Bench Talk

Circuit protection is like car insurance. Some people feel like they don’t need it if they are driving a very cheap car. I can understand that, as long as you don’t hit a pedestrian or someone else’s car. Car insurance is mandatory in many places for good reason. It’s possible that nothing bad can happen, but if it does, a circuit can bring an entire system down if it doesn’t have circuit protection.

I was looking for circuit protection for a video device earlier today and decided on a Transient Voltage Suppressor (TVS). A TVS is pretty quick at responding to protect a circuit, so delicate circuits that can experience a lightning strike could benefit from a TVS because it would act fast enough to avoid damage. Each scenario is unique, so apply what makes sense in your application.  So I knew a TVS was a good choice for my application.

But should my TVS be “unidirectional” or “bidirectional”?  I had forgotten. Had someone asked me to guess, I would’ve thought it meant that you should apply bidirectional TVS whenever you know the voltage will go above and below ground. This is sort of right, and definitely applies to protecting circuits that experience AC voltage. But “sort of right” isn’t something I am willing to live with, so I checked it out.

Figure 1: Unidirectional TVS from wonderful app note by On Semi: https://www.mouser.com/pdfdocs/AND8424-D.PDF

Choosing a unidirectional or bidirectional TVS device depends on the circuit being protected. Yes, use a bidirectional TVS for AC power because it has bidirectional voltage swings. But this not completely right, because if I am throwing bidirectional TVS at everything “just to be safe,” I am missing something crucial: a bidirectional TVS assumes that going below ground is OK.  This is not the case with a positive DC voltage circuit, where even though 0 to +5 volts might be ok, -5 volts could damage the circuit. This is when a unidirectional TVS is used, because a bi-directional 5V TVS will allow a range of -5 to +5 volts through to your circuit without regret. Choose a clamping voltage that you know will be OK for your circuit and anything downstream (as a good neighbor) to experience. The datasheets might not state what your components can withstand, and do not assume that the max voltage rating is OK. Max voltage rating on a datasheet is a tested value and may not apply perfectly to every single component every time. You need to either run a smoke test on your circuit, or assume the worst and go with the clamping voltages that you know will protect you but will not cramp the functioning of the circuit, either. If you test your circuit, keep in mind that TVS protects against transients and damage can occur that results in a delayed degradation, if not just plain failure.

Thus:

Bidirectional TVSes are used with AC circuits and with any DC circuit that is supposed to experience +/- DC Voltage.

Unidirectional is for something that can’t handle voltages in both directions. TVS for pretty much all ranges are available, so you can tailor it to your application.

Choosing clamping voltages can be difficult, so being conservative isn’t a bad idea. Place a TVS nearest to the input of where the transient is most likely to occur, such as the connector entry point on a PCB, keeping connective traces short to avoid parasitic capacitance. With high speed circuits, choose the lowest capacitance TVS you can find, because capacitance has a definite relationship with frequency and parasitic capacitance can be inadvertently generated.