Zeners in Series

Suppose one of the zeners in the stack has very low leakage. So low that th e others in the stack don't turn on. The bleeder resistors would then allo w the other diodes to turn on and eventually the bad actor would turn on to o. Once the series string of diodes turns on they can 'share the same damn current'. Bleeders among the cells in a battery would be counter productive wasting t he energy with no beneficial purpose.

the others in the stack don't turn on. The bleeder resistors would then al low the other diodes to turn on and eventually the bad actor would turn on too. Once the series string of diodes turns on they can 'share the same dam n current'.

Zener current increases rapidly with voltage around the rated breakdown vol tage.

Look at the data sheets.

In so far as zener diodes have a "leakage current", it has to measured at v oltages well below the voltage at which is designed to operate.

If one zener in a string has a lower "leakage current" than the others in a nominally identical string it will have a slightly higher operating voltag e at whatever current you chose to run the string at.

The only way that the other diodes in the stack "wouldn't turn on" would be if the offending diode had blown open circuit. Bleeder resistor wouldn't b e a good way to solve that problem.

the energy with no beneficial purpose.

Equally true of a zener diode string.

Yes, it is. In fact, many TVSs in higher ratings (>=48V maybe?) are stacked die construction!

The absence of a safe breakdown mechanism is precisely what causes leakage problems for some devices. You're literally using the best possible device to stack in series. ;-)

The opposite sutation is of a bit more concern: TVSs in parallel. But there too, they are pretty good. The only downside is, at modest currents (where quiescent power dissipation is the most likely failure mode, i.e., a given part just eventually overheats), parallel diodes will tend not to share current. And that's only down to mismatch in breakdown voltage.

Avalanche breakdown has a positive tempco, so it's not a current-hogging issue like it is for paralleled transistors or (forward biased) diodes. And at high peak currents, internal resistance dominates, so the sharing under surge is okay.

Tim

I made these current-limiting switches:

It's also got a ground-return connection, so if you want to use it as a current-limiting buck converter instead, you can save a lot of power dissipation and operate at higher duty cycle. Otherwise, it works as a two-terminal, bidirectional current-regulating diode, gobbling the excess power internally.

Tim

-- Seven Transistor Labs, LLC Electrical Eng>> Whilst mere hobbyists like myself can easily get away with making up a

Oh, the shame!

BAV23S, dual series, 250V, 4 cents

BAV99, dual series, 70V, 1.4 cents

The BAV23S is great in C-W high voltage boosted flyback supplies. I'd post an example but people would whine.

But isn't the anser the same? Obviously it is done routinely.

Some big diode packages are multiple paralleled diodes. Diodes get ohmic, and have positive TCs, at high current, so it works out.

ISTR you already did a few months ago; one of your 'Dremmelboard' [TM] prototypes. I hope you take proper precautions with the dust!

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Maybe he struggles with the concept of current.

..... Phil

Get out the swords, let's duel!

Here's my programmable power supply for an extreme-UV (13 nm) photodiode. It has a dual diode AND zeners in series.

Try to surrender with dignity.

Why the wacky transformer connection?

Is Q3 really supposed to short the output or is that a stray dot at that junction?

It's a stock, cheap ISDN transformer. The AC is stolen from another circuit, an isolated dc-dc converter.

Stray dot? The mosfet gate drive net name is a hint. The powerup default is zero volts.

Oh that's fun and just my 'speed'. Thanks. George H.

Shouldn't C73, C77 be rated at least 500V? More like 1000V for safety? At least C64 is protected by R88 (pop). And....those BAV23S seem to be rated at a paltry 200V which will surely fry shorted. FURthermore, will that transformer withstand AC line voltage?

Gurr.

for a 75V supply?

AC line voltage?

The voltage at TP15 is about +120.

The input AC is high frequency, swiped from the main DC-DC converter circuit. Here's the whole sheet:

We've sold about 3500 of these with roughly zero failures.

Schematic DOES say AC..

Interesting. But I couldn't find any information about the T932 on your website.