On Mon, 14 Jul 2008 19:44:02 -0400, Fred McKenzie wrote:

In article ,
cliff wright wrote:

If a search tells you that they are Darlington devices get
rid of them right off!!!
Darlingtons have a minimum collector/emitter voltage of ~1.5 volts or
more so they will get very hot indeed at 30 amps or so.

Cliff-

Are you sure that applies to Astron-type power supplies? The ones I'm
familiar with do not operate with the pass transistors near saturation.

Perhaps a Darlington device is not the best choice for some other
reason, but I would think a pair of 50 Amp devices could handle 15 Amps
each (30 Amps total) with sufficient cooling, if they are rated for at
least 120 watts each. (I assume 22 volts input, 14 volts output, 8 * 15
= 120 Watts per device.)

Fred
K4DII

--------------------------------------------------------------------------------------

Fred is absolutely correct in saying that the pass transistors in most linear
regulators such as those in the Astron supplies do not operate as saturated
elements. Therefore, the higher CE saturation voltage of a darlington has no
bearing in this discussion. On another point, the 120 watts per device that
Fred mentions is at a case temperature of 25 deg, C. Heatsinks of reasonable
size can't dissipate that much power and keep the case temperature at 25C.

I have a few talking points that nobody has touched on directly.

If the regulator was designed to use normal (non-darlington) pass transistors,
and you substitute darlingtons in their place, some very bad things could (will)
happen.

The output of the regulator will likely try to go to its full input voltage rail
(because of the high gain of the darlingtons). The feedback loop in the
regulator will try to bring it back down to the set output voltage. It probably
can't because the gain of the darlingtons is too high, and the output stays
banged against the input rail (22V).

Even if the regulator loop can bring it down, the control loop isn't compensated
correctly for the high gain of the pass elements, and the regulator will
oscillate wildly. Both conditions are catastrophic for a power supply.

Another point that I would like to mention is that of a transistor's SOA or Safe
Operating Area. There are two graphs in the datasheet for the transistors. The
SOA graph shows the safe combinations of collector current and collector-emitter
voltage AT A CASE TEMPERATURE OF 25 DEG C. The power derating graph shows how
much power the transistor can safely dissipate at a given temperature.

You MUST use thesse graphs when designing a pass element. They will help you
choose the appropriate transistor types and will tell you how many transistors
you have to use in the pass element to safely supply the design load current.

In almost all cases, the current through each transistor in the pass element is
limited to much less than the absolute maximum collector current spec. Thus,
although a 2N3771 transistor has an absolute max Ic of 30A, it won't be able to
pass that amount of current at any value of CE voltage. The datasheet gives the
conditions for that current; the higher the collector current, the lower the CE
voltage allowed.

Sorry for being long-winded, but it's not a 2-sentence discussion.

Cheers!!!
==============

Dave M
Remove all of the Xs from the reply address Dave M
Never take a laxative and a sleeping pill at the same time!!