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Apr 11 2014

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Never say “NEVER”

Did you ever hear about Murphy’s Laws ?
Did you ever experience a “perfectly” designed and assembled electronic circuit “explode” in your Face ?
Do you remember what you said then ?

” …. Why did this happen ??
This should have not happened.
It was Impossible to happen !
This circuit should have worked as per design !
This explosion / wreckage should **NEVER** have happened …

When creating high voltage, (and/or high current) circuits, it is a good practice to …..

NEVER SAY NEVER !!!

Specifically, make sure that you often use the concept of so called “assertions” – meaning any and all “irrational” additional elements, that are supposed to protect your circuit against conditions, that will never(*)  occur. Especially if these are high voltage and/or high current circuits.

Be **Protected** from the **Unexpected** .   

Use the True Malcontents Wunderwaffe:   Use a healthy dose of mistrust.

Use ASSERTIONS.  Test for the occurrences of conditions that are impossible to happen.

Assertions, in the form of protective add-ons, are additional elements mounted into the circuit, ones that seemingly do “nothing at all”.  Ones that will “never” have a chance to … operate. Ones that upon first glance may be considered as “nonsense” elements.

A very popular example: look for them in places such as output stages of high power silicon based amplifiers, especially those with bipolar transistor based output stages … Imagine something like a pair of “NON-Sense” power diodes, which are polarized in a direction, in which they can “NEVER” conduct:

a). from the negative supply rail to the speaker output,  and
b). from the speaker output to the positive supply rail.

Seemingly bizarre ?
Seemingly absurd ?

Not necessarily.

You could get the impression, that such elements are utter nonsense, as they will virtually “NEVER” be active.  And yet they are THERE, within the circuit !!!

Some designer has put them in, for some unobvious purpose.

They are there “just in case”.  You may ask: in case of WHAT ?

Well, in case that a highly inductive woofer coil in your speaker will generate a back EMF voltage spike, of a voltage very much exceeding the voltage value of the supply rail voltages. If the spike is of a sufficient magnitude, it could easily destroy your output transistors.

NEVER say NEVER. 

Do you remember that CCS current source that I presented some time ago ?  This circuit works at high voltages.  This is where our Caution / Red Signal should start mentally flashing …

High Voltages means a higher chance of Murphy’s Laws to happen …

DN2540 CCS Cascaded

This circuit as above, the DN2540 HV CCS, can also be additionally equipped with seemingly “nonsense” elements, ones that seemingly provide no value added at all ….

Have a closer look at the specifications of the maximum permissible Gate Voltages (V_gs MAX) within the pdf specifications of the DN2540 semiconductor chip.  The specification says that the maximum gate voltage can not exceed +/-20V.  Nonsense you say. Such voltage extremes can physically “never” exist on those gates within this circuit.

Did I hear you say “never” ?   …..  So what do you intend to do about that ?

Easy. We shall Never say Never, and hence we shall **NOT** assume that such an absurd voltage condition is impossible.

How do we protect the gate from “impossible” gate voltages, ones that exceed +/- 20V ?

Easy. We piggy-back two Zener Diodes, in reverse directions, both Zener’s with a zener voltage of say 12V, or better yet, maybe even 15V. We connect them in parallel to the resistor that takes care of the Gate bias voltage.

 

DN2540 CCS safe

This is as if we were to say: “…. whatever happens in this circuit, the voltage across this particular resistor  bias resistor R1 for Q2,  (or this “resistor”-Q2, acting as a cascaded bias resistor for Q1),  the voltage across this resistor shall “Never” {{  ;)  }} … ever … be greater than 12 Volts ….

{{ Pun – Intended }}.

This way, we shall “guarantee” (within reason), that the DN2540 shall never be destroyed by an excessive gate voltage, a voltage that would exceed 20V.  If such a voltage condition comes to exist, the zeners will “activate” and kick in a bit earlier, limiting the maximum gate voltage to a safe value of no more than 12 or 15V.

But will these extra zener diodes have an impact of the sound and the workings of the circuit during “normal operating conditions” ?  Rather not.  Whilst “normal operating conditions” exist, such voltages will “NEVER” be possible to occur on the gate.  In other words, these zeners will remain inactive. They remain “invisible”.  They literally fall out of the circuit. They will “never” conduct.

But having said the former, I shall reiterate with full force:  Never say Never.

I suggest that you do use those protection zeners and provide for a long and healthy life of your DN2540′s within this high voltage circuit.

The same applies to the other CCS circuit, that I suggested elsewhere within these pages.

Better be safe than sorry.

Cheers,

zjj_wwa

 

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