fast, small low-capacitance zener to protect MOSFET gates

The Central Semi CSL05 looks decent at 1.2 pF, but it has a series diode to prevent forward current. How about the ESD0P2RF-02LS at 0.25 pF?

Cheers

Phil Hobbs

Missed the CSL05, SL05, a typo? Anyway, yes, dozens of amazing low- capacitance TVS protection devices available, handling high peak currents for a few microseconds. But my designs (similar to AoE III Fig 3.111, page 209) must handle 150mA continuous forward current, ordinary zeners can do it just fine. That's what the added Schottky diode is for. It contributes most of the capacitance, but increasing the node capacitance from 38pF to 50pF is an acceptable penalty. Maybe some small unipolar TVS can handle that, 10pF capacitance max?

You could put a schottky in series with a zener. The first couple of pulses will charge up the zener capacitance.

Huh, I was going to suggest that, but I figured the series schottky would be forward biased, and so have a lot of capacitance. Is that 'wrong' thinking on my part?

George H.

It wouldn't be forward biased much unless the zener were conducting, which would mean it's protecting the fet.

One could add a resistor to keep the zener conducting, so that the schottky was usually reverse biased. Or, equivalently, anchor the schottky to some power supply.

Interesting idea!

I looked at the AoE page and see that this is a linear amp, not the Pockels cell driver. So charging the zener isn't as obvious.

I like to use PV optocouplers as gate drivers, because they float on the fet source and don't have the huge gate over-voltage hazards.

With the schottky+zener, you could use a PV to keep the zener on, but that may be overkill.

I guess the output fet never gets too close to the V+ rail, so a resistor from V+ to the zener could keep the zener conducting, so the schottky is usually back-biased and low capacitance. Higher voltage zeners will zener at nanoamps current, so the resistor value is reasonable, megohms.

Can you tune out the Schottky diode's capacitance with a small series L?

Is there anything like a string of PIN diodes in a single package?

I've just used Skyworks SMP1330 to protect an RF LNA. The combination of low capacitance (0.7pF) and ability to sink power (almost 1 watt) is remarkable. If I understand it, as the device approaches conduction the I layer thins, increasing capacitance rapidly, so it becomes "conductive" even before the diode turns on.

Clifford Heath.

I wondered about a diode string instead of a zener, but it ups cost & adds bulk.

NT

Hmm, I guess I was thinking that it would be charging the zener C. How fast does it leak off through the schottky? (~1uA?)

Ahh... I'm not seeing something, if the zener is always conducting then isn't the series shocttky also?

I'm not sure how to think about diodes on short time scales. A switch, with appropriate RCL, and then charge conservation.?

George H.

Winfield Hill wrote in news: snipped-for-privacy@drn.newsguy.com:

I thought this item was commonly known as a tranzorb.

The PV drivers are slow, I'm dealing with ns time-scales here.

Both schemes involve two parts, so I'll stick with the parallel Schottky approach. In many linear amplifier applications, one has no control over when the device operation is invoked.

Thanks for the suggestions. Your PV coupler scheme is frequently useful. We suggest it as well, AoE III, Fig 107, page 205, and Table 3.5, shows various floating low-resistance bipolar switches.

Not if the resistor connects to the schottky-zener junction. Keep the zener voltage up, but don't forward bias the schittky.

John Larkin wrote in news: snipped-for-privacy@4ax.com:

Oh Look! Larkin is into scatology.

Oh... that's right... that is the stupid s*it he accuses others of.

Right, I saw this in bed last night... but didn't get up to correct my post :^)

George H.

I'm still hoping someone will try to answer these questions.

Winfield perhaps?

Clifford Heath.

You can only tune out by about 50%, corresponding to the famous doubling of bandwidth for peaked amplifiers. The ideal is closer to 3x for an infinite order network, but those are hard to build and tune, ;-) so we settle for

In this case, you can't argue with the capacitance being a parallel impedance, and all you can do is flatten the impedance with respect to frequency, i.e., swamp it with resistance.

Class A gate drive? The solution is worse than the disease. :^)

String of diodes, no idea. High voltage rectifiers (over 2kV or so). Think I've seen "thermistors" like that, for use in temperature compensation of audio amplifiers (a stack of diodes bonded to the heatsink, instead of a Vbe multiplier). Dunno if that's a standard product.

Low voltage zeners work about as well as a stack of diodes, so *shrug*. There's also LEDs, which probably work better, though they aren't rated for all that much peak forward current (like a few amps for less than a microsecond?). Maybe a big one, but the capacitance will be high yet again.

Tim

A stack of low v zeners gives a very soft knee. The softness only adds. And a lot more C than vanilla diodes.

NT

Hmmm, fair enough.

A stack of low-C PIN diodes, to be specific, with the sudden C change around threshold.

Those actually are PIN, but just too big (too much C, even in series).

As a stack of PIN diodes? These don't turn on like ordinary diodes, is my point. I bet there's a package that has four pairs that could be strung together.

Clifford Heath.