of pins? then send the wires to the printed-circuit board?

I don't know what the capacitors are for, just forget that idea. (IMHO)

directly to the printed-circuit board?

es) will be much appreciated!

Art of Electronics is a good book. It has a chapter on various consutrctio n techniques.

George H.

Reply to
George Herold

?? well, i have to confess that the mechanical parts of these projects are the the big nightmare for me. but still, i don't think it is as easy as y ou suggest. well, or not inexpensive in any case. simple calculations (as per LM3886 datasheet) indicate a 0.5 deg per W heat sink (and that is for just 50W). a quick lookup through digikey.com shows just two suitable uni ts: one at $130 and the other one at $250. the rest are all intel CPU heat skinks between $30 and $40 (which i guess having a perfectly flat side, i could figure out a way to attach the 3886 to them)

the thing is: if i was thinking mass-production, then yes, i would find ex trusions (even custom-made ones) from whatever manufacturer at a reasonable price. but as a hobbyist project, i'm finding heat sinks to be the big ni ghtmare of the whole project.

btw, the thermal conductivity of the TO-3s is much better (though i haven't looked up the thermal resistance of the mica film that i need to place to electrically isolate the TO-3 case from the heat sink), plus the max. junct ion temperature is also better. on the other hand, it's true that for thes e transistors i'm looking at, each of them is more expensive than the LM388

6, and i need two of them in addition to the LME!! so that's a good point t o consider.

well, for example, through digikey:

selecting all items that contain TO-220 I get 501 results -- the lowest the rmal conductivity (wihtout forced convection) is 2.6 degrees per W. if i include every item that contains some TO-2XX, i get two that report 1.1 and 0.9 but that clearly are mistakes (they are very small pieces).

if i select TO-3 i get, well, ok, not a ton, but i do get six items below 1 degree per W. ok, three of them are above $100, but one of them, with .96 deg per W, is just $22.

anyway, i'm probably digressing much more than planned. not sure whether i should just start with the LM3886. still, the advantages of the LMEs seem quite solid.

?? ok, now you confused me. i see plenty of them available, and i see ple nty of heat sinks for TO-3. what makes them obsolete? are there other alt ernatives that offer better performance or better convenience? (i guess i could partly answer my own question, in that things with the three pins goi ng parallel and thus easy to solder to a printed-circuit board is a lot mor e convenient than the TO-3 -- precisely the reason that prompted me to post this question here!)

ok (i actually thought so -- i was double checking)

i guess i haven't had many of those; but the ones i've seen blowing up they all end up an open circuit (essentially, the coil acts as a fuse). that's why i wasn't too worried about it.

e

so? the comparator would produce a rising edge within a microsecond or may be even less (no?). is there an additional delay from the shutdown input p in to actual shutdown? or something else that i'm missing?

well, i'm sure i can try :) certainly the comparator circuit that produc es a pulse in the shutdown input is trivial to design and implement. thoug h granted, originally i wasn't even considering the need for such aspects i n the design (i was assuming i'd go with the design as suggested in the LM Es datasheets -- which btw, i haven't entirely discarded this option yet; y ou're certainly giving some good arguments against it)

as Bob posted, one issue is certainly the lots of empty board space that wo uld be needed. plus the lack of flexibility in terms of locating the heat sinks vs. the location of the printed-circuit board -- i'm worried about th is because of the flow of hot air from the heat sink. i would like to have flexibility to locate those so that the heat goes away. i guess i'll conti nue to think about this.

thanks,

-Zico

Reply to
zico

TO3s aren't used nearly as much as they once were, for many of the same reasons you're finding: they're hard to mount to both heatsink and board at once. The obvious method is to put the HS on the back of the board, and run the leads straight through plated-through holes. Unfortunately, since there's very little give in those thick straight leads, this tends to lead to cracking the copper or the solder due to temperature cycling.

TO220s aren't as good thermally, but (a) they're much cheaper, so you can use more of them and spread the heat out, and (b) the leads have a nice bend to them when you mount them, so they're much more forgiving of temperature cycling. You do have to be careful about mounting them, because the single screw tends to make the clamping force very uneven across the copper plate. This is especially important with newer TO-220s, which have little wimpy tabs. A spring clamp right on the epoxy part is generally better than screws.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
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Reply to
Phil Hobbs

?? well, i have to confess that the mechanical parts of these projects are the the big nightmare for me. but still, i don't think it is as easy as you suggest. well, or not inexpensive in any case. simple calculations (as per LM3886 datasheet) indicate a 0.5 deg per W heat sink (and that is for just

50W).

** What utter crap !!

A heatsink with 5 deg C per watt is sufficient.

Look at what others are using you damn fool.

?? ok, now you confused me. i see plenty of them available, and i see plenty of heat sinks for TO-3. what makes them obsolete?

** Err - large flat pack devices.

Like TO3P and TO247.

i guess i haven't had many of those; but the ones i've seen blowing up they all end up an open circuit (essentially, the coil acts as a fuse). that's why i wasn't too worried about it.

** Then you are a complete fool.

Voice coils are double layer wound, so the first insulation failure is layer to layer shorting making the impedance drop to half or less. Then the ( now misshaped) voice coil starts rubbing on the magnet making complete shorts.

well, i'm sure i can try :)

** Trying is all you are, very trying.

as Bob posted,

** Fuck Bob.

You are a clueless, raving lunatic - piss off.

.... Phil

Reply to
Phil Allison

"George Herold"

** Audio frequencies are not the problem - f****it.

Somebody should tatoo " Public Menace" on your forehead as a warning.

.... Phil

Reply to
Phil Allison

Why use ancient TO3s? TO247 types are cheaper and better thermally.

You can do something like this:

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or

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and avoid all those wires.

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John Larkin         Highland Technology, Inc 

jlarkin att highlandtechnology dott com 
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John Larkin

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