ESD protection / detection - Page 1
Forum section: electronics / projects

author: DzAnej
author_rank: Contributor
timestamp: December 23, 2021, 07:56:32 pm
content: Hello there,I am working on a product that has an integrated TFT display, which is controlled by a custom made PCB with a microcontroller on it. I've sent the product to an institution to get a CE certificate and there they conduct a few tests, one of which is an ESD event. The product doesn't pass the ESD test because it freezes though it should continue normally after the event.I found out by testing it that it is not the microcontroller that freezes but the TFT screen does. I have already made a watchdog timer on the microcontroller so that if it freezes it gets reset. I would now need to make a watchdog for the TFT display. I've tried making a re-initialisation every few seconds so that way it cannot freeze for longer than that period - but that makes the display very hard to watch because it is constantly refreshinh (black screen and rewriting/drawing everything again).I was now thinking the solution would be to make a circuit that detects an ESD event and triggers an interrupt on the microcontroller so that he restarts the screen when an ESD event odccurs.I am now asking for any advice or schematic for such circuit that detects very short pulses of high voltage spikes. I am attaching a picture of the event captured with an oscilloscope - i've made a recreation of the event with a 5-7kV spark near the product. From the scope it is clear that the event lasts about 200ns (sometimes less, sometimes more) and there are a few spikes with the voltages going up to 240V (again that may differ from event to event of course).To sum up what I want: a circuit that detects souch ESD event and outputs a 5V signal that is latched or a few milliseconds long (a few 10ms).Thank you all.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: December 24, 2021, 05:21:18 pm
content: I've had LCD displays latch up in a couple projects when hit with the ESD test.There are many variations to this problem. The path to the LCD controller can be capacitive through the glass, through the circuit board, to the case, etc.1. I put a ribbon-cable ferrite on the control cable. This helped with capacitive coupling at the cutout of the metal enclosure. Depending on the construction, this may not be possible.2. If the LCD responds to a software reset, review the controller datasheet for any registers you can read back. It may be that you can read corrupt data, get a timeout, etc. when the LCD is latched. However, this might not work and the registers read fine. In that case, the line drivers might be the problem.3. Because there is an LCD, it's likely you have user-interface buttons. Instead of a timing-based reset cycle, run the reset cycle when a combination is hit. I used the "Clear" or "Enter" buttons being hit 3 or 4 times in a row.4. One variation of the problem was the display would be rendered upside-down. Otherwise it worked. Instead of re-initializing every few seconds, perhaps do it after 10 minutes or an hour of no activity.5. If you have a backlight timer, initialize when activity turns it back on.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 24, 2021, 06:20:24 pm
content: For the detector, that's easy enough; an RF detector (little more than a wire in a likely location, inductor, coupling capacitors, schottky diodes..) and maybe a pulse stretcher will make it readable by MCU.Maybe even simpler than that, like -- put the antenna wire on the MCU pin directly, attach a pin change interrupt, and let it cook. Likely either the induced latchup crashes the device, or the rapid-fire series of interrupts overflows the stack, to the same end.Mind that, if the pin structures latch up, a soft reset won't fix it, it needs to be power cycled; using a LDO with a fairly tight current limit may be useful here (e.g. a 50-100mA limit so that the MCU's ballpark >100mA latchup causes itself to brown out and clear the latchup).Mind, these aren't at all reliable methods, there's no guarantee that your detector fires from the same stimulus that knocks out the LCD.As for mitigation:If this is a plastic enclosure, there's still plenty that can be done to improve immunity. The display itself can be screened (use a window with an extremely fine mesh covering, or ITO coating), though at notable expense to visibility. If the display has metal brackets/bezel/mounts, ground them. Construct a metallic shell around, and/or including, the PCBs, so that ESD washes over them, preventing induced fields in wires, traces, etc. The enclosure can be lined with foil or formed sheetmetal, or painted with conductive paint; PCBs can be designed with internal ground planes (or stitched outer layers, doing about as good a job for 2-layer builds, when you don't need the extra density of a 4+ layer build).The shield must be reasonably solid; ESD easily passes through gaps/slots of 5cm+ length. So, EMI springs, screws, whatever kind of contact points, should be at least as frequent.And yeah, if it's the LCD driver latching up altogether, you really don't have much recourse but power cycling. Maybe the failure is apparent on the interface (reads are corrupt -- if your panel has a read function at all).And you don't have to do the full reset cycle, it may do just to reset the config registers (e.g. screen size, orientation, etc. -- this would address the above-mentioned flip symptom, presumably), and try to avoid chain-writing the screen forever, do a location reset every so often so the pixel position doesn't get out of sync.A common symptom is extra clocks, for any kind of interface really, but especially SPI being clock sequential. If you're doing continuous pixel updates, just copying from an internal frame buffer -- it can easily get off by a pixel here or there. Just resetting the frame every time, isn't a bad idea.Quote from: Tomorokoshi on December 24, 2021, 05:21:18 pmGotta say, this is a surprisingly intuitive solution. Like, "alright fine, we expected this might happen, so here's a freebie for you", *mashes button* and there you go. Like, you might not even need to put that in the manual for users to discover it.Tim

author: Aheld
author_rank: Regular Contributor
timestamp: December 26, 2021, 07:01:31 pm
content: Do you have the possibility to add ESD protection devices to your PCB design?This devices will detect over voltages of ESD strikes and can reduce this voltage to uncritical levels.Companies like Nexperia offers a lange bandwidth of very specialized protection solutions.Do you know if the data lines are the issue or maybe the power supply of display?BrAxel