TVS diode as flyback diode

[halexa]

Hi,

Im designing a curcuit with an 12V 2A motor driver. The motor is driven by an HS-Driver.
On the output from the driver to the motor i have an bidercitional TVS diode to protect from transients and ESD (SM6TY).
My question is do I also ned a Flyback diode or is the TVS Diode enough?

See attached AAD (analog aided design)

[madires]

For a bidirectional TVS I'd add a flyback diode because the breakdown voltage of the bidirectional TVS is higher than 0.7V in both directions. Or you could use an unidirectional TVS.

[T3sl4co1l]

You neither want nor need it; the flyback diode alone is what you want.

I assume this is a DC supply and the switching frequency is high enough to maintain continuous motor current?

Tim

[halexa]

Hi,

Thanks for feedback

Maybe I should have mention that this shall be automotive rated protection.
I think I'm gonna need the TVS diode when they zap the output at ISO7637-2 verification. 

[tggzzz]

Hi,

Thanks for feedback

Maybe I should have mention that this shall be automotive rated protection.
I think I'm gonna need the TVS diode when they zap the output at ISO7637-2 verification.

That information makes it simpler: many components are designed specifically for automotive applications, and you can easily search for them and their application notes.

[T3sl4co1l]

What magnitude are those surges, and where are they applied?  (Motor to ground?  Supply input?  Everything?)

How does the ratings of your switching transistor(s) come into play?

Power switching circuits are rarely if ever given additional protection, because the natural properties of the circuit (such as MOSFET body diodes) tends to clamp surges into stiff supply rails which absorb small and medium surges easily.

Tim

[halexa]

What magnitude are those surges, and where are they applied?  (Motor to ground?  Supply input?  Everything?)

How does the ratings of your switching transistor(s) come into play?

Power switching circuits are rarely if ever given additional protection, because the natural properties of the circuit (such as MOSFET body diodes) tends to clamp surges into stiff supply rails which absorb small and medium surges easily.

Tim

Hi as i understand it both the supply, input and outputs will be tested for surges.
Herse a easy to read app note from littelfuse:
http://www.littelfuse.com/~/media/electronics/application_notes/tvs_diodes/littelfuse_tvs_diode_automotive_circuit_protection_using_automotive_tvs_diodes_application_note.pdf.pdf

[T3sl4co1l]

That document seems to claim that all the pulses are constant voltage, and long duration, which is absurd.  I don't see any measure of impedance or time/energy until they talk, at length, about only the load dump pulse.

Your product only has to withstand load dump on the input.  It probably has to withstand only the switching transients on the output.

Tim

[halexa]

I had design review with our clients today.
And they wanted both TVS and flyback diode on the output, based on their experience with other designs.

[Seekonk]

That mirrors my experience.  The customer is always wrong!