Voltage spike protection - Page 1
Forum section: electronics / projects

author: Kay
author_rank: Contributor
timestamp: July 08, 2014, 06:57:13 am
content: I am designing a small handheld LED light charged via a USB port, only power lines of the USB are used in the circuit.The device can be charged with a USB charger ( computer USB, etc..) or with a AC/DC adapter. The maximum charge input voltage is about 11V.I would like to have protection on the charge port for ESD human touch model and also voltage transients, spikes on the AC/DC adapter , USB port.A TVS such as SMAJ series (400W) , TranZorb, would provide ESD human model but would it be sufficient for transient voltage spikes on the USB, hot plugs, AC/DC adapter etc ? a reasonable protection.There is a PIC (SO14) and some other sensitive components such as battery protection IC on the PCB.Thanks

author: AndyC_772
author_rank: Super Contributor
timestamp: July 08, 2014, 07:40:08 am
content: ESD events don't contain much energy in total, but they are very fast and high voltage.The key thing to keep in mind, therefore, is that the total inductance in series between the external pin (that might get subjected to an ESD event) and the protection component you choose needs to be kept to an absolute minimum.A bit of inductance in the wrong place will limit the rate of rise of current through the protection device, which in turn will allow a much higher voltage to appear across it than the ideal. This 'excess' voltage is simply a result of the inductor equation V = L dI/dt. If you need dI/dt to be high, and V to be small, you need L to be small.I've used SMAJ series TVS diodes a lot, and they work well. You might not need something so physically big, though, if you're only looking to absorb ESD strikes rather than higher energy surges. (I tend to use them in automotive projects, where there are much lower voltage but higher energy spikes to worry about).Even a capacitor will help a lot, and a well-placed one will do a good job of absorbing high frequency spikes as their response time is pretty much limited by the ESL of the package and layout. If you're worried, there are any number of small TVS diodes out there which are designed to respond to ESD events.Effective suppression is less about the devices you use than it is about the physical layout. Look up typical values for parasitic trace inductance, and have a play in Spice if you haven't already... it can be quite the eye opener.

author: Kay
author_rank: Contributor
timestamp: July 09, 2014, 02:06:19 am
content: AndyC_722Thanks for your insights. The trace length from the diode to the external PIN is 6mm.Also concerned about the voltage spikes, overshoots caused by the chargers once plugged in or out, wondering if a SMAJ or something similar chosen for ESD be sufficient for such spikes? still looking for characteristics of such spikes on the web though..the load is not inductive so so inrush currents on the PCB

author: AndyC_772
author_rank: Super Contributor
timestamp: July 09, 2014, 06:22:54 am
content: The trace length from the diode to the pin doesn't matter. The pin is nothing special electrically, it's just a continuation of the wire, which is very much longer.The pin, the diode and the rest of the circuit (that needs protecting) should form a 'T', with the protection diode (or capacitor, or whatever) as the vertical. Lay out your board such that there's a trace from the pin to the diode, and then from the diode to everything else. The 'horizontal' arms of the T can be as long as you like.I'm assuming you have a proper, solid GND plane, of course.

author: Kay
author_rank: Contributor
timestamp: July 18, 2014, 01:36:40 am
content: Andyc_772Thank you for your feedback. also for USB voltage spike protection, TI has some parts such as bq24305, ON also has some similar parts, most likely will use one of these parts.