# Transient Voltage Suppression diodes

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?

1. 8/20μsec

2. 10/1000μsec or

3. 10/150msec

and to derate the power of the diode, at what pulse width I have to read? At 0.5ms or at 1ms?

• Power derating of diodes is continuous vs time. Not 0.5 or 1ms . Perhaps consider MOV 15 V 100A
– user40708
Commented Apr 25, 2014 at 14:32
• I'm not talking about the steady state power dissipation Commented Apr 25, 2014 at 15:11
• MOVs will wear out -- they are consumables. If you can use a high-power TVS, it will likely have a design life longer than your shortest-life component (likely some capacitor,) which is a good property. Commented Apr 26, 2014 at 2:09

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.

Time duration depends on storage capacitance of loads and resistance. So the exact load dump depends on the load, not the ISO spec for industry standard applications. Your app. is non-std.

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

• Thanks Joe. With dissipation power I mean the power that absorbs by the diode during transient and not by a series resitance. Second question is how to derate the stated power capability from 1ms to let's say 100ms. Sorry for the cofusion. Commented Apr 25, 2014 at 11:59
• TVS diode must handle current from inductive load dump. Energy is VI*t
– user40708
Commented Apr 25, 2014 at 13:58
• Your guess is corect! According to your reccomendations, the energy is less than 1 Joule, so a 600W TVS and a 2,2Ω resistor can do a reliable job. Commented Apr 25, 2014 at 15:05
• Good stuff.. Or a 2kW MOV for 0.50\$ which may work better
– user40708
Commented Apr 25, 2014 at 15:10
• I would go with the TVS, not the MOV, for reasons above. Also a 5 kW TVS vs a 600W TVS is not a cost difference that matters if you build one (or a handful) of the thing. maybe capacitance matters, but only on signalling lines. Commented Apr 26, 2014 at 2:11