TVS diode selection for automotive

Question

I am looking for advice on protecting my circuit from both transient voltages and reverse polarity. I plan to use a unidirectional TVS diode along with a fuse (fuse directly off the battery, and TVS in parallel with the load), the idea being: normal transients found in an automotive environment will be sent to ground as the TVS clamps, and if the power is connected backwards it will blow the fuse. Then I can easily replace the fuse.

I chose this method due to its simplicity, and because this is a relatively high power circuit (15 A continuous current, fused at 20 A).

The normal operating voltage I expect is around 14.6 V. The lowest rated component in my circuit is an IC with a 40 V rating. As for selecting the TVS voltage ratings - I think this is the easy part. Pick a reverse-standoff voltage around 20 V, breakdown voltage around 25 V, and clamping voltage around 30 V. This gives me some wiggle room on the input voltage, and still protects the IC.

My main concern surrounds the reverse-polarity part. How do I go about selecting the diode power based on the few seconds it will be shorted before the fuse blows? I take it that TVS diodes are not made for sustained current of any kind and might be damaged while it waits for a fuse to blow.

Is there a better, similarly simple way to protect the circuit without introducing any losses or parasitic drains?

Answer 1

You can just use a normal diode (well not exactly normal but close) in parallel for that, however, it may be better to consider a reverse polarity protection circuit rather than trying to blow a huge fuse.

If your fuse takes a few seconds to blow at say, 50A, thats around 1V * 50A * 3 sec = 150J, which is a pretty large amount of heat. At that level, you will have to consider physical factors like proximity of diode to other components, heatsinking, etc.

This is a simple circuit, there are more complex ones, or ICs that handle it for you. Your high current means you will need a good mosfet, but thats not really a problem (for example).

Another thing you may want to consider is load dumps. These high voltage pulses carry huge amounts of energy, and will explode most TVS devices you would reasonably use. I would do some research into your system to see if these can exist on your device, and look into ISO16750-2. Is this something you're actually going to sell? Or just a one off thing you want to make? If the latter, you might be able to get away with ignoring load dumps.