Author Topic: EEVblog #1157 - Transistor Zener Clamp Circuit  (Read 2987 times)

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Offline T3sl4co1l

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #25 on: December 15, 2018, 06:40:16 pm »
Incidentally, if you do need low switch resistance in a BJT, "low Vce(sat)" types usually have very good saturation resistance -- including high inverted hFE (though they aren't rated for it -- do measure this yourself on whatever parts you get!).  Veb is not very high though.

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Offline free_electron

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #26 on: December 16, 2018, 08:56:45 am »
You really shouldn't be doing this. If you drive a B-e junction into reverse you create hot electrons that collide with atoms outside the the depletion zone. They can create new hole-electorn pairs or can collide with the oxide round the junction. ( making it resistive) . This causes permanent, irreversible damage. They can penetrate the oxide and become trapped. IF they have enough energy they create something called an 'interface state' in the silicon/silicondioxide contact layer. This has an effect on the b-e junction. It increases the recombination speed of the charge carriers in the depletion layer and this creates additional base current... collector current remains unchanged. Ic/Vbe remains stable , but at low currents the current gain collapses (ib/ic) due to the added base current caused by the trapped charge carriers.

Problem two is that this is an avalanche behavior. Every electron-hole pair will generate another pair and things avalanche very fast.
The trapped electrons collapse the hfe.

Problem three is that this stuff increases the noise. you get effects like popcorn noise, RTN ( random telegraph noise) cause by the trapped stuff.

You can partially reverse this by heating the transistor to 300+ degree centigrade which will 'heal the interface states' but the traps are lost forever...

Certain RF transistors can be damaged with as little as 2 volts reverse across their b-e junction.

In situations where a b-e junction can see reverse voltage you will find that the designers put two silicon diodes (two diodes in series ) in antiparallel with the b-e junction. That way the reverse voltage can not go above 2vbe.
There exists a special diode stack for that functions.

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Any comments, or points of view expressed, are my own and not endorsed , induced or compensated by my employer(s).
 
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Online nctnico

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #27 on: December 16, 2018, 01:47:06 pm »
But wouldn't this effect depend very much on the type of transistor process used and maximum current? It even seems some transistors are made for this purpose.

Years ago I experimented with trying to get BJT transistors to avalanche. The more modern ones just didn't want to avalanche at all.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline drtaylor

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #28 on: December 16, 2018, 05:50:17 pm »
Almost everyone is missing the point of this circuit. It is a protection circuit for the ohms function of a DMM. In normal operation, no current flows through the transistors. As long as the normal ohms excitation voltage is present, the bipolar clamp is not conducting. The main selling point is the leakage current is so low, that in parallel with the unknown resistor, it does not cause any errors. If you used standard zeners, you'd never get a 20Megohm circuit to be accurate!

When an external voltage is applied to a DMM in the ohms mode, this is a fault that can damage the excitation circuitry, which has to be fairly low impedance. So a PTC device is placed in series with the excitation source. The dual transistor clamp will zener at around 6-7V (it does not matter how accurate it is) from the externally applied fault voltage. The clamp circuit will heat up, but so will the PTC. Once the PTC switches to its high impedance state, the clamp is safe, the DMM is safe. The clamp circuit does not have to dissipate power for long because the PTC switches.

This circuit, built with 3904 transistors, is proven reliable, and can protect the ohms circuitry for thousands of events without failure. There is no noise generated in normal operation. Noise generation does not matter when the circuit clamps, because the ohms function will overload. If you use two back to back, it is a bipolar clamp that protects the ohms circuitry from the accidental application of AC line voltage.

All that being said, you can use 3904s and similar transistors as low leakage diodes in a lot of circuits. Using just the BC junction, you can select devices that leak in the low pA range. This is very advantageous in circuits like sample and holds where even a low leakage diode would not suffice.

I do not suggest or recommend that you would ever use a forward biased BE junction as a zener. It would not be capable of much power. But given sufficient resistance in front of it, it is a great clamp...a low leakage diode in one direction, and a zener in the other. Emphasize clamp! Not a full time conducting zener.

A major advantage of this circuit is that it is dirt cheap!
 

Offline NANDBlog

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #29 on: December 16, 2018, 06:57:45 pm »
I thought the BE junction gets damaged over time, with a reverse current?

It does but this does not matter for a low leakage clamp or Vbe zener diode.
Well, there goes my idea of using it.
 

Offline Dr. Frank

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #30 on: December 16, 2018, 09:15:11 pm »
After reviewing the circuit diagram of the 121GW, i come to this concluion:

The usage of this clamping circuit as protection of the 121GW  O/B/D/C modes is a complete fail.

It does not protect the Y port of the 4053 MUX, as the clamping / zener voltage is too high or un-specified, as the circuit clamps in both directions at > +/-25V only.

So it's letting Y / pin 15 go far beyond its supply voltage (+4 or +15V), if positive over voltage occurs, but also lets the  Y / pin 15 go far below -0.5V if negative voltages to the input jacks are applied, so damaging the 4053 in both cases.
Especially in 'normal' mode, when NOT using the 15V supply, but +4V only, the 4053 is completely unprotected now.

To have low leakage protection, you should only use the BE diode of a bipolar transistor in diode mode, but not in zenering mode, as zenering might damage the BE junction.

In other words, for an effective protection, you should have replaced both 1N4007 of the original circuit by two separate BE diodes of transistors (or n-JFETs) with > 20V reverse voltage specification.

Frank
« Last Edit: December 16, 2018, 09:38:39 pm by Dr. Frank »
 
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Offline Dr. Frank

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #31 on: December 16, 2018, 10:49:54 pm »
OK,
here's the analysis schematics.

The DUT Rx receives either a constant current or test voltage from the MUX, pin15 (or 15V from the SMPSU, U10, not displayed here) via SW 36-37, and over current protection R16 / PTC 3.
The generated voltage over Rx is routed to voltage input, PB0, of the HY3131.



In the original circuit, in case of (over) voltage to V/Ohm jack. D7 will clamp positive voltages beyond VDD into pin 15 of the MUX U9, HEF4053, and D8 will clamp negative over voltages  below VSS, or GND.

As can be seen in Dave's video #1158, 15:46 min onwards, D7 is removed, and D8 is replaced by Dave's zenering circuit, as shown in the changed circuit diagram.
This assembly will zener at >+25V or < -25V only, so exceeding the absolute limits of pin15 in any case of over voltage on the Ohm input jacks.



Effectively, there is now no protection any more.

Frank
« Last Edit: December 17, 2018, 02:30:50 am by Dr. Frank »
 

Offline mjs

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #32 on: December 17, 2018, 03:08:13 am »
I did some Vebo damage tests 2 months ago with HP 4142B I bought. Here's Vebo change https://plot.ly/~msyrjala/11 and hFE damage https://plot.ly/~msyrjala/9 from one of the tests.

I think I'll find my scripts and check if the leakage current is affected by the damage, too!
 
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Offline Kleinstein

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #33 on: December 17, 2018, 05:29:05 am »
The protection with a kind of +-25 V zener clamp in deed look odd for an input that might withstand something like -1 V to + 16 V.  For a positive over-voltage the  chip internal "diodes" would direct some current to rise the supply and depending on the version some 15 or maybe 18 V are still OK, though the digital signals would be no longer valid - so temporary male-function cold happen.
With to the negative side even -7 V would be too much, as there is no negative supply.

Too much current to the chip internal protection "diodes" could cause latchup and thus permanent damage if the supply is strong enough. So a weak supply could safe the chip.
 

It is known that reverse (zener) current to the BE junction can cause some more or less permanent damage.  A similar hot electron effect is also suggested to happening in normal zener diodes and possible process to produce 1/f and popcorn noise.

If this damage also causes an increased leakage current is a good question - it at least seems possible, though it may no effect PNP and NPN transistors in the same way.
 

Online nctnico

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #34 on: December 17, 2018, 05:35:20 am »
I probably missed this but shouldn't the HEF4053 have internal ESD protection diodes? Why aren't these working? There is enough resistance in front of it to limit the current. I'd just remove the 1N4007s. Who thought those where a good idea anyway? And clamping to the supply voltage? :palm:
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline HKJ

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #35 on: December 17, 2018, 06:00:23 am »
I probably missed this but shouldn't the HEF4053 have internal ESD protection diodes? Why aren't these working? There is enough resistance in front of it to limit the current. I'd just remove the 1N4007s. Who thought those where a good idea anyway? And clamping to the supply voltage? :palm:

The transistor clamp may have to handle 0.5A until PTC heats up, that is probably too much for the internal diodes. The smart way to do it is to have a resistor between the transistor clamp and the chip, then you can control how much current the internal diodes must handle.
 

Online nctnico

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #36 on: December 17, 2018, 06:14:21 am »
I probably missed this but shouldn't the HEF4053 have internal ESD protection diodes? Why aren't these working? There is enough resistance in front of it to limit the current. I'd just remove the 1N4007s. Who thought those where a good idea anyway? And clamping to the supply voltage? :palm:
The transistor clamp may have to handle 0.5A until PTC heats up, that is probably too much for the internal diodes. The smart way to do it is to have a resistor between the transistor clamp and the chip, then you can control how much current the internal diodes must handle.
Such a current will likely kill a transistor clamp as well. The HEF4053 from NXP is specified to have a maximum clamping current of 10mA.
« Last Edit: December 17, 2018, 06:23:24 am by nctnico »
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline HKJ

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #37 on: December 17, 2018, 06:20:39 am »
The transistor clamp may have to handle 0.5A until PTC heats up, that is probably too much for the internal diodes. The smart way to do it is to have a resistor between the transistor clamp and the chip, then you can control how much current the internal diodes must handle.
Such a current will likely kill a transistor clamp as well. The HEF4053 from NXP is specified to have a maximum clamping current of 10mA.

I do not know it, but they can handle about 200mA, that is about what they get when you plug a typically cheap meter into 230V mains when in ohm range and then usual survive that.
 

Offline David Hess

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Re: EEVblog #1157 - Transistor Zener Clamp Circuit
« Reply #38 on: December 17, 2018, 07:39:18 am »
But wouldn't this effect depend very much on the type of transistor process used and maximum current? It even seems some transistors are made for this purpose.

A lot of transistors are made for this purpose; they are called zener diodes.

Gold doped and high power transistors are largely immune because they already suffer from low minority carrier lifetime.  Reverse breakdown of the base-emitter junction most seriously affects low current gain.  At high currents, the loss of gain from the junction damage becomes insignificant.

Quote
Years ago I experimented with trying to get BJT transistors to avalanche. The more modern ones just didn't want to avalanche at all.

Avalanche operation is a completely different thing where the base-emitter junction never goes into breakdown.  It relies on the negative resistance of the Vces curve (shown in my third link above) at low currents which not all transistors display.

If this damage also causes an increased leakage current is a good question - it at least seems possible, though it may no effect PNP and NPN transistors in the same way.

I have never noticed that it affects leakage unless of course the transistor gets destroyed and then it first becomes a short.  This is how zener zapping works.
« Last Edit: December 17, 2018, 07:41:02 am by David Hess »
 


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