TVS diode as flyback diode - Page 1
Forum section: electronics / projects

author: halexa
author_rank: Regular Contributor
timestamp: December 17, 2014, 12:37:01 pm
content: 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)

author: madires
author_rank: Super Contributor
timestamp: December 17, 2014, 01:24:45 pm
content: 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 17, 2014, 02:29:48 pm
content: 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

author: halexa
author_rank: Regular Contributor
timestamp: December 17, 2014, 07:25:46 pm
content: Hi,Thanks for feedbackMaybe 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.

author: tggzzz
author_rank: Super Contributor
timestamp: December 17, 2014, 07:51:18 pm
content: Quote from: halexa on December 17, 2014, 07:25:46 pmThat information makes it simpler: many components are designed specifically for automotive applications, and you can easily search for them and their application notes.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 17, 2014, 11:14:43 pm
content: 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

author: halexa
author_rank: Regular Contributor
timestamp: December 18, 2014, 08:41:38 am
content: Quote from: T3sl4co1l on December 17, 2014, 11:14:43 pmHi 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

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 18, 2014, 12:54:19 pm
content: 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

author: halexa
author_rank: Regular Contributor
timestamp: December 18, 2014, 06:18:18 pm
content: 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.

author: Seekonk
author_rank: Super Contributor
timestamp: December 18, 2014, 07:18:54 pm
content: That mirrors my experience. The customer is always wrong!