MOSFET and zener to clamp inductive spikes - 10-20W of power - Page 1
Forum section: electronics / projects

author: Miyuki
author_rank: Frequent Contributor
timestamp: October 29, 2020, 12:42:51 pm
content: Hi folks,I need a snubber/clamp for inductive spikes, I need to burn about 10-20W of powerThis high power zeners aren't available, so I want to use MOSFET with zener to gate insteadShould I use any capacitor parallel to MOSFET to "smooth" pulses as they will be sharp like 30 V/ns with 600V clamp voltage and about 15A peak currentOr use some smarter way?

author: sandalcandal
author_rank: Unknown
timestamp: October 29, 2020, 01:26:58 pm
content: As in an active clamp?https://www.ti.com/lit/ug/tidu185/tidu185.pdfAs far as I know they're synchronously turned on and have a capacitor in series to help "catch" the energy of the spike. If you need fast turn off to actually dissipate the ringing as heat you could add a series resistor to behave more like a typical RC snubber but switched.

author: jonpaul
author_rank: Super Contributor
timestamp: October 29, 2020, 02:23:49 pm
content: Bonjour, Zeners up to 50-100W exist, and need a heatsink.Clamping "inductive spikes" is vague: What PRF, energy, inductance and current?Normally a reverse diode is used across the inductor, not a Zener.Finally an R-C snubber is another solution.Any design that needs watts of snub/diode clamp dissipation may benefit from a review.Bon Chance,Jon

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 29, 2020, 03:36:32 pm
content: What is generating the spikes?Tim

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 29, 2020, 04:29:51 pm
content: MOSfet's are rarely optimised for operation in their lineair region. I'd think I would prefer to use a good old BJT here, together with a TL431TL431 is an IC that "emulates" a zener diode, has a much sharper knee, and it is also designed to be adjustable with a few resistors.

author: Miyuki
author_rank: Frequent Contributor
timestamp: October 29, 2020, 05:14:31 pm
content: Flyback for test-measure purposesI can easily trade efficiency for simplicity, that is why I want this huge clampIt will run just for short timeAnd I need quick and cheap solution

author: sandalcandal
author_rank: Unknown
timestamp: October 29, 2020, 05:52:55 pm
content: Quote from: Miyuki on October 29, 2020, 05:14:31 pmWell active clamp systems like my earlier link are designed specifically for this purpose of replacing a snubber in a flyback converter.

author: David Hess
author_rank: Super Contributor
timestamp: October 30, 2020, 12:47:07 am
content: P600-2 axial packaged TVSes can handle 30 kilowatts peak and up to 8 watts continuous power if heat sinked well through the leads. Depending on your regulation requirements, several lower voltage parts can be used in series to get up to 20 watts or more. They do however cost $20 each.https://www.mouser.com/datasheet/2/240/Littelfuse_TVS_Diode_30KPA_Datasheet_pdf-269535.pdf600 volts is well within the thermal instability zone of a power MOSFET however this only applies at DC and low frequencies. Peak power capability is more limited by the heat capacity of the junction itself. Look for datasheets which specify peak power capability for different pulse widths. The same applies for bipolar transistors and secondary breakdown at high voltages however the base current drive requirements will be more difficult to meet.

author: jonpaul
author_rank: Super Contributor
timestamp: October 30, 2020, 12:47:34 pm
content: Dear Miyuki: " I need quick and cheap solution"Experience proves that "quick and cheap" is "pennywise and pound foolish".If you took the time to document the exact issue, eg scope photos, schematic, inductor design or spec, you will receive better answers from our mavens, than with a vague question.The cheapest solution is an RC snubber, depending on the pulse width, PRF, energy, etc.Certain converter topologies have inherent clamping eg double foward.Transient Absorbing Zeners are most costly and clamp to voltages like 150V etc.The steady state and peak power of the TAZ is well specified in the mfg sheets.Best source of TAZ data is at the original site, General Semiconductor in Tempe AZ, now part of Vishay.https://www.vishay.com/diodes/protection-tvs-esd/I humbly suggest that you spend the time to read spec sheets, design guide and the old app notes and papers on TAZ.We have used GI TAZ since 1970s in demanding designs, eg high voltage arc lamp ballasts, medical electronics, avionics.Learning about the technical problems and researching the optimum solutions is the joy (and suffering!) of our work as electronics experts.Just the ramblings of an old retired EE.Enjoy and Bon Chance!Jon (in Paris under National confinement!)

author: Pawelr98
author_rank: Regular Contributor
timestamp: October 31, 2020, 11:02:07 pm
content: You could try a different snubber topology.In my simple flyback design I faced a similar problem, too high power dissapation.Looked up some different designs and found an interesting alternative to the classic RCD one.Capacitor goes to ground and is charged through diode that goes to the drain.Resitor goes from the capacitor to the main tank capacitor.All the voltage spikes charge the capacitor and the energy is slowly going back to the tank capacitor through the resistor.Identical resistor in "classic" topology would be burning from excessive losses while in this topology it's just fine, providing good spike supression.The main tradeoff is that the capacitor has to handle higher voltage.In my case with 230V mains there's around 300V DC constant (even if the converter is not working) + the voltage spikes. 450V average and around 500-550V peak.630V capacitor seems to be fine but 1kV would be better.