# How to design a discrete flat-clamp TVS circuit

I need something similar to the Texas Instruments TVS3300 but for a higher voltage. The TVS3300 is basically a drop-in replacement for a TVS diode that clamps the voltage using an internal mosfet and has a very small window between fully off and fully on.

My maximum working voltage is 60-65V (pref 65V) and I need a max clamping voltage of below 70V. I will use a discrete mosfet for the clamping. One option is to use a zener regulator directly driving the gate of an N channel mosfet. However this has a fairly wide window between off and on and a ~60V zener doesn't have a precise voltage, especially when driven at only a few microamps.

I understand the TVS3300 uses a comparator. Most comparators are only available up to 40V. How can I power a comparator so that it will activate fast enough to block a fast transient when the input voltage is otherwise 0V? An LDO will often have a small start up time.

I also need a low cost (below \$1 USD), low quiescent current, preferably below 10uA and a very small footprint (pref lower than 10mm2 not including mosfet).

• What does this mean: when the input voltage is otherwise 0V Jun 14, 2021 at 11:08
• @Andyaka I mean when the circuit is unpowered and suddenly is exposed to a transient Jun 14, 2021 at 11:30
• You give us voltages and what you want but you missed one minor detail, how much current are looking at?
– Gil
Jun 15, 2021 at 1:13
• @Gil as much as possible. But that's more to do with mosfet choice? Device will be used in varied environments with long leads, low impedance sources and inductive loads. Continuous currents (non-transient) could be 30-60A at 60V. Given the size I can't protect against everything. Jun 15, 2021 at 10:29

I understand the TVS3300 uses a comparator.

yep, that's the magic here: a fixed voltage reference, a voltage divider to divide your input voltage exactly such that at the clamping voltage, it is equal to the reference; both fed into a comparator which drives the gate of your MOSFET or the base of a transistor. Classic crowbar!

Most comparators are only available up to 40V. How can I power a comparator so that it will activate fast enough to block a fast transient when the input voltage is otherwise 0V?

well, build one yourself. Your comparator doesn't need to have fantastic accuracy, right.

simulate this circuit – Schematic created using CircuitLab

You can buy Q1+Q2 as dual transistor in the same package, somewhat matched, thermally well-coupled. R1,R2,R5, Q1,Q2 form a comparator that compares the base voltages of the two transistors (the one with the higher base voltage conducts exponentially more, thereby raising the voltage across R5, thereby reducing the current through the other, thereby raising the collector voltage of the other, all very steeply).

R4-D1 form a voltage reference (you're encouraged to use better references than a simple Zener diode). R3 is a voltage divider adjusted exactly such that at 67 V, the voltage at Q1's base is equal to that at Q2's base.

None of these components are especially hard to buy for voltages > 100 V.

• Thank you, this looks like what I need. However I think it might be hard to obtain such a low quiescent current with transistors? I'll look at implementing something similar using mosfets. Jun 14, 2021 at 12:03
• that should work, indeed. Don't have experience buying matched mosfets, though. Probably they exist. But before that, let's see: when we're close to (but not yet) clamping, at 65 V, 10µA current: that'd be ca 5 µA per NPN, which sounds doable, if overly sensitive to noise and whatnot. Jun 14, 2021 at 12:05
• Yeah, matched MOSFET pairs do exist. Jun 14, 2021 at 12:07
• @MarcusMüller Good to know of these matched MOSFET pairs! Thanks. -- Note: Be careful that they do not go beyond 30V. The best ones as for matching are limited to 10 V of VDS. Those going up to 100V do not seem so matched at a first look (just in the same package). Jun 14, 2021 at 15:00
• @andrea you're right! I just found the category, but I didn't check the datasheets! Jun 14, 2021 at 17:11