Avalanche diodes?

I see that avalanche diodes have a "knee" in the reverse-bias region when you look at the V-I characteristics.

What is the "knee" voltage? I see ratings of maximum reverse voltage and current for the diode, but nowhere is it mentioned at what voltage the diode starts to conduct in the reverse bias direction.

For example:

Another Q:

If used to protect a thyristor, for example, is the avalanche diode installed reverse polarity compared to a standard rectifier (ie, 1N400x)? (since the avalanche diode is conducting current in the reverse direction and the rectifier is conducting in the forward direction).

Another Q:

Why use an avalanche diode rather than a rectifier of similar max voltage & current & recovery speed to protect a thyristor? Is it that the avalanche diode will work less hard because it will not conduct below its operating voltage (fairly high, I'm guessing), whereas the rectifier conducts above ~0.7v?

Thank you.

Reply to
Gloria West
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You should really do your own homework.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

Maybe you can fix this motor controller for me?

I love usent. It attracts the full spectrum. Even the bottom of the barrel can respond to queries.

Cheers.

Reply to
Gloria West

Since Phil is one of the 'top of the barrel' folks on this group I guess that places you, eh?

You come to a newsgroup when school is starting and ask a question that sounds like homework -- exactly what do you expect the response to be?

--
www.wescottdesign.com
Reply to
Tim Wescott

From what I've seen, avalanche rectifiers have waned in popularity - giving ground to MOVs - which in their turn have stared to give way to SIDACs.

MOVs generally have higher spike quenching capability than avalanche and are less likely to fail S/C, they can however shatter and arc when damaged by extreme surges.

Reply to
Ian Field

The reasons I suspected homework are: (a) you posted it to the design and basics group, not the repair group; (b) the questions are phrased very much like a problem set for a circuits class for technicians.

Without meaning any disrespect, if you don't already know the answer to these questions, your chances of fixing a motor controller successfully appear poor, and you may cause additional damage.

That is, assuming it actually isn't homework. See (b).

Cheers

Phil Hobbs

Reply to
Phil Hobbs

Junky Vishay data sheet! This is better...

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See figs 7/8/9. The lower voltage parts have softer knees. Knee behavior isn't actually specified very clearly; the zener impedance and leakage current specs give hints.

(Some of the fig 9 curves are interesting.)

???

Rectifiers don't spec a max breakdown voltage... just a min; a "50 volt" rectifier may not zener at 200 volts. And some rectifiers can be damaged by modest reverse current; zeners are designed to conduct backwards.

Transzorbs are good for protecting thyristors. They are just brutal zeners.

John

Reply to
John Larkin

Top and bottom barrels?

Sounds like kraut to me! Yum!

Jamie

Reply to
Jamie

"Gloria West"

** That is a "controlled avalanche" rectifier diode - which merely distinguishes it from standard types were reverse breakdown behaviours are unspecified and to be avoided.

Such diodes have been around for decades and are useful where one wishes to build up the max reverse voltage by putting several diodes in a series string. Another use is mentioned in the link you quoted under "pulse energy", where very small amounts of energy can be absorbed during reverse breakdown.

It is not a form of zener diode and cannot be used as a voltage clamp and hence there is no specification for reverse breakdown voltage.

** That is an extraordinarily naive question.
** Even worse.

.... Phil

Reply to
Phil Allison

So your point is.. "I know more than you do"?

Well done.

Reply to
Gloria West

"Gloria West"

** No.

You have made absurd assumptions.

And you over snip.

And you are bad mannered.

And I bet you are not female.

... Phil

Reply to
Phil Allison

Thanks, Ian, for the information. I did not know that MOVs can take the place of avalanche dioces. I had a difficult time locating avalanche diodes locally but MOVs I can find and have bought a few.

Re. sizing: the MOV should be sized such that the clamping voltage is above the working voltage of the target device (that it's protecting) but less than the failure voltage of the device, yes? And the balance between these two is the trick?

Thanks for not playing the BS game that is so prevalent in usenet.

Cheers.

Reply to
Gloria West

"Gloria West"

** Well, that is because they cannot. MOVs are not diodes.

And you are still using the term ambiguously.

Try reading the part of my first post you rudely snipped.

** The bullshit is all coming directly from you.

.... Phil

Reply to
Phil Allison

I like using TVS diodes, the 1.5k series to start with. They make them larger of course.

Mov's have a limited number cycles before they become permanently clamped in the circuit or blow apart. I guess if you have enough energy in the over load, both would destroy themselves how ever, I find that TVS diodes recover more often and they are more compact.

Then again, maybe you should correct what is causing the issue in the first play that is pushing you to this route.

Using MOV's is a good way of getting the equipment on the bench to further investigate problems since they will eventually fail, and if that being the case, it keeps the simple guys from replacing fuses or resetting breakers. If things get that back, something is wrong.

Jamie

Reply to
Jamie

you

ode

For the devices you referenced there is a table for Electrical Characteristics. It provides a value for the Breakdown Voltage, 1600 volts max This is the knee voltage. However for the devices you referenced the typical value is not defined. The actual Breakdown Voltage will be some place between VR, the repetitive reverse voltage rating, and the max Breakdown voltage rating of 16000 volts. The devices you referenced are designed to be used as rectifiers and not in the avalanche mode.

lled

Yes

(since the

&

The following is from

"Avalanche type diodes can result in increased reliability in many applications, particularly those where voltage transients are expected. Due to its high speed and ability to withstand large numbers of transients, avalanche diodes are used to protect circuits against surges, lightning and other transients. They are faster than MOV=92s, zeners, and gas tubes."

Howard

Reply to
hrh1818

"hrh1818"

For the devices you referenced there is a table for Electrical Characteristics. It provides a value for the Breakdown Voltage, 1600 volts max This is the knee voltage. However for the devices you referenced the typical value is not defined. The actual Breakdown Voltage will be some place between VR, the repetitive reverse voltage rating, and the max Breakdown voltage rating of 16000 volts. The devices you referenced are designed to be used as rectifiers and not in the avalanche mode.

** Correct.

Controlled avalanche rectifier diodes are NOT the same as "avalanche" diodes use for transient suppression.

The OP has fallen for one of the perils of doing Google searches under a single word.

.... Phil

Reply to
Phil Allison

Hi Gloria,

After a diode starts to conduct, in either the forward or reverse direction, any further increases in voltage are marked by huge increases in current (a sharply rising curve on the current versus voltage graph).

So for all practical purposes, the knee voltage and the max are the same. These V{sub}R values are basically the parameter you are looking for. That's why they are listed for the various models of the diode in the very first table in the datasheet: the basic info everyone wants is up front.

If the avalanche diode has a VR of 200V, and you put, say, 210V across it, it is toast. Diodes are used with some kind of current limiting resistance (e.g. resistor). If a voltage in excess of the diode's voltage is placed across this circuit consisting of the diode and the resistor, then (approximately) the diode will drop its 200V, and the resistor takes the rest. Current is determined by the resistor's I = V/R. (A more accurate solution demands that we draw the load line: the graph of the resistor current versus the diode's voltage, rather than its own. Where the load line intersects the diode curve is the operating point, or Q point, which tells us the voltage and current.)

Basically the whole point of using an avalanche or zener type diode is to take advantage of its reverse breakdown behavior, so it is hooked up such that a reverse voltage is applied.

I'm not sure that the comparison makes sense.

A rectifier is a circuit that uses one or more diodes to turn AC into DC. It doesn't protect anything.

We should not say "rectifier" to mean "rectifier diode".

A rectifier diode doesn't protect anything either. Well, it protect some part of a circuit from the ingress of a voltage of the wrong polarity! (But if that voltage is too great, the rectifier diode will suffer reverse breakdown, which, unlike for an avalanche diode, is probably fatal for the diode.)

A rectifier does not protect from overvoltage. If a spike of excess voltage comes into a rectifier, it generally goes out. (Unless it's a half-wave rectifier, and the spike is on the suppressed half.)

In rectifiers, the forward drop across the diodes does not go above that

0.7. Again, there is a load line there: the diodes supply current into some load which drops most of the voltage. If the load resistance is too small for the diode to handle, it will fry, but it will not attain, say 1.5V.

The diodes conduct above 0.7V, but they never go far above that. If the voltage in the circuit is much more than 0.7, something else has to pick up the remainder of that voltage.

Reply to
Kaz Kylheku

Maybe I should have used clearer wording.

MOVs don't replace rectifiers, they protect them.

There are various MOV manufacturers who publish datasheets/appnotes/graphs to help you select the best device for your application - usually the maximum VRMS + a holdoff margin - then you select ordinary (not avalanche) rectifier diodes with a generous margin above what the MOV clamps at.

Reply to
Ian Field

0

des

Thank you for your comments. Perhaps you can clear up some confusion. I found this definition for controlled avalanche rectifier. "A silicon rectifier in which carefully controlled, nondestructive internal avalanche breakdown across the entire junction area protects the junction surface, thereby eliminating local heating that would impair or destroy the reverse blocking ability of the rectifier." Whereas Wikipedia says "An avalanche diode is a diode (usually made from silicon, but can be made from another semiconductor) that is designed to go through avalanche breakdown at a specified reverse bias voltage. The junction of an avalanche diode is designed to prevent current concentration at hot spots, so that the diode is undamaged by the breakdown."

These two definitions leave me confused about what is the real difference between a controlled avalanche rectifier and an avalanche diode. The only difference I have found so far is the reverse breakdown voltage for an avalanche diode has tighter specifications than a controlled avalanche rectifier. What else is different and makes one device an avalanche diode and the other a controlled avalanche rectifier?

Howard

Reply to
hrh1818

Thank you for your comments. Perhaps you can clear up some confusion. I found this definition for controlled avalanche rectifier. "A silicon rectifier in which carefully controlled, nondestructive internal avalanche breakdown across the entire junction area protects the junction surface, thereby eliminating local heating that would impair or destroy the reverse blocking ability of the rectifier." Whereas Wikipedia says "An avalanche diode is a diode (usually made from silicon, but can be made from another semiconductor) that is designed to go through avalanche breakdown at a specified reverse bias voltage. The junction of an avalanche diode is designed to prevent current concentration at hot spots, so that the diode is undamaged by the breakdown."

These two definitions leave me confused about what is the real difference between a controlled avalanche rectifier and an avalanche diode. The only difference I have found so far is the reverse breakdown voltage for an avalanche diode has tighter specifications than a controlled avalanche rectifier. What else is different and makes one device an avalanche diode and the other a controlled avalanche rectifier?

Howard

It should also be remembered that zener diodes are only correctly named upto

5V - above that they are avalanche regulator diodes.

Such diodes are not unknown upto about 180V and even higher on occasion.

For controlled avalanche rectifiers you'd probably expect from about 350V upwards.

Reply to
Ian Field

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