0
\$\begingroup\$

As im searching for TVS diodes, I see a 30KV rated diode array has pins RIGHT next to each other. Aren't they so close that a spark can easily jump between pins??

I read a few time that you need a certain case size for protection components so that Sparks don't jump the components, yet I see 30KV rated bidirectional TVS diodes with a 0402 case size!

I need to make my protection circuit as compact as possible and as high rated as possible. I'm having a hard time finding component size, and spacing guide. Anyone can help? Thanks!

\$\endgroup\$
4
  • 1
    \$\begingroup\$ Somewhat facetious: The TVS diodes are there to prevent the energy from going further 'downstream'. They do that by going conductive at high voltages. Any sparking along the outside is great as all that energy no longer has to go through the diodes. \$\endgroup\$
    – Oldfart
    Oct 9, 2019 at 12:24
  • \$\begingroup\$ I guess that does make some sense. But do you know of any guides or recommendations for other component spacing? I was going to do series resistor --> cap --> TVS --> series resistor --> cap --> input. Should I remove the ground copper pour under each component? These protected inputs are just monitoring switch closures. Recognition time isn't important \$\endgroup\$
    – James Pie
    Oct 9, 2019 at 12:34
  • \$\begingroup\$ Smaller packages for TVS are actually an advantage, since they have lower inductance. Helpful hint: don't thwart this advantage (with any size TVS) by attaching it with long signal/ground lines. Best practice is to have one pad on the signal line (so that no signal can get onto the board without passing that pad) and the other on the shortest possible path to the ground plane...and the TVS as close to the entry point as possible, so the ESD can't arc to another node before getting to the TVS. If you're going to put a guard at the door, you make sure all visitors have to go past the guard. \$\endgroup\$ Oct 9, 2019 at 15:04
  • \$\begingroup\$ Right, I understand this, but id need to lower and slow the power entering the TVS, I will have a series high pulse resistor with parallel high voltage capacitor before TVS. My main concern is arch jumping across resistor rather than through it. Is there a case code and distance I should maintain between input terminal, resistor and capacitor? \$\endgroup\$
    – James Pie
    Oct 9, 2019 at 15:10

1 Answer 1

1
\$\begingroup\$

The "30kV" refers to a ESD test voltage with a specified circuit as in EN/IEC 61000-4-2 standard. At no time will 30kV appear across the device if it is within the specified limits and working- it's intended to clamp the voltage to a relatively safe level assuming a limited current.

Here is the circuit used in the standard (which is not free), from this application note: enter image description here

As you can see, at a 30kV test voltage, the current through the device will be approximately 30kV/300\$\Omega\$= 91A, which typically corresponds to Ipp (peak pulse current) in the datasheet. For example a 3.0SMCJ20AQ with a standoff voltage of 20V will clamp an Ipp of 92.6A to less than 32.4V. So, for example, a voltage regulator with an absolute maximum input voltage of 35V should be protected in the test situation.

\$\endgroup\$
5
  • \$\begingroup\$ Thanks for the answer! Since I will have series resistor and parallel cap between input and TVS, does this RC circuit need specific case code sizes and specific spacing? \$\endgroup\$
    – James Pie
    Oct 9, 2019 at 12:41
  • \$\begingroup\$ The parts shown are not on your PCB, they're in the test ESD generator, and yes they're going to be special. If the circuit shown causes 90A to pass through some RC circuit you would have to ensure that all the parts you specify are not damaged by a pulse of the desired level. Clearly if you put 330 ohms in series it will see about 15kV @45A, for example. \$\endgroup\$ Oct 9, 2019 at 12:55
  • \$\begingroup\$ I know that the components in your diagram are just for the ESD gun. But in my circuit, I figured an RC before the TVS will allow slower, less power to be handled by the diode. Certainly I don't need a resistor and capacitor rated for the power for the pulse right? I assume the resistor and capacitor specs are for sustained power, not a few ns pulse? I was going to use 1/4 - 1/2 resistors and 630v caps. Would this not be enough? \$\endgroup\$
    – James Pie
    Oct 9, 2019 at 13:16
  • \$\begingroup\$ Maybe. You can get pulse withstanding resistors with clear ratings. With the test circuit if your capacitor is 100nF less than 50V will appear across the cap, and resistor sees virtually the whole pulse voltage. Do you actually need to comply with a 30kV rating? \$\endgroup\$ Oct 9, 2019 at 13:24
  • \$\begingroup\$ No I don't need to comply with 30kV. I'm trying to keep the protection circuit compact as possible with high as possible ratings. This is industrial application. Lightning is main concern. Some switches are connected to uC input via wire runs up to 1000' \$\endgroup\$
    – James Pie
    Oct 9, 2019 at 13:34

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.