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Old 15th February 2006, 10:57 AM   #11
amperex is offline amperex  United States
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Exactly, linear PS low voltage & high current design requires large amounts of pass transistors & are vulnerable to failure if one uses non-matched pass transistors. No other way to design it, yet nothing short of a PS with a battery will outperform linear type design.
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Old 15th February 2006, 11:43 AM   #12
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Quote:
Originally posted by amperex
Exactly, linear PS low voltage & high current design requires large amounts of pass transistors & are vulnerable to failure if one uses non-matched pass transistors. No other way to design it, yet nothing short of a PS with a battery will outperform linear type design.
You use low value emitter resistors in each transistor to balance the load - it's a standard technique, and would be FAR better than attempting to use matched transistors (because there's no way they will be matched accurately enough).

Did you discount that method for some reason? (although I can't think of one?).
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Old 16th February 2006, 02:57 AM   #13
Enzo is offline Enzo  United States
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8,000,000 audio amplifiers can't be wrong. Those fractional ohm resistors are called "ballast" resistors in that use. They allow transistors to share current. the transistors all ought to be the same type, but they don't then have to be exactly matched.
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Old 16th February 2006, 01:32 PM   #14
amperex is offline amperex  United States
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Default Disagree........

13.8 vdc @ 130 amperes regulated power supply here- hello.

Typical bipolar power transistors vary in hfe from say 20 to 150. We match closely for a lot of pass transistors. Do some simple ohms law math & you will find to use any hfe from far less a spread of 20 to 150 would require such a high value load sharing lot of resistors that little current would be available at the output. Like I said, we supplied the Military & NASA.

You obviously have no experience in high current linear power supplies. Enzo, your fired.
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Old 16th February 2006, 02:39 PM   #15
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Default Re: Disagree........

Quote:
Originally posted by amperex
13.8 vdc @ 130 amperes regulated power supply here- hello.

Typical bipolar power transistors vary in hfe from say 20 to 150. We match closely for a lot of pass transistors. Do some simple ohms law math & you will find to use any hfe from far less a spread of 20 to 150 would require such a high value load sharing lot of resistors that little current would be available at the output. Like I said, we supplied the Military & NASA.

You obviously have no experience in high current linear power supplies. Enzo, your fired.
I suggest YOU try applying ohms law, I can't believe you didn't use emitter resistors! - and (from what you've posted!) didn't even consider doing it that way? (the CORRECT way!).

It's an incredibly simple concept, the resistors basically give local negative feedback. To give a simple example - based on just two transistors, although it makes no difference how many you use, the principle is identical.

OK, two transistors in parallel, with 0.1 ohm emitter resistors. because transistors aren't perfectly matched (even with your selected ones!), one will turn ON before the other one - fairly obviously. You pick the value of the resistors to reduce dissipation and voltage loss, the lower the value the more current the 'best' transistor will provide on it's own at low loads.

So we'll assume that one turns ON at 0.7V, and the other at 0.75V (nice round figures). As the load increases from zero the first transistor will provide ALL of the power to the load, as it's 0.7V Vbe is preventing the second transistors 0.75V Vbe being reached.

However, the effective Vbe of the first transistor is 0.7V PLUS the voltage drop across it's 0.1 ohm emitter resistor. So once 500mA is flowing through it then it's effective Vbe becomes 0.75V, enough for the second transistor to start doing some of the work.

This bypasses some of the current from the first transistor, which then takes more of the current back. In this way every transistor takes it's share of the load (except at very small currents when you only need one transistor anyway).

The gain of the individual transistors is irrelevent, as long as the drivers have enough current capacity to feed them all - making the circuit device insensitive, as any good design should be.

I'm absolutely staggered that a commercial manufacturer would try and match devices rather than using standard design techniques, and I'm even MORE staggered that the US Military and NASA would use such designs?.

BTW, it's common practice to provide a current meter by measuring the voltage drop across any one of the resistors - obviously this won't work for very small loads - but you choose the resistors accordingly (no point measuring 0.5A with a 130A PSU).
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Old 17th February 2006, 03:15 PM   #16
amperex is offline amperex  United States
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Default to Nigel

I did not say we do not use emitter balancing resistors of fractional ohms. For you to suggest matching hfe is not good design practice means you also obviously have no experience designing or manufacturing high power linear power supplies.

Not one idiot would not close match pass transistors AND use emitter resistors. You suggest using whatever transistor comes out of the freakin box be it an hfe of 15 or 150 & simply toss a lot of 16 of these in a high current PS?

You have no experience in high current power supplies. Do you have any idea of what is required to be Mil-Spec Cage certified?

Your out to lunch here big time here. Go back to audio applications as I do believe you are well versed.

BTW- The original discussion was the tolerance of solid-state devices varies & wildly so with discrete transistors compared to vacuum tubes.
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Old 17th February 2006, 03:37 PM   #17
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Default Re: to Nigel

Quote:
Originally posted by amperex
I did not say we do not use emitter balancing resistors of fractional ohms.
So WHY has it taken you so long to confirm that?, my entire point of view has been that these are essential, and you have always given the impression that didn't use any, instead relying on matching the transistors!.

As for your assertion that you need to match the transistors as well?, I would still say that correct design would prevent that requirement!. However, as your application was to Military spec (with a corresponding massive price tag), there's no harm to a 'belt and braces' approach.
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Old 18th February 2006, 02:43 PM   #18
amperex is offline amperex  United States
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I have been designing high current low noise power supplies since 1973. Why would I not use current balancing resistors? Many PS I designed up to 150 amperes output from the 1970s are still functioning in automotive plants today without failure.

Why would one used unmatched pass transistors & triple the wasted heat thru those transistors to force matching with large value matching resistors?

Should one design & produce a KW of wasted heat? Match the transistors, document the installed hfe right on the PS itself & make for a more efficient design.

Per Motorola, for every 10 degree C drop in junction temperature doubles the life of a transistor. One can not afford improper matching with one transistor operating at a junction temperature of over 200 degree C in the lot.

Anyways, this is really an audio site & I am sure you know electronics design as applied to audio. Time to move on, good day.
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Old 18th February 2006, 02:46 PM   #19
amperex is offline amperex  United States
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Default Found answere to original question....

Yes, different manufacture ICs with the same part number do sound different as noted at Prodigy Professional web site. Look up 5532 under 'The Lab' forum.
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