diodestvs
I'm going to design a protection system for a 12V boat GPS using TVS diodes. I choose the ISO7637 pulse model in severity III, against of ignition pulse and battery disconnection.
To calculate the energy for this particular application, which curve I have to use?
8/20μsec
10/1000μsec or
10/150msec
and to derate the power of the diode, at what pulse width I have to read? At 0.5ms or at 1ms?
Maybe, but probably not. A key feature of decoupling capacitors is that they are able to quickly supply pulsed currents as close as possible to the chip - without the inherent inductance of copper planes or traces that carry the average power from your power supply to the chip.
When you are talking about picofarads decoupling capacitors, you're talking about extremely high frequency decoupling and thus extremely short and thin leads from your decoupling capacitor to the chip. TVS diodes are probably
1) physically too big to be suitable for this purpose (for this kind of decoupling even 0402 is big) as physical size will inherently create a current loop with some inductance, dampening the effect at the desired frequency and
2) will not be able to supply surge currents at the frequencies of interest, or at least will not be well-characterized at such frequencies.
Very often the parasitic impedance between the lines and diode is some nanohenries. This caused by the length of diode leads.
This impedance slowing the response of TVS diode by nanoseconds, thus the transient pulse can pass.
You can use a TVS diode with so called “flow through design” in which you have to pass rail and ground through the same package.
Of course this needs a proper design of PCB.
Best Answer
Load dump is based on load during disconnect. Power to dissipate existing load may be quite small unless you have a big boat load of accessories.
So it depends. Normally use Joules of stored energy in inductive load. . Even a big cap on 12V may resonate.
so, energy = Joules = Watt-sec.
Power in inductive load (trolling motor?) =W?
1st approximation is quench the same current as the worst case load during disconnect and duration depends on stored energy in a system with a disconnected battery. 1/2 CV^2 and this energy will resonate with open circuit inductance of powered devices but get dumped into TVS. Intermittent contacts giving the worst case condition.
Either measure or guess . My guess is you have a small boat, so just ignition and low inductance wiring.
1ms should be fine. (1/1000us would be my choice of test condition)
Biggest category is for AIr conditioner clutch in a car.
More details here from ON semi. http://www.onsemi.com/pub_link/Collateral/AND9098-D.PDF