not totally repulsive

On Tue, 06 Nov 2007 13:01:51 -0800, BobW wrote: ...

If Satan and Lucifer showed up at the inn, and the proprietor rented them each a room, would he be the lessor of two evils? ;-)

Cheers! Rich

Or, as Captain Jack Aubrey said, the proper British sailor always chooses the lesser of two weevils.

John

The chances of this not working are nil; I'd be more worried about meteor damage in shipping. But in fact my customers are great: when we do have a problem, we tell them the truth, we work with them to fix it, we ship them replacement products, whatever it takes. They know that complicated things sometimes go wrong. What they appreciate is an honest, energetic fix.

John

In fact I would think it is more likely that the "proper" LDO solution will e.g. start oscillating, for some reason.

For example, if it prefers black capacitors over orange ones.

Best regards, Spehro Pefhany

If the LDO starts oscillating "for some reason", it is either a faulty (i.e. not meeting its specs) component, or the design was incorrect (failing to take into account the entire range of operating conditions, tolerances, tempcos, drift/aging, etc.) which is much more common, particularly among advocates of using forward biased zener diodes for power supplies.

The diode circuit cannot be designed correctly (because it is being operated outside its specifications).

If you are saying it is easier to "get it working" for a diode circuit than an LDO, you may be correct, under the right conditions (which generally disappear the moment the product is shipped).

As mentioned earlier, continuously forward biasing a diode designed to operate reverse-biased may cause long term problems.

Andy

What makes you think zeners should not be operated in forward bias? The normal zener supply off line voltage switches between forward and reverse bias at line frequency. Do you have any reason to suspect there would be problem?

Best regards, Spehro Pefhany

^^^ any

(Maybe I'm dealing with too many people these days who are not fortunate enough to have the Queen's English as their mother tongue). ;-)

Best regards, Spehro Pefhany

There are lots of tricky/flakey/poorly specified LDOs being sold in volume. Many/most are very sensitive to the capacitive+esr load environment, and many of the datasheets are fuzzy about that. Ditto switchers.

What a bizarre thing to say. I'm an electrical engineer, and I do stuff like this all the time. The behavior of forward-biased PN junctions is fairly well known by now. And as Austin pointed out, the actual operating range of Vccaux is huge.

Why would it do that? What would be the failure mechanism? What would be the failure mode?

Incidentally, zeners are use in the forward direction all the time, like in bidirectional clippers and transzorbs.

John

In the FPGA the engineering is already done for you!

The point is current spreading. Because they aren't intended for handling large forward currents, their junctions aren't designed to handle the thermal effects. Like an SCR's dI/dt rating, local heating can cause failure not expected for that current level.

Tim

-- Deep Fryer: A very philosophical monk. Website @

I'm not all that concerned about dissipating 40 milliwatts in a 1-watt zener.

John

You have a degree in Electrical Engineering. So do I.

When you design electronics that has to work right every time or the wrong people will die, you learn to do it right.

The forward biased behavior of PN junctions designed to be operated primarily (albeit not exclusively) in the negative biased mode is not nearly so simple. I'm not an expert on forward biased zener diodes, and I wouldn't use it that way unless I was. Apparently, from other posts on this, there are potential problems that you and I are not fully aware of. I won't design a product that way. I may design an experiment that way, but I will specify additional analysis, screening, and source control measures to ensure that, once proven to be effective over the entire operating range, a product will continue to operate correctly over its entire range of operating conditions, over the duration of its production and useful life. Anything less is hacking, not engineering. And the cost of those additional measures nearly always exceeds the cost of doing it right in the first place.

Andy

I didn't design the diode in originally; it was a hack to fix a problem on these boards. People who are "not an expert on forward biased zener diodes" are conjecturing problems without suggesting what they might actually be, except prissy comments about "not the intended use", as if the silicon cares.

All rectifier diodes are zener diodes, and they seem to work in the forward direction for a long time.

The only thing remotely like this that I've ever heard of is the fact that longterm zenering of a transistor b-e junction can reduce beta. I've never heard that forward biasing a diode can damage its forward-bias performance. [1]

Hell, I'll probably use the diode next rev, too. I'm starting to like it.

John