Steering diode array + TVS vs. TVS array... - Page 1
Forum section: electronics / projects

author: alank2
author_rank: Super Contributor
timestamp: March 26, 2015, 02:28:36 am
content: Hi,Looking at esd protection I have seen both types, one type has steering diodes and a single tvs and other implementations have an independent tvs for each io being protected. What are the pros and cons of each type and when it would be better implemented?Thanks,Alan

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 26, 2015, 04:08:37 am
content: Clamp diodes: fairly fast, low capacitance, low cost.Clamp schottky: very fast, moderate capacitance, low cost.Direct TVS: very fast, moderate to high capacitance, moderate cost.Combining options in the same package keeps the speed about constant, without compromising performance due to trace lengths and stuff. So by using PN junction diodes (fairly fast) with a TVS, it doesn't get any slower.Note that TVS and schottky diodes, intrinsically speaking, don't exhibit any overshoot when clamping transients. PN junction diodes do: this is called forward recovery. Typically, a PN clamp diode, even a very high speed type, will overshoot 20-30V when clamping an ESD transient. It looks like series inductance, but it's physically voltage appearing across the junction.Clamp diodes, with or without an integrated supply TVS, are handy when you have supplies available. The clamping is relative to the supply, therefore, so are your signals once they pass, so they don't go screwing up your ICs that require signals limited to the supply rails. Lone TVS are good when you have a signal level different from the supplies. Examples: suppose you have a +/-15V analog circuit, which interfaces a few 0-5V digital signals -- not enough to be worth adding a whole set of logic interface chips and a supply. These kinds of signals need to be protected with lone TVSs, because there's no supply to clamp to. (If they are high speed signals, you can still use the low-capacitance clamp method, but the TVS needs to be pre-biased, like you would use a shunt regulator.)Tim

author: alank2
author_rank: Super Contributor
timestamp: March 26, 2015, 12:02:40 pm
content: Thanks Tim, I appreciate the knowledge!

author: ajb
author_rank: Super Contributor
timestamp: March 26, 2015, 06:35:44 pm
content: Using a separate TVS has the additional advantage of not disturbing your supply rails when it kicks in the way that clamping diodes would--provided the TVS keeps the transient voltage from forward biasing any clamping diodes built into your ICs. Or you could use clamping diodes to a dedicated clamping node instead of the supplies, as Tim suggests with a pre-biased TVS. You probably don't particularly care about a momentary disturbance in the supply voltage during an ESD event (which are by their nature relatively infrequent), but this could be a serious consideration if you expect your system to encounter more frequent over voltage events and require a particularly clean supply..

author: alank2
author_rank: Super Contributor
timestamp: March 26, 2015, 06:55:22 pm
content: This is probably a dumb question, I've tried reading some info about this and perhaps I am just not getting it.A TVS diode just allows current to flow when it exceeds the breakdown voltage, right? So if you have a TVS reverse biased to ground and connected to a signal line, so long as that signal line stays between ground and the breakdown voltage, nothing happens.If the signal goes lower than ground, then the diode will conduct from ground, right?If the signal goes higher than the breakdown, then the diode will conduct to try to bring it to the breakdown voltage?The electricity is never consumed inside the tvs like it would be in a resistor as heat for example, right?What is the point of adding a TVS to a steering diode array? To have the option to breakdown at a lower voltage than vcc might be at?

author: ajb
author_rank: Super Contributor
timestamp: March 26, 2015, 07:32:07 pm
content: Here's a good app note that goes over the basics:http://www.onsemi.com/pub_link/Collateral/AND8231-D.PDFA TVS will absolutely consume power when it operates, just like any other diode. Part of what distinguishes a TVS from a zener, which behaves basically the same way, is that the TVS isdesignedto absorb high peak energy, which winds up as heat in the junction. How much total energy the TVS will dissipate depends on the amount of energy the transient event involves and the impedances involved.As breakdown voltages go, it's important to understand the shape of the TVS's VI curve (figure 3 in the link). VBR is at the 'knee' of the curve, but note that this is not a sharp right angle--the TVS doesn't go from infinite to zero impedance right at VBR. What this means is that there will be a certain (minute) amount of current flowing through the diode below VBR, and above VBR, as the current through the diode increases the voltage can creep above VBR. You'll need to look at the specific curves for a particular TVS to get an idea of what this really means for your circuit, but the biggest takeaway is that the actual clamped voltage during a transient event may be above VBR.The reason for including a TVS in a steering array is because when the clamping diodes kick in, the current that's injected into the supply rail causes its voltage to increase. In the case of a slow, sustained overvoltage, the current that gets shunted through the diodes into the supply basically replaces some of the current that your power supply would otherwise supply to your circuit. That's fine, as long as the shunted current is below what your circuit consumes, the supply voltage won't budge all that much. But if that supply comes from a series regulator, once the shunted current exceeds what your circuit consumes, the supply voltage will start going up--and there's nothing that your series regulator can do to stop it! So you need a TVS to keep the supply voltage from getting out of hand. If you're using a shunt regulator, then as long as the current doesn't exceed the regulator's capacity, the regulator should be able to sink the excess fault current to ground to keep the supply voltage in regulation.In the case of a faster transient, you have to consider the parasitic inductance and capacitance in the circuit. Having a TVS right in the steering array limits the impedance in the clamping network, and allows it to kick in faster. It also allows you to put the entire clamping network right at the input, which in turn limits the peak voltages that your more sensitive parts, which are further away, will see during a transient.

author: alank2
author_rank: Super Contributor
timestamp: March 26, 2015, 07:57:43 pm
content: Quote from: ajb on March 26, 2015, 07:32:07 pmSo a TVS _does_ try to "eat up" the energy instead of passing it?Thanks for the doc - I'll check it out!