ESD protecting MOSFETs

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ESD protecting MOSFETs

Postby smeezekitty » Sun Nov 22, 2020 8:27 pm

I'm putting this in off topic because even though it is NBTV related, it is more of an electronics question.

I am using a pair of Klaas Robert's LED drivers for my bicolor mechanical TV. They work amazingly!
Except I have gone through two MOSFETs now (they're stuck on) due to (I assume) ESD damage. They don't get hot so its the only thing I can think of.

Is there any way to protect the gates of the MOSFET without sacrificing too much NBTV bandwidth?

Thank you
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Re: ESD protecting MOSFETs

Postby Steve Anderson » Mon Nov 23, 2020 3:48 pm

I've seen more than one version of a MOSFET LED driver, maybe not all done by Klaas, but if you could post up the circuit here that may help.

I assume it/they worked for a while and the next time you powered up they're dead, correct? Grounding and the power supply would be my first thoughts. Did you do anything else than power-down then power-up again? i.e. plug/unplug, solder/unsolder anything?...basically any changes aside from power-off and power-on? Is the power supply switched-mode or linear? Also anything 'dirty' sharing the same power supply? e.g. a brushed DC motor?

I'm sure Klaas will have his thoughts as well...

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Re: ESD protecting MOSFETs

Postby smeezekitty » Tue Nov 24, 2020 12:49 pm

Maybe it wasn't actually Klaas who designed the circuit. But in any case, that's where I got it from. viewtopic.php?f=1&t=2557#p23409

I had touched the input lead at one point so that's why I suspect ESD. The power supply is an HP adjustable linear supply (fairly high quality).
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Re: ESD protecting MOSFETs

Postby Steve Anderson » Tue Nov 24, 2020 6:02 pm

smeezekitty wrote:I had touched the input lead at one point so that's why I suspect ESD.

Most discrete MOSFET devices are fairly well protected from ESD these days but it still is wise to take precautions. That also applies to CMOS and all other varieties of static-sensitive devices.

I know I bang on about this seemingly endlessly, but supply decoupling/bypassing is essential. Even if you've only got 30cm (12") of wire from the PSU to the application, add the supply caps. Even without zapping chips or discretes through handling, they're still at the mercy of cool and low-humidity conditions. that's Europe and the US at this time of year. Thunderstorms...another subject. (I'm in the tropics).

The gates to static-sensitive devices can be protected externally depending on the application. But as you assemble the final circuit you need to be careful.

It might be worth investing in an wrist-earthing/grounding strap, I haven't, but maybe where you are it's a good insurance policy. Make sure your soldering iron is properly earthed/grounded as well...in this case mine is...

https://www.amazon.com/earthing-bracele ... g+bracelet

Many, many others out there too...

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Re: ESD protecting MOSFETs

Postby smeezekitty » Mon Nov 30, 2020 6:25 am

Does anyone have experience with TVS diodes?

I agree decoupling is important but that isn't likely to lead to the death of a power MOSFET. And I would like to have it so I don't need to wear an anti-static device just handling the device.
To be clear, I only touched the audio input, before the input coupling cap
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Re: ESD protecting MOSFETs

Postby Steve Anderson » Mon Nov 30, 2020 3:25 pm

smeezekitty wrote:Does anyone have experience with TVS diodes?

Yes, I have used them on my RS485 house 'network', so far after at least 16 years nothing has failed due to static/thunderstorms. Or me extending or adding stuff to it, plugging in or removing items.

As RS485 is only a 5V system I chose a bi-directional TVS rated at 5.8V and 600A pulse current. You can view them as a brute-force Zener. I used CAT5/6 UTP cable, four twisted pairs, and cheap. The length end-to-end is around 75-80 metres, some of that is outside. How CAT5/6 cable would cope with European or US sub-zero winter temperatures I don't know...it never happens here.

But...these devices do have a lot of capacitance, so they're not suited for high speed networks or data links. My 'network' is only 57kbs, there's no need for faster...although screw-terminals are shown in the scrap circuit, they were all changed to 15-way D-types.

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Re: ESD protecting MOSFETs

Postby smeezekitty » Mon Nov 30, 2020 7:08 pm

Steve Anderson wrote:But...these devices do have a lot of capacitance, so they're not suited for high speed networks or data links. My 'network' is only 57kbs, there's no need for faster...although screw-terminals are shown in the scrap circuit, they were all changed to 15-way D-types.

Do you think the capactence would be a problem for a 32 line NBTV application? Since it works on a 57k network, I would guess it may be okay for audio signals below 48KHz. But I guess that depends on the impedences and drive strengths involved.

Because I'm only trying to protect against human body discharges and not lightning or anything like that, maybe if I use a smaller diode, the capacitance isn't as large?

Thanks for the info!
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Re: ESD protecting MOSFETs

Postby Steve Anderson » Tue Dec 01, 2020 11:16 am

smeezekitty wrote:...maybe if I use a smaller diode, the capacitance isn't as large?

...seems entirely reasonable...

But why/how are you blowing these devices in first place? They're almost bullet-proof these days. Many have handled them (including myself) without a care in the world, with no earth-straps etc., - never a problem! Maybe a defective batch? Most unlikely, but possible...

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